| perlfunc - Perl builtin functions |
perlfunc - Perl builtin functions
The functions in this section can serve as terms in an expression.
They fall into two major categories: list operators and named unary
operators. These differ in their precedence relationship with a
following comma. (See the precedence table in the perlop manpage.) List
operators take more than one argument, while unary operators can never
take more than one argument. Thus, a comma terminates the argument of
a unary operator, but merely separates the arguments of a list
operator. A unary operator generally provides a scalar context to its
argument, while a list operator may provide either scalar or list
contexts for its arguments. If it does both, the scalar arguments will
be first, and the list argument will follow. (Note that there can ever
be only one such list argument.) For instance, splice() has three scalar
arguments followed by a list, whereas gethostbyname() has four scalar
arguments.
In the syntax descriptions that follow, list operators that expect a list (and provide list context for the elements of the list) are shown with LIST as an argument. Such a list may consist of any combination of scalar arguments or list values; the list values will be included in the list as if each individual element were interpolated at that point in the list, forming a longer single-dimensional list value. Elements of the LIST should be separated by commas.
Any function in the list below may be used either with or without parentheses around its arguments. (The syntax descriptions omit the parentheses.) If you use the parentheses, the simple (but occasionally surprising) rule is this: It looks like a function, therefore it is a function, and precedence doesn't matter. Otherwise it's a list operator or unary operator, and precedence does matter. And whitespace between the function and left parenthesis doesn't count--so you need to be careful sometimes:
print 1+2+4; # Prints 7.
print(1+2) + 4; # Prints 3.
print (1+2)+4; # Also prints 3!
print +(1+2)+4; # Prints 7.
print ((1+2)+4); # Prints 7.
If you run Perl with the -w switch it can warn you about this. For example, the third line above produces:
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
A few functions take no arguments at all, and therefore work as neither
unary nor list operators. These include such functions as time
and endpwent. For example, time+86_400 always means
time() + 86_400.
For functions that can be used in either a scalar or list context, nonabortive failure is generally indicated in a scalar context by returning the undefined value, and in a list context by returning the null list.
Remember the following important rule: There is no rule that relates the behavior of an expression in list context to its behavior in scalar context, or vice versa. It might do two totally different things. Each operator and function decides which sort of value it would be most appropriate to return in scalar context. Some operators return the length of the list that would have been returned in list context. Some operators return the first value in the list. Some operators return the last value in the list. Some operators return a count of successful operations. In general, they do what you want, unless you want consistency.
A named array in scalar context is quite different from what would at
first glance appear to be a list in scalar context. You can't get a list
like (1,2,3) into being in scalar context, because the compiler knows
the context at compile time. It would generate the scalar comma operator
there, not the list construction version of the comma. That means it
was never a list to start with.
In general, functions in Perl that serve as wrappers for system calls
of the same name (like chown(2), fork(2), closedir(2), etc.) all return
true when they succeed and undef otherwise, as is usually mentioned
in the descriptions below. This is different from the C interfaces,
which return -1 on failure. Exceptions to this rule are wait,
waitpid, and syscall. System calls also set the special $!
variable on failure. Other functions do not, except accidentally.
Here are Perl's functions (including things that look like functions, like some keywords and named operators) arranged by category. Some functions appear in more than one place.
chomp, chop, chr, crypt, hex, index, lc, lcfirst,
length, oct, ord, pack, q/STRING/, qq/STRING/, reverse,
rindex, sprintf, substr, tr///, uc, ucfirst, y///
m//, pos, quotemeta, s///, split, study, qr//
abs, atan2, cos, exp, hex, int, log, oct, rand,
sin, sqrt, srand
pop, push, shift, splice, unshift
grep, join, map, qw/STRING/, reverse, sort, unpack
delete, each, exists, keys, values
binmode, close, closedir, dbmclose, dbmopen, die, eof,
fileno, flock, format, getc, print, printf, read,
readdir, rewinddir, seek, seekdir, select, syscall,
sysread, sysseek, syswrite, tell, telldir, truncate,
warn, write
pack, read, syscall, sysread, syswrite, unpack, vec
-X, chdir, chmod, chown, chroot, fcntl, glob,
ioctl, link, lstat, mkdir, open, opendir,
readlink, rename, rmdir, stat, symlink, sysopen,
umask, unlink, utime
caller, continue, die, do, dump, eval, exit,
goto, last, next, redo, return, sub, wantarray
caller, import, local, my, our, package, use
defined, dump, eval, formline, local, my, our, reset,
scalar, undef, wantarray
alarm, exec, fork, getpgrp, getppid, getpriority, kill,
pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
times, wait, waitpid
do, import, no, package, require, use
bless, dbmclose, dbmopen, package, ref, tie, tied,
untie, use
accept, bind, connect, getpeername, getsockname,
getsockopt, listen, recv, send, setsockopt, shutdown,
socket, socketpair
msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
shmctl, shmget, shmread, shmwrite
endgrent, endhostent, endnetent, endpwent, getgrent,
getgrgid, getgrnam, getlogin, getpwent, getpwnam,
getpwuid, setgrent, setpwent
endprotoent, endservent, gethostbyaddr, gethostbyname,
gethostent, getnetbyaddr, getnetbyname, getnetent,
getprotobyname, getprotobynumber, getprotoent,
getservbyname, getservbyport, getservent, sethostent,
setnetent, setprotoent, setservent
gmtime, localtime, time, times
abs, bless, chomp, chr, exists, formline, glob,
import, lc, lcfirst, map, my, no, our, prototype,
qx, qw, readline, readpipe, ref, sub*, sysopen, tie,
tied, uc, ucfirst, untie, use
* - sub was a keyword in perl4, but in perl5 it is an
operator, which can be used in expressions.
dbmclose, dbmopen
Perl was born in Unix and can therefore access all common Unix system calls. In non-Unix environments, the functionality of some Unix system calls may not be available, or details of the available functionality may differ slightly. The Perl functions affected by this are:
-X, binmode, chmod, chown, chroot, crypt,
dbmclose, dbmopen, dump, endgrent, endhostent,
endnetent, endprotoent, endpwent, endservent, exec,
fcntl, flock, fork, getgrent, getgrgid, gethostbyname,
gethostent, getlogin, getnetbyaddr, getnetbyname, getnetent,
getppid, getprgp, getpriority, getprotobynumber,
getprotoent, getpwent, getpwnam, getpwuid,
getservbyport, getservent, getsockopt, glob, ioctl,
kill, link, lstat, msgctl, msgget, msgrcv,
msgsnd, open, pipe, readlink, rename, select, semctl,
semget, semop, setgrent, sethostent, setnetent,
setpgrp, setpriority, setprotoent, setpwent,
setservent, setsockopt, shmctl, shmget, shmread,
shmwrite, socket, socketpair,
stat, symlink, syscall, sysopen, system,
times, truncate, umask, unlink,
utime, wait, waitpid
For more information about the portability of these functions, see the perlport manpage and other available platform-specific documentation.
$_, except for -t, which tests STDIN.
Unless otherwise documented, it returns 1 for true and '' for false, or
the undefined value if the file doesn't exist. Despite the funny
names, precedence is the same as any other named unary operator, and
the argument may be parenthesized like any other unary operator. The
operator may be any of:
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
-R File is readable by real uid/gid.
-W File is writable by real uid/gid.
-X File is executable by real uid/gid.
-O File is owned by real uid.
-e File exists.
-z File has zero size (is empty).
-s File has nonzero size (returns size in bytes).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named pipe (FIFO), or Filehandle is a pipe.
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-t Filehandle is opened to a tty.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-T File is an ASCII text file (heuristic guess).
-B File is a "binary" file (opposite of -T).
-M Script start time minus file modification time, in days.
-A Same for access time.
-C Same for inode change time (Unix, may differ for other platforms)
Example:
while (<>) {
chomp;
next unless -f $_; # ignore specials
#...
}
The interpretation of the file permission operators -r, -R,
-w, -W, -x, and -X is by default based solely on the mode
of the file and the uids and gids of the user. There may be other
reasons you can't actually read, write, or execute the file. Such
reasons may be for example network filesystem access controls, ACLs
(access control lists), read-only filesystems, and unrecognized
executable formats.
Also note that, for the superuser on the local filesystems, the -r,
-R, -w, and -W tests always return 1, and -x and -X return 1
if any execute bit is set in the mode. Scripts run by the superuser
may thus need to do a stat() to determine the actual mode of the file,
or temporarily set their effective uid to something else.
If you are using ACLs, there is a pragma called filetest that may
produce more accurate results than the bare stat() mode bits.
When under the use filetest 'access' the above-mentioned filetests
will test whether the permission can (not) be granted using the
access() family of system calls. Also note that the -x and -X may
under this pragma return true even if there are no execute permission
bits set (nor any extra execute permission ACLs). This strangeness is
due to the underlying system calls' definitions. Read the
documentation for the filetest pragma for more information.
Note that -s/a/b/ does not do a negated substitution. Saying
-exp($foo) still works as expected, however--only single letters
following a minus are interpreted as file tests.
The -T and -B switches work as follows. The first block or so of the
file is examined for odd characters such as strange control codes or
characters with the high bit set. If too many strange characters (>30%)
are found, it's a -B file, otherwise it's a -T file. Also, any file
containing null in the first block is considered a binary file. If -T
or -B is used on a filehandle, the current IO buffer is examined
rather than the first block. Both -T and -B return true on a null
file, or a file at EOF when testing a filehandle. Because you have to
read a file to do the -T test, on most occasions you want to use a -f
against the file first, as in next unless -f $file && -T $file.
If any of the file tests (or either the stat or lstat operators) are given
the special filehandle consisting of a solitary underline, then the stat
structure of the previous file test (or stat operator) is used, saving
a system call. (This doesn't work with -t, and you need to remember
that lstat() and -l will leave values in the stat structure for the
symbolic link, not the real file.) (Also, if the stat buffer was filled by
a lstat call, -T and -B will reset it with the results of stat _).
Example:
print "Can do.\n" if -r $a || -w _ || -x _;
stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;
$_.
accept(2) system call
does. Returns the packed address if it succeeded, false otherwise.
See the example in Sockets: Client/Server Communication in the perlipc manpage.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor, as determined by the value of $^F. See $^F in the perlvar manpage.
$_ is used. (On some machines,
unfortunately, the elapsed time may be up to one second less or more
than you specified because of how seconds are counted, and process
scheduling may delay the delivery of the signal even further.)
Only one timer may be counting at once. Each call disables the
previous timer, and an argument of 0 may be supplied to cancel the
previous timer without starting a new one. The returned value is the
amount of time remaining on the previous timer.
For delays of finer granularity than one second, you may use Perl's
four-argument version of select() leaving the first three arguments
undefined, or you might be able to use the syscall interface to
access setitimer(2) if your system supports it. The Time::HiRes
module (from CPAN, and starting from Perl 5.8 part of the standard
distribution) may also prove useful.
It is usually a mistake to intermix alarm and sleep calls.
(sleep may be internally implemented in your system with alarm)
If you want to use alarm to time out a system call you need to use an
eval/die pair. You can't rely on the alarm causing the system call to
fail with $! set to EINTR because Perl sets up signal handlers to
restart system calls on some systems. Using eval/die always works,
modulo the caveats given in Signals in the perlipc manpage.
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
if ($@) {
die unless $@ eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
For more information see the perlipc manpage.
For the tangent operation, you may use the Math::Trig::tan
function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
undef and sets $! (errno).
On some systems (in general, DOS and Windows-based systems) binmode()
is necessary when you're not working with a text file. For the sake
of portability it is a good idea to always use it when appropriate,
and to never use it when it isn't appropriate. Also, people can
set their I/O to be by default UTF-8 encoded Unicode, not bytes.
In other words: regardless of platform, use binmode() on binary data,
like for example images.
If LAYER is present it is a single string, but may contain multiple directives. The directives alter the behaviour of the file handle. When LAYER is present using binmode on text file makes sense.
If LAYER is omitted or specified as :raw the filehandle is made
suitable for passing binary data. This includes turning off possible CRLF
translation and marking it as bytes (as opposed to Unicode characters).
Note that, despite what may be implied in ``Programming Perl'' (the
Camel) or elsewhere, :raw is not the simply inverse of :crlf
-- other layers which would affect binary nature of the stream are
also disabled. See the PerlIO manpage, the perlrun manpage and the discussion about the
PERLIO environment variable.
The :bytes, :crlf, and :utf8, and any other directives of the
form :..., are called I/O layers. The open pragma can be used to
establish default I/O layers. See the open manpage.
The LAYER parameter of the binmode() function is described as ``DISCIPLINE'' in ``Programming Perl, 3rd Edition''. However, since the publishing of this book, by many known as ``Camel III'', the consensus of the naming of this functionality has moved from ``discipline'' to ``layer''. All documentation of this version of Perl therefore refers to ``layers'' rather than to ``disciplines''. Now back to the regularly scheduled documentation...
To mark FILEHANDLE as UTF-8, use :utf8.
In general, binmode() should be called after open() but before any I/O
is done on the filehandle. Calling binmode() will normally flush any
pending buffered output data (and perhaps pending input data) on the
handle. An exception to this is the :encoding layer that
changes the default character encoding of the handle, see the open manpage.
The :encoding layer sometimes needs to be called in
mid-stream, and it doesn't flush the stream. The :encoding
also implicitly pushes on top of itself the :utf8 layer because
internally Perl will operate on UTF-8 encoded Unicode characters.
The operating system, device drivers, C libraries, and Perl run-time
system all work together to let the programmer treat a single
character (\n) as the line terminator, irrespective of the external
representation. On many operating systems, the native text file
representation matches the internal representation, but on some
platforms the external representation of \n is made up of more than
one character.
Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
character to end each line in the external representation of text (even
though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
on Unix and most VMS files). In other systems like OS/2, DOS and the
various flavors of MS-Windows your program sees a \n as a simple \cJ,
but what's stored in text files are the two characters \cM\cJ. That
means that, if you don't use binmode() on these systems, \cM\cJ
sequences on disk will be converted to \n on input, and any \n in
your program will be converted back to \cM\cJ on output. This is what
you want for text files, but it can be disastrous for binary files.
Another consequence of using binmode() (on some systems) is that
special end-of-file markers will be seen as part of the data stream.
For systems from the Microsoft family this means that if your binary
data contains \cZ, the I/O subsystem will regard it as the end of
the file, unless you use binmode().
binmode() is not only important for readline() and print() operations,
but also when using read(), seek(), sysread(), syswrite() and tell()
(see the perlport manpage for more details). See the $/ and $\ variables
in the perlvar manpage for how to manually set your input and output
line-termination sequences.
bless is often the last thing in a constructor,
it returns the reference for convenience. Always use the two-argument
version if the function doing the blessing might be inherited by a
derived class. See the perltoot manpage and the perlobj manpage for more about the blessing
(and blessings) of objects.
Consider always blessing objects in CLASSNAMEs that are mixed case. Namespaces with all lowercase names are considered reserved for Perl pragmata. Builtin types have all uppercase names, so to prevent confusion, you may wish to avoid such package names as well. Make sure that CLASSNAME is a true value.
eval or require, and the undefined value
otherwise. In list context, returns
($package, $filename, $line) = caller;
With EXPR, it returns some extra information that the debugger uses to print a stack trace. The value of EXPR indicates how many call frames to go back before the current one.
($package, $filename, $line, $subroutine, $hasargs,
$wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
Here $subroutine may be (eval) if the frame is not a subroutine
call, but an eval. In such a case additional elements $evaltext and
$is_require are set: $is_require is true if the frame is created by a
require or use statement, $evaltext contains the text of the
eval EXPR statement. In particular, for an eval BLOCK statement,
$filename is (eval), but $evaltext is undefined. (Note also that
each use statement creates a require frame inside an eval EXPR
frame.) $subroutine may also be (unknown) if this particular
subroutine happens to have been deleted from the symbol table.
$hasargs is true if a new instance of @_ was set up for the frame.
$hints and $bitmask contain pragmatic hints that the caller was
compiled with. The $hints and $bitmask values are subject to change
between versions of Perl, and are not meant for external use.
Furthermore, when called from within the DB package, caller returns more
detailed information: it sets the list variable @DB::args to be the
arguments with which the subroutine was invoked.
Be aware that the optimizer might have optimized call frames away before
caller had a chance to get the information. That means that caller(N)
might not return information about the call frame you expect it do, for
N > 1. In particular, @DB::args might have information from the
previous time caller was called.
$ENV{HOME}, if set; if not,
changes to the directory specified by $ENV{LOGDIR}. (Under VMS, the
variable $ENV{SYS$LOGIN} is also checked, and used if it is set.) If
neither is set, chdir does nothing. It returns true upon success,
false otherwise. See the example under die.
0644 is okay, '0644' is not. Returns the number of files
successfully changed. See also oct, if all you have is a string.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
$mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
# --w----r-T
$mode = '0644'; chmod oct($mode), 'foo'; # this is better
$mode = 0644; chmod $mode, 'foo'; # this is best
You can also import the symbolic S_I* constants from the Fcntl
module:
use Fcntl ':mode';
chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
# This is identical to the chmod 0755 of the above example.
$/ (also known as
$INPUT_RECORD_SEPARATOR in the English module). It returns the total
number of characters removed from all its arguments. It's often used to
remove the newline from the end of an input record when you're worried
that the final record may be missing its newline. When in paragraph
mode ($/ = ""), it removes all trailing newlines from the string.
When in slurp mode ($/ = undef) or fixed-length record mode ($/ is
a reference to an integer or the like, see the perlvar manpage) chomp() won't
remove anything.
If VARIABLE is omitted, it chomps $_. Example:
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
# ...
}
If VARIABLE is a hash, it chomps the hash's values, but not its keys.
You can actually chomp anything that's an lvalue, including an assignment:
chomp($cwd = `pwd`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and the total number of characters removed is returned.
If the encoding pragma is in scope then the lengths returned are
calculated from the length of $/ in Unicode characters, which is not
always the same as the length of $/ in the native encoding.
Note that parentheses are necessary when you're chomping anything
that is not a simple variable. This is because chomp $cwd = `pwd`;
is interpreted as (chomp $cwd) = `pwd`;, rather than as
chomp( $cwd = `pwd` ) which you might expect. Similarly,
chomp $a, $b is interpreted as chomp($a), $b rather than
as chomp($a, $b).
s/.$//s because it neither
scans nor copies the string. If VARIABLE is omitted, chops $_.
If VARIABLE is a hash, it chops the hash's values, but not its keys.
You can actually chop anything that's an lvalue, including an assignment.
If you chop a list, each element is chopped. Only the value of the
last chop is returned.
Note that chop returns the last character. To return all but the last
character, use substr($string, 0, -1).
See also chomp.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
Here's an example that looks up nonnumeric uids in the passwd file:
print "User: ";
chomp($user = <STDIN>);
print "Files: ";
chomp($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
@ary = glob($pattern); # expand filenames
chown $uid, $gid, @ary;
On most systems, you are not allowed to change the ownership of the file unless you're the superuser, although you should be able to change the group to any of your secondary groups. On insecure systems, these restrictions may be relaxed, but this is not a portable assumption. On POSIX systems, you can detect this condition this way:
use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
$can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
chr(65) is "A" in either ASCII or Unicode, and
chr(0x263a) is a Unicode smiley face. Note that characters from 128
to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
backward compatibility reasons (but see the encoding manpage).
If NUMBER is omitted, uses $_.
For the reverse, use ord.
Note that under the bytes pragma the NUMBER is masked to
the low eight bits.
See the perlunicode manpage and the encoding manpage for more about Unicode.
/ by your process and all its children. (It doesn't
change your current working directory, which is unaffected.) For security
reasons, this call is restricted to the superuser. If FILENAME is
omitted, does a chroot to $_.
You don't have to close FILEHANDLE if you are immediately going to do
another open on it, because open will close it for you. (See
open.) However, an explicit close on an input file resets the line
counter ($.), while the implicit close done by open does not.
If the file handle came from a piped open, close will additionally
return false if one of the other system calls involved fails, or if the
program exits with non-zero status. (If the only problem was that the
program exited non-zero, $! will be set to 0.) Closing a pipe
also waits for the process executing on the pipe to complete, in case you
want to look at the output of the pipe afterwards, and
implicitly puts the exit status value of that command into $?.
Prematurely closing the read end of a pipe (i.e. before the process writing to it at the other end has closed it) will result in a SIGPIPE being delivered to the writer. If the other end can't handle that, be sure to read all the data before closing the pipe.
Example:
open(OUTPUT, '|sort >foo') # pipe to sort
or die "Can't start sort: $!";
#... # print stuff to output
close OUTPUT # wait for sort to finish
or warn $! ? "Error closing sort pipe: $!"
: "Exit status $? from sort";
open(INPUT, 'foo') # get sort's results
or die "Can't open 'foo' for input: $!";
FILEHANDLE may be an expression whose value can be used as an indirect filehandle, usually the real filehandle name.
opendir and returns the success of that
system call.
continue BLOCK attached to a BLOCK (typically in a while or
foreach), it is always executed just before the conditional is about to
be evaluated again, just like the third part of a for loop in C. Thus
it can be used to increment a loop variable, even when the loop has been
continued via the next statement (which is similar to the C continue
statement).
last, next, or redo may appear within a continue
block. last and redo will behave as if they had been executed within
the main block. So will next, but since it will execute a continue
block, it may be more entertaining.
while (EXPR) {
### redo always comes here
do_something;
} continue {
### next always comes here
do_something_else;
# then back the top to re-check EXPR
}
### last always comes here
Omitting the continue section is semantically equivalent to using an
empty one, logically enough. In that case, next goes directly back
to check the condition at the top of the loop.
$_.
For the inverse cosine operation, you may use the Math::Trig::acos()
function, or use this relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
crypt(3) function in the C library
(assuming that you actually have a version there that has not been
extirpated as a potential munition). This can prove useful for checking
the password file for lousy passwords, amongst other things. Only the
guys wearing white hats should do this.
Note that crypt is intended to be a one-way function, much like
breaking eggs to make an omelette. There is no (known) corresponding
decrypt function (in other words, the crypt() is a one-way hash
function). As a result, this function isn't all that useful for
cryptography. (For that, see your nearby CPAN mirror.)
When verifying an existing encrypted string you should use the
encrypted text as the salt (like crypt($plain, $crypted) eq
$crypted). This allows your code to work with the standard crypt
and with more exotic implementations. In other words, do not assume
anything about the returned string itself, or how many bytes in
the encrypted string matter.
Traditionally the result is a string of 13 bytes: two first bytes of
the salt, followed by 11 bytes from the set [./0-9A-Za-z], and only
the first eight bytes of the encrypted string mattered, but
alternative hashing schemes (like MD5), higher level security schemes
(like C2), and implementations on non-UNIX platforms may produce
different strings.
When choosing a new salt create a random two character string whose
characters come from the set [./0-9A-Za-z] (like join '', ('.',
'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]). This set of
characters is just a recommendation; the characters allowed in
the salt depend solely on your system's crypt library, and Perl can't
restrict what salts crypt() accepts.
Here's an example that makes sure that whoever runs this program knows their own password:
$pwd = (getpwuid($<))[1];
system "stty -echo";
print "Password: ";
chomp($word = <STDIN>);
print "\n";
system "stty echo";
if (crypt($word, $pwd) ne $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in your own password to whoever asks you for it is unwise.
The crypt function is unsuitable for encrypting large quantities of data, not least of all because you can't get the information back. Look at the by-module/Crypt and by-module/PGP directories on your favorite CPAN mirror for a slew of potentially useful modules.
If using crypt() on a Unicode string (which potentially has
characters with codepoints above 255), Perl tries to make sense
of the situation by trying to downgrade (a copy of the string)
the string back to an eight-bit byte string before calling crypt()
(on that copy). If that works, good. If not, crypt() dies with
Wide character in crypt.
untie function.]
Breaks the binding between a DBM file and a hash.
tie function.]
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
hash. HASH is the name of the hash. (Unlike normal open, the first
argument is not a filehandle, even though it looks like one). DBNAME
is the name of the database (without the .dir or .pag extension if
any). If the database does not exist, it is created with protection
specified by MASK (as modified by the umask). If your system supports
only the older DBM functions, you may perform only one dbmopen in your
program. In older versions of Perl, if your system had neither DBM nor
ndbm, calling dbmopen produced a fatal error; it now falls back to
sdbm(3).
If you don't have write access to the DBM file, you can only read hash
variables, not set them. If you want to test whether you can write,
either use file tests or try setting a dummy hash entry inside an eval,
which will trap the error.
Note that functions such as keys and values may return huge lists
when used on large DBM files. You may prefer to use the each
function to iterate over large DBM files. Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);
See also the AnyDBM_File manpage for a more general description of the pros and cons of the various dbm approaches, as well as the DB_File manpage for a particularly rich implementation.
You can control which DBM library you use by loading that library before you call dbmopen():
use DB_File;
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
or die "Can't open netscape history file: $!";
undef. If EXPR is not present, $_ will be
checked.
Many operations return undef to indicate failure, end of file,
system error, uninitialized variable, and other exceptional
conditions. This function allows you to distinguish undef from
other values. (A simple Boolean test will not distinguish among
undef, zero, the empty string, and "0", which are all equally
false.) Note that since undef is a valid scalar, its presence
doesn't necessarily indicate an exceptional condition: pop
returns undef when its argument is an empty array, or when the
element to return happens to be undef.
You may also use defined(&func) to check whether subroutine &func
has ever been defined. The return value is unaffected by any forward
declarations of &func. Note that a subroutine which is not defined
may still be callable: its package may have an AUTOLOAD method that
makes it spring into existence the first time that it is called -- see
the perlsub manpage.
Use of defined on aggregates (hashes and arrays) is deprecated. It
used to report whether memory for that aggregate has ever been
allocated. This behavior may disappear in future versions of Perl.
You should instead use a simple test for size:
if (@an_array) { print "has array elements\n" }
if (%a_hash) { print "has hash members\n" }
When used on a hash element, it tells you whether the value is defined, not whether the key exists in the hash. Use exists for the latter purpose.
Examples:
print if defined $switch{'D'};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;
Note: Many folks tend to overuse defined, and then are surprised to
discover that the number 0 and "" (the zero-length string) are, in fact,
defined values. For example, if you say
"ab" =~ /a(.*)b/;
The pattern match succeeds, and $1 is defined, despite the fact that it
matched ``nothing''. But it didn't really match nothing--rather, it
matched something that happened to be zero characters long. This is all
very above-board and honest. When a function returns an undefined value,
it's an admission that it couldn't give you an honest answer. So you
should use defined only when you're questioning the integrity of what
you're trying to do. At other times, a simple comparison to 0 or "" is
what you want.
element(s) from the hash or array.
In the case of an array, if the array elements happen to be at the end,
the size of the array will shrink to the highest element that tests
true for exists() (or 0 if no such element exists).
Returns a list with the same number of elements as the number of elements for which deletion was attempted. Each element of that list consists of either the value of the element deleted, or the undefined value. In scalar context, this means that you get the value of the last element deleted (or the undefined value if that element did not exist).
%hash = (foo => 11, bar => 22, baz => 33);
$scalar = delete $hash{foo}; # $scalar is 11
$scalar = delete @hash{qw(foo bar)}; # $scalar is 22
@array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
Deleting from %ENV modifies the environment. Deleting from
a hash tied to a DBM file deletes the entry from the DBM file. Deleting
from a tied hash or array may not necessarily return anything.
Deleting an array element effectively returns that position of the array
to its initial, uninitialized state. Subsequently testing for the same
element with exists() will return false. Note that deleting array
elements in the middle of an array will not shift the index of the ones
after them down--use splice() for that. See exists.
The following (inefficiently) deletes all the values of %HASH and @ARRAY:
foreach $key (keys %HASH) {
delete $HASH{$key};
}
foreach $index (0 .. $#ARRAY) {
delete $ARRAY[$index];
}
And so do these:
delete @HASH{keys %HASH};
delete @ARRAY[0 .. $#ARRAY];
But both of these are slower than just assigning the empty list or undefining %HASH or @ARRAY:
%HASH = (); # completely empty %HASH
undef %HASH; # forget %HASH ever existed
@ARRAY = (); # completely empty @ARRAY
undef @ARRAY; # forget @ARRAY ever existed
Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash element, array element, hash slice, or array slice lookup:
delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
delete $ref->[$x][$y][$index];
delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
eval, prints the value of LIST to STDERR and
exits with the current value of $! (errno). If $! is 0,
exits with the value of ($? >> 8) (backtick `command`
status). If ($? >> 8) is 0, exits with 255. Inside
an eval(), the error message is stuffed into $@ and the
eval is terminated with the undefined value. This makes
die the way to raise an exception.
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
If the last element of LIST does not end in a newline, the current
script line number and input line number (if any) are also printed,
and a newline is supplied. Note that the ``input line number'' (also
known as ``chunk'') is subject to whatever notion of ``line'' happens to
be currently in effect, and is also available as the special variable
$.. See $/ in the perlvar manpage and $. in the perlvar manpage.
Hint: sometimes appending ", stopped" to your message will cause it
to make better sense when the string "at foo line 123" is appended.
Suppose you are running script ``canasta''.
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
See also exit(), warn(), and the Carp module.
If LIST is empty and $@ already contains a value (typically from a
previous eval) that value is reused after appending "\t...propagated".
This is useful for propagating exceptions:
eval { ... };
die unless $@ =~ /Expected exception/;
If LIST is empty and $@ contains an object reference that has a
PROPAGATE method, that method will be called with additional file
and line number parameters. The return value replaces the value in
$@. ie. as if $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };
were called.
If $@ is empty then the string "Died" is used.
die() can also be called with a reference argument. If this happens to be
trapped within an eval(), $@ contains the reference. This behavior permits
a more elaborate exception handling implementation using objects that
maintain arbitrary state about the nature of the exception. Such a scheme
is sometimes preferable to matching particular string values of $@ using
regular expressions. Here's an example:
eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if ($@) {
if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
# handle Some::Module::Exception
}
else {
# handle all other possible exceptions
}
}
Because perl will stringify uncaught exception messages before displaying them, you may want to overload stringification operations on such custom exception objects. See the overload manpage for details about that.
You can arrange for a callback to be run just before the die
does its deed, by setting the $SIG{__DIE__} hook. The associated
handler will be called with the error text and can change the error
message, if it sees fit, by calling die again. See
$SIG{expr} in the perlvar manpage for details on setting %SIG entries, and
eval BLOCK for some examples. Although this feature was meant
to be run only right before your program was to exit, this is not
currently the case--the $SIG{__DIE__} hook is currently called
even inside eval()ed blocks/strings! If one wants the hook to do
nothing in such situations, put
die @_ if $^S;
as the first line of the handler (see $^S in the perlvar manpage). Because this promotes strange action at a distance, this counterintuitive behavior may be fixed in a future release.
do BLOCK does not count as a loop, so the loop control statements
next, last, or redo cannot be used to leave or restart the block.
See the perlsyn manpage for alternative strategies.
SUBROUTINE(LIST)
do 'stat.pl';
is just like
eval `cat stat.pl`;
except that it's more efficient and concise, keeps track of the current
filename for error messages, searches the @INC libraries, and updates
%INC if the file is found. See Predefined Names in the perlvar manpage for these
variables. It also differs in that code evaluated with do FILENAME
cannot see lexicals in the enclosing scope; eval STRING does. It's the
same, however, in that it does reparse the file every time you call it,
so you probably don't want to do this inside a loop.
If do cannot read the file, it returns undef and sets $! to the
error. If do can read the file but cannot compile it, it
returns undef and sets an error message in $@. If the file is
successfully compiled, do returns the value of the last expression
evaluated.
Note that inclusion of library modules is better done with the
use and require operators, which also do automatic error checking
and raise an exception if there's a problem.
You might like to use do to read in a program configuration
file. Manual error checking can be done this way:
# read in config files: system first, then user
for $file ("/share/prog/defaults.rc",
"$ENV{HOME}/.someprogrc")
{
unless ($return = do $file) {
warn "couldn't parse $file: $@" if $@;
warn "couldn't do $file: $!" unless defined $return;
warn "couldn't run $file" unless $return;
}
}
goto LABEL (with all the restrictions that goto suffers).
Think of it as a goto with an intervening core dump and reincarnation.
If LABEL is omitted, restarts the program from the top.
WARNING: Any files opened at the time of the dump will not be open any more when the program is reincarnated, with possible resulting confusion on the part of Perl.
This function is now largely obsolete, partly because it's very
hard to convert a core file into an executable, and because the
real compiler backends for generating portable bytecode and compilable
C code have superseded it. That's why you should now invoke it as
CORE::dump(), if you don't want to be warned against a possible
typo.
If you're looking to use dump to speed up your program, consider
generating bytecode or native C code as described in perlcc. If
you're just trying to accelerate a CGI script, consider using the
mod_perl extension to Apache, or the CPAN module, CGI::Fast.
You might also consider autoloading or selfloading, which at least
make your program appear to run faster.
Entries are returned in an apparently random order. The actual random
order is subject to change in future versions of perl, but it is
guaranteed to be in the same order as either the keys or values
function would produce on the same (unmodified) hash. Since Perl
5.8.1 the ordering is different even between different runs of Perl
for security reasons (see Algorithmic Complexity Attacks in the perlsec manpage).
When the hash is entirely read, a null array is returned in list context
(which when assigned produces a false (0) value), and undef in
scalar context. The next call to each after that will start iterating
again. There is a single iterator for each hash, shared by all each,
keys, and values function calls in the program; it can be reset by
reading all the elements from the hash, or by evaluating keys HASH or
values HASH. If you add or delete elements of a hash while you're
iterating over it, you may get entries skipped or duplicated, so
don't. Exception: It is always safe to delete the item most recently
returned by each(), which means that the following code will work:
while (($key, $value) = each %hash) {
print $key, "\n";
delete $hash{$key}; # This is safe
}
The following prints out your environment like the printenv(1) program,
only in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
ungetcs it, so isn't very useful in an
interactive context.) Do not read from a terminal file (or call
eof(FILEHANDLE) on it) after end-of-file is reached. File types such
as terminals may lose the end-of-file condition if you do.
An eof without an argument uses the last file read. Using eof()
with empty parentheses is very different. It refers to the pseudo file
formed from the files listed on the command line and accessed via the
<> operator. Since <> isn't explicitly opened,
as a normal filehandle is, an eof() before <> has been
used will cause @ARGV to be examined to determine if input is
available. Similarly, an eof() after <> has returned
end-of-file will assume you are processing another @ARGV list,
and if you haven't set @ARGV, will read input from STDIN;
see I/O Operators in the perlop manpage.
In a while (<>) loop, eof or eof(ARGV) can be used to
detect the end of each file, eof() will only detect the end of the
last file. Examples:
# reset line numbering on each input file
while (<>) {
next if /^\s*#/; # skip comments
print "$.\t$_";
} continue {
close ARGV if eof; # Not eof()!
}
# insert dashes just before last line of last file
while (<>) {
if (eof()) { # check for end of last file
print "--------------\n";
}
print;
last if eof(); # needed if we're reading from a terminal
}
Practical hint: you almost never need to use eof in Perl, because the
input operators typically return undef when they run out of data, or if
there was an error.
$_. This form is typically used to
delay parsing and subsequent execution of the text of EXPR until run time.
In the second form, the code within the BLOCK is parsed only once--at the same time the code surrounding the eval itself was parsed--and executed within the context of the current Perl program. This form is typically used to trap exceptions more efficiently than the first (see below), while also providing the benefit of checking the code within BLOCK at compile time.
The final semicolon, if any, may be omitted from the value of EXPR or within the BLOCK.
In both forms, the value returned is the value of the last expression evaluated inside the mini-program; a return statement may be also used, just as with subroutines. The expression providing the return value is evaluated in void, scalar, or list context, depending on the context of the eval itself. See wantarray for more on how the evaluation context can be determined.
If there is a syntax error or runtime error, or a die statement is
executed, an undefined value is returned by eval, and $@ is set to the
error message. If there was no error, $@ is guaranteed to be a null
string. Beware that using eval neither silences perl from printing
warnings to STDERR, nor does it stuff the text of warning messages into $@.
To do either of those, you have to use the $SIG{__WARN__} facility, or
turn off warnings inside the BLOCK or EXPR using no warnings 'all'.
See warn, the perlvar manpage, the warnings manpage and the perllexwarn manpage.
Note that, because eval traps otherwise-fatal errors, it is useful for
determining whether a particular feature (such as socket or symlink)
is implemented. It is also Perl's exception trapping mechanism, where
the die operator is used to raise exceptions.
If the code to be executed doesn't vary, you may use the eval-BLOCK
form to trap run-time errors without incurring the penalty of
recompiling each time. The error, if any, is still returned in $@.
Examples:
# make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = }; # WRONG
# a run-time error
eval '$answer ='; # sets $@
Due to the current arguably broken state of __DIE__ hooks, when using
the eval{} form as an exception trap in libraries, you may wish not
to trigger any __DIE__ hooks that user code may have installed.
You can use the local $SIG{__DIE__} construct for this purpose,
as shown in this example:
# a very private exception trap for divide-by-zero
eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
warn $@ if $@;
This is especially significant, given that __DIE__ hooks can call
die again, which has the effect of changing their error messages:
# __DIE__ hooks may modify error messages
{
local $SIG{'__DIE__'} =
sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
eval { die "foo lives here" };
print $@ if $@; # prints "bar lives here"
}
Because this promotes action at a distance, this counterintuitive behavior may be fixed in a future release.
With an eval, you should be especially careful to remember what's
being looked at when:
eval $x; # CASE 1
eval "$x"; # CASE 2
eval '$x'; # CASE 3
eval { $x }; # CASE 4
eval "\$$x++"; # CASE 5
$$x++; # CASE 6
Cases 1 and 2 above behave identically: they run the code contained in
the variable $x. (Although case 2 has misleading double quotes making
the reader wonder what else might be happening (nothing is).) Cases 3
and 4 likewise behave in the same way: they run the code '$x', which
does nothing but return the value of $x. (Case 4 is preferred for
purely visual reasons, but it also has the advantage of compiling at
compile-time instead of at run-time.) Case 5 is a place where
normally you would like to use double quotes, except that in this
particular situation, you can just use symbolic references instead, as
in case 6.
eval BLOCK does not count as a loop, so the loop control statements
next, last, or redo cannot be used to leave or restart the block.
Note that as a very special case, an eval '' executed within the DB
package doesn't see the usual surrounding lexical scope, but rather the
scope of the first non-DB piece of code that called it. You don't normally
need to worry about this unless you are writing a Perl debugger.
exec function executes a system command and never returns--
use system instead of exec if you want it to return. It fails and
returns false only if the command does not exist and it is executed
directly instead of via your system's command shell (see below).
Since it's a common mistake to use exec instead of system, Perl
warns you if there is a following statement which isn't die, warn,
or exit (if -w is set - but you always do that). If you
really want to follow an exec with some other statement, you
can use one of these styles to avoid the warning:
exec ('foo') or print STDERR "couldn't exec foo: $!";
{ exec ('foo') }; print STDERR "couldn't exec foo: $!";
If there is more than one argument in LIST, or if LIST is an array
with more than one value, calls execvp(3) with the arguments in LIST.
If there is only one scalar argument or an array with one element in it,
the argument is checked for shell metacharacters, and if there are any,
the entire argument is passed to the system's command shell for parsing
(this is /bin/sh -c on Unix platforms, but varies on other platforms).
If there are no shell metacharacters in the argument, it is split into
words and passed directly to execvp, which is more efficient.
Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
If you don't really want to execute the first argument, but want to lie to the program you are executing about its own name, you can specify the program you actually want to run as an ``indirect object'' (without a comma) in front of the LIST. (This always forces interpretation of the LIST as a multivalued list, even if there is only a single scalar in the list.) Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
When the arguments get executed via the system shell, results will be subject to its quirks and capabilities. See `STRING` in the perlop manpage for details.
Using an indirect object with exec or system is also more
secure. This usage (which also works fine with system()) forces
interpretation of the arguments as a multivalued list, even if the
list had just one argument. That way you're safe from the shell
expanding wildcards or splitting up words with whitespace in them.
@args = ( "echo surprise" );
exec @args; # subject to shell escapes
# if @args == 1
exec { $args[0] } @args; # safe even with one-arg list
The first version, the one without the indirect object, ran the echo
program, passing it "surprise" an argument. The second version
didn't--it tried to run a program literally called ``echo surprise'',
didn't find it, and set $? to a non-zero value indicating failure.
Beginning with v5.6.0, Perl will attempt to flush all files opened for
output before the exec, but this may not be supported on some platforms
(see the perlport manpage). To be safe, you may need to set $| ($AUTOFLUSH
in English) or call the autoflush() method of IO::Handle on any
open handles in order to avoid lost output.
Note that exec will not call your END blocks, nor will it call
any DESTROY methods in your objects.
print "Exists\n" if exists $hash{$key};
print "Defined\n" if defined $hash{$key};
print "True\n" if $hash{$key};
print "Exists\n" if exists $array[$index];
print "Defined\n" if defined $array[$index];
print "True\n" if $array[$index];
A hash or array element can be true only if it's defined, and defined if it exists, but the reverse doesn't necessarily hold true.
Given an expression that specifies the name of a subroutine,
returns true if the specified subroutine has ever been declared, even
if it is undefined. Mentioning a subroutine name for exists or defined
does not count as declaring it. Note that a subroutine which does not
exist may still be callable: its package may have an AUTOLOAD
method that makes it spring into existence the first time that it is
called -- see the perlsub manpage.
print "Exists\n" if exists &subroutine;
print "Defined\n" if defined &subroutine;
Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash or array key lookup or subroutine name:
if (exists $ref->{A}->{B}->{$key}) { }
if (exists $hash{A}{B}{$key}) { }
if (exists $ref->{A}->{B}->[$ix]) { }
if (exists $hash{A}{B}[$ix]) { }
if (exists &{$ref->{A}{B}{$key}}) { }
Although the deepest nested array or hash will not spring into existence
just because its existence was tested, any intervening ones will.
Thus $ref->{"A"} and $ref->{"A"}->{"B"} will spring
into existence due to the existence test for the $key element above.
This happens anywhere the arrow operator is used, including even:
undef $ref;
if (exists $ref->{"Some key"}) { }
print $ref; # prints HASH(0x80d3d5c)
This surprising autovivification in what does not at first--or even second--glance appear to be an lvalue context may be fixed in a future release.
See Pseudo-hashes: Using an array as a hash in the perlref manpage for specifics
on how exists() acts when used on a pseudo-hash.
Use of a subroutine call, rather than a subroutine name, as an argument
to exists() is an error.
exists ⊂ # OK
exists &sub(); # Error
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
See also die. If EXPR is omitted, exits with 0 status. The only
universally recognized values for EXPR are 0 for success and 1
for error; other values are subject to interpretation depending on the
environment in which the Perl program is running. For example, exiting
69 (EX_UNAVAILABLE) from a sendmail incoming-mail filter will cause
the mailer to return the item undelivered, but that's not true everywhere.
Don't use exit to abort a subroutine if there's any chance that
someone might want to trap whatever error happened. Use die instead,
which can be trapped by an eval.
The exit() function does not always exit immediately. It calls any
defined END routines first, but these END routines may not
themselves abort the exit. Likewise any object destructors that need to
be called are called before the real exit. If this is a problem, you
can call POSIX:_exit($status) to avoid END and destructor processing.
See the perlmod manpage for details.
exp($_).
fcntl(2) function. You'll probably have to say
use Fcntl;
first to get the correct constant definitions. Argument processing and
value return works just like ioctl below.
For example:
use Fcntl;
fcntl($filehandle, F_GETFL, $packed_return_buffer)
or die "can't fcntl F_GETFL: $!";
You don't have to check for defined on the return from fcntl.
Like ioctl, it maps a 0 return from the system call into
"0 but true" in Perl. This string is true in boolean context and 0
in numeric context. It is also exempt from the normal -w warnings
on improper numeric conversions.
Note that fcntl will produce a fatal error if used on a machine that
doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
manpage to learn what functions are available on your system.
Here's an example of setting a filehandle named REMOTE to be
non-blocking at the system level. You'll have to negotiate $|
on your own, though.
use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
$flags = fcntl(REMOTE, F_GETFL, 0)
or die "Can't get flags for the socket: $!\n";
$flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
or die "Can't set flags for the socket: $!\n";
select and low-level POSIX tty-handling operations.
If FILEHANDLE is an expression, the value is taken as an indirect
filehandle, generally its name.
You can use this to find out whether two handles refer to the same underlying descriptor:
if (fileno(THIS) == fileno(THAT)) {
print "THIS and THAT are dups\n";
}
(Filehandles connected to memory objects via new features of open may
return undefined even though they are open.)
fcntl(2) locking, or lockf(3).
flock is Perl's portable file locking interface, although it locks
only entire files, not records.
Two potentially non-obvious but traditional flock semantics are
that it waits indefinitely until the lock is granted, and that its locks
merely advisory. Such discretionary locks are more flexible, but offer
fewer guarantees. This means that files locked with flock may be
modified by programs that do not also use flock. See the perlport manpage,
your port's specific documentation, or your system-specific local manpages
for details. It's best to assume traditional behavior if you're writing
portable programs. (But if you're not, you should as always feel perfectly
free to write for your own system's idiosyncrasies (sometimes called
``features''). Slavish adherence to portability concerns shouldn't get
in the way of your getting your job done.)
OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
you can use the symbolic names if you import them from the Fcntl module,
either individually, or as a group using the ':flock' tag. LOCK_SH
requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
LOCK_SH or LOCK_EX then flock will return immediately rather than blocking
waiting for the lock (check the return status to see if you got it).
To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE before locking or unlocking it.
Note that the emulation built with lockf(3) doesn't provide shared
locks, and it requires that FILEHANDLE be open with write intent. These
are the semantics that lockf(3) implements. Most if not all systems
implement lockf(3) in terms of fcntl(2) locking, though, so the
differing semantics shouldn't bite too many people.
Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
be open with read intent to use LOCK_SH and requires that it be open
with write intent to use LOCK_EX.
Note also that some versions of flock cannot lock things over the
network; you would need to use the more system-specific fcntl for
that. If you like you can force Perl to ignore your system's flock(2)
function, and so provide its own fcntl(2)-based emulation, by passing
the switch -Ud_flock to the Configure program when you configure
perl.
Here's a mailbox appender for BSD systems.
use Fcntl ':flock'; # import LOCK_* constants
sub lock {
flock(MBOX,LOCK_EX);
# and, in case someone appended
# while we were waiting...
seek(MBOX, 0, 2);
}
sub unlock {
flock(MBOX,LOCK_UN);
}
open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
or die "Can't open mailbox: $!";
lock();
print MBOX $msg,"\n\n";
unlock();
On systems that support a real flock(), locks are inherited across fork()
calls, whereas those that must resort to the more capricious fcntl()
function lose the locks, making it harder to write servers.
See also the DB_File manpage for other flock() examples.
fork(2) system call to create a new process running the
same program at the same point. It returns the child pid to the
parent process, 0 to the child process, or undef if the fork is
unsuccessful. File descriptors (and sometimes locks on those descriptors)
are shared, while everything else is copied. On most systems supporting
fork(), great care has gone into making it extremely efficient (for
example, using copy-on-write technology on data pages), making it the
dominant paradigm for multitasking over the last few decades.
Beginning with v5.6.0, Perl will attempt to flush all files opened for
output before forking the child process, but this may not be supported
on some platforms (see the perlport manpage). To be safe, you may need to set
$| ($AUTOFLUSH in English) or call the autoflush() method of
IO::Handle on any open handles in order to avoid duplicate output.
If you fork without ever waiting on your children, you will
accumulate zombies. On some systems, you can avoid this by setting
$SIG{CHLD} to "IGNORE". See also the perlipc manpage for more examples of
forking and reaping moribund children.
Note that if your forked child inherits system file descriptors like STDIN and STDOUT that are actually connected by a pipe or socket, even if you exit, then the remote server (such as, say, a CGI script or a backgrounded job launched from a remote shell) won't think you're done. You should reopen those to /dev/null if it's any issue.
write function. For
example:
format Something =
Test: @<<<<<<<< @||||| @>>>>>
$str, $%, '$' . int($num)
.
$str = "widget";
$num = $cost/$quantity;
$~ = 'Something';
write;
See the perlform manpage for many details and examples.
formats, though you may call it,
too. It formats (see the perlform manpage) a list of values according to the
contents of PICTURE, placing the output into the format output
accumulator, $^A (or $ACCUMULATOR in English).
Eventually, when a write is done, the contents of
$^A are written to some filehandle, but you could also read $^A
yourself and then set $^A back to "". Note that a format typically
does one formline per line of form, but the formline function itself
doesn't care how many newlines are embedded in the PICTURE. This means
that the ~ and ~~ tokens will treat the entire PICTURE as a single line.
You may therefore need to use multiple formlines to implement a single
record format, just like the format compiler.
Be careful if you put double quotes around the picture, because an @
character may be taken to mean the beginning of an array name.
formline always returns true. See the perlform manpage for other examples.
$! is set). If FILEHANDLE is omitted, reads from
STDIN. This is not particularly efficient. However, it cannot be
used by itself to fetch single characters without waiting for the user
to hit enter. For that, try something more like:
if ($BSD_STYLE) {
system "stty cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", '-icanon', 'eol', "\001";
}
$key = getc(STDIN);
if ($BSD_STYLE) {
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", 'icanon', 'eol', '^@'; # ASCII null
}
print "\n";
Determination of whether $BSD_STYLE should be set is left as an exercise to the reader.
The POSIX::getattr function can do this more portably on
systems purporting POSIX compliance. See also the Term::ReadKey
module from your nearest CPAN site; details on CPAN can be found on
CPAN in the perlmodlib manpage.
getpwuid.
$login = getlogin || getpwuid($<) || "Kilroy";
Do not consider getlogin for authentication: it is not as
secure as getpwuid.
use Socket;
$hersockaddr = getpeername(SOCK);
($port, $iaddr) = sockaddr_in($hersockaddr);
$herhostname = gethostbyaddr($iaddr, AF_INET);
$herstraddr = inet_ntoa($iaddr);
0 to get the current process group for the
current process. Will raise an exception if used on a machine that
doesn't implement getpgrp(2). If PID is omitted, returns process
group of current process. Note that the POSIX version of getpgrp
does not accept a PID argument, so only PID==0 is truly portable.
Note for Linux users: on Linux, the C functions getpid() and
getppid() return different values from different threads. In order to
be portable, this behavior is not reflected by the perl-level function
getppid(), that returns a consistent value across threads. If you want
to call the underlying getppid(), you may use the CPAN module
Linux::Pid.
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
($name,$passwd,$gid,$members) = getgr*
($name,$aliases,$addrtype,$length,@addrs) = gethost*
($name,$aliases,$addrtype,$net) = getnet*
($name,$aliases,$proto) = getproto*
($name,$aliases,$port,$proto) = getserv*
(If the entry doesn't exist you get a null list.)
The exact meaning of the $gcos field varies but it usually contains the real name of the user (as opposed to the login name) and other information pertaining to the user. Beware, however, that in many system users are able to change this information and therefore it cannot be trusted and therefore the $gcos is tainted (see the perlsec manpage). The $passwd and $shell, user's encrypted password and login shell, are also tainted, because of the same reason.
In scalar context, you get the name, unless the function was a lookup by name, in which case you get the other thing, whatever it is. (If the entry doesn't exist you get the undefined value.) For example:
$uid = getpwnam($name);
$name = getpwuid($num);
$name = getpwent();
$gid = getgrnam($name);
$name = getgrgid($num);
$name = getgrent();
#etc.
In getpw*() the fields $quota, $comment, and $expire are special
cases in the sense that in many systems they are unsupported. If the
$quota is unsupported, it is an empty scalar. If it is supported, it
usually encodes the disk quota. If the $comment field is unsupported,
it is an empty scalar. If it is supported it usually encodes some
administrative comment about the user. In some systems the $quota
field may be $change or $age, fields that have to do with password
aging. In some systems the $comment field may be $class. The $expire
field, if present, encodes the expiration period of the account or the
password. For the availability and the exact meaning of these fields
in your system, please consult your getpwnam(3) documentation and your
pwd.h file. You can also find out from within Perl what your
$quota and $comment fields mean and whether you have the $expire field
by using the Config module and the values d_pwquota, d_pwage,
d_pwchange, d_pwcomment, and d_pwexpire. Shadow password
files are only supported if your vendor has implemented them in the
intuitive fashion that calling the regular C library routines gets the
shadow versions if you're running under privilege or if there exists
the shadow(3) functions as found in System V ( this includes Solaris
and Linux.) Those systems which implement a proprietary shadow password
facility are unlikely to be supported.
The $members value returned by getgr*() is a space separated list of the login names of the members of the group.
For the gethost*() functions, if the h_errno variable is supported in
C, it will be returned to you via $? if the function call fails. The
@addrs value returned by a successful call is a list of the raw
addresses returned by the corresponding system library call. In the
Internet domain, each address is four bytes long and you can unpack it
by saying something like:
($a,$b,$c,$d) = unpack('C4',$addr[0]);
The Socket library makes this slightly easier:
use Socket;
$iaddr = inet_aton("127.1"); # or whatever address
$name = gethostbyaddr($iaddr, AF_INET);
# or going the other way
$straddr = inet_ntoa($iaddr);
If you get tired of remembering which element of the return list
contains which return value, by-name interfaces are provided
in standard modules: File::stat, Net::hostent, Net::netent,
Net::protoent, Net::servent, Time::gmtime, Time::localtime,
and User::grent. These override the normal built-ins, supplying
versions that return objects with the appropriate names
for each field. For example:
use File::stat; use User::pwent; $is_his = (stat($filename)->uid == pwent($whoever)->uid);
Even though it looks like they're the same method calls (uid),
they aren't, because a File::stat object is different from
a User::pwent object.
use Socket;
$mysockaddr = getsockname(SOCK);
($port, $myaddr) = sockaddr_in($mysockaddr);
printf "Connect to %s [%s]\n",
scalar gethostbyaddr($myaddr, AF_INET),
inet_ntoa($myaddr);
Socket module) will exist. To query options at another level the
protocol number of the appropriate protocol controlling the option
should be supplied. For example, to indicate that an option is to be
interpreted by the TCP protocol, LEVEL should be set to the protocol
number of TCP, which you can get using getprotobyname.
The call returns a packed string representing the requested socket option,
or undef if there is an error (the error reason will be in $!). What
exactly is in the packed string depends in the LEVEL and OPTNAME, consult
your system documentation for details. A very common case however is that
the option is an integer, in which case the result will be an packed
integer which you can decode using unpack with the i (or I) format.
An example testing if Nagle's algorithm is turned on on a socket:
use Socket;
defined(my $tcp = getprotobyname("tcp"))
or die "Could not determine the protocol number for tcp";
# my $tcp = Socket::IPPROTO_TCP; # Alternative
my $packed = getsockopt($socket, $tcp, Socket::TCP_NODELAY)
or die "Could not query TCP_NODELAY SOCKEt option: $!";
my $nodelay = unpack("I", $packed);
print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
<*.c> operator, but you can use it directly. If
EXPR is omitted, $_ is used. The <*.c> operator is discussed in
more detail in I/O Operators in the perlop manpage.
Beginning with v5.6.0, this operator is implemented using the standard
File::Glob extension. See the File::Glob manpage for details.
# 0 1 2 3 4 5 6 7
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
gmtime(time);
All list elements are numeric, and come straight out of the C `struct
tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
specified time. $mday is the day of the month, and $mon is the month
itself, in the range 0..11 with 0 indicating January and 11
indicating December. $year is the number of years since 1900. That
is, $year is 123 in year 2023. $wday is the day of the week, with
0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
the year, in the range 0..364 (or 0..365 in leap years.)
Note that the $year element is not simply the last two digits of the year. If you assume it is, then you create non-Y2K-compliant programs--and you wouldn't want to do that, would you?
The proper way to get a complete 4-digit year is simply:
$year += 1900;
And to get the last two digits of the year (e.g., '01' in 2001) do:
$year = sprintf("%02d", $year % 100);
If EXPR is omitted, gmtime() uses the current time (gmtime(time)).
In scalar context, gmtime() returns the ctime(3) value:
$now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
If you need local time instead of GMT use the localtime builtin.
See also the timegm function provided by the Time::Local module,
and the strftime(3) and mktime(3) functions available via the the POSIX manpage module.
This scalar value is not locale dependent (see the perllocale manpage), but is instead a Perl builtin. To get somewhat similar but locale dependent date strings, see the example in localtime.
goto-LABEL form finds the statement labeled with LABEL and resumes
execution there. It may not be used to go into any construct that
requires initialization, such as a subroutine or a foreach loop. It
also can't be used to go into a construct that is optimized away,
or to get out of a block or subroutine given to sort.
It can be used to go almost anywhere else within the dynamic scope,
including out of subroutines, but it's usually better to use some other
construct such as last or die. The author of Perl has never felt the
need to use this form of goto (in Perl, that is--C is another matter).
(The difference being that C does not offer named loops combined with
loop control. Perl does, and this replaces most structured uses of goto
in other languages.)
The goto-EXPR form expects a label name, whose scope will be resolved
dynamically. This allows for computed gotos per FORTRAN, but isn't
necessarily recommended if you're optimizing for maintainability:
goto ("FOO", "BAR", "GLARCH")[$i];
The goto-&NAME form is quite different from the other forms of
goto. In fact, it isn't a goto in the normal sense at all, and
doesn't have the stigma associated with other gotos. Instead, it
exits the current subroutine (losing any changes set by local()) and
immediately calls in its place the named subroutine using the current
value of @_. This is used by AUTOLOAD subroutines that wish to
load another subroutine and then pretend that the other subroutine had
been called in the first place (except that any modifications to @_
in the current subroutine are propagated to the other subroutine.)
After the goto, not even caller will be able to tell that this
routine was called first.
NAME needn't be the name of a subroutine; it can be a scalar variable containing a code reference, or a block which evaluates to a code reference.
grep(1) and its
relatives. In particular, it is not limited to using regular expressions.
Evaluates the BLOCK or EXPR for each element of LIST (locally setting
$_ to each element) and returns the list value consisting of those
elements for which the expression evaluated to true. In scalar
context, returns the number of times the expression was true.
@foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
@foo = grep {!/^#/} @bar; # weed out comments
Note that $_ is an alias to the list value, so it can be used to
modify the elements of the LIST. While this is useful and supported,
it can cause bizarre results if the elements of LIST are not variables.
Similarly, grep returns aliases into the original list, much as a for
loop's index variable aliases the list elements. That is, modifying an
element of a list returned by grep (for example, in a foreach, map
or another grep) actually modifies the element in the original list.
This is usually something to be avoided when writing clear code.
See also map for a list composed of the results of the BLOCK or EXPR.
$_.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
Hex strings may only represent integers. Strings that would cause integer overflow trigger a warning. Leading whitespace is not stripped, unlike oct().
import function. It is just an ordinary
method (subroutine) defined (or inherited) by modules that wish to export
names to another module. The use function calls the import method
for the package used. See also use, the perlmod manpage, and the Exporter manpage.
0 (or whatever
you've set the $[ variable to--but don't do that). If the substring
is not found, returns one less than the base, ordinarily -1.
$_.
You should not use this function for rounding: one because it truncates
towards 0, and two because machine representations of floating point
numbers can sometimes produce counterintuitive results. For example,
int(-6.725/0.025) produces -268 rather than the correct -269; that's
because it's really more like -268.99999999999994315658 instead. Usually,
the sprintf, printf, or the POSIX::floor and POSIX::ceil
functions will serve you better than will int().
ioctl(2) function. You'll probably first have to say
require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
to get the correct function definitions. If ioctl.ph doesn't
exist or doesn't have the correct definitions you'll have to roll your
own, based on your C header files such as <sys/ioctl.h >>.
(There is a Perl script called h2ph that comes with the Perl kit that
may help you in this, but it's nontrivial.) SCALAR will be read and/or
written depending on the FUNCTION--a pointer to the string value of SCALAR
will be passed as the third argument of the actual ioctl call. (If SCALAR
has no string value but does have a numeric value, that value will be
passed rather than a pointer to the string value. To guarantee this to be
true, add a 0 to the scalar before using it.) The pack and unpack
functions may be needed to manipulate the values of structures used by
ioctl.
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus Perl returns true on success and false on failure, yet you can still easily determine the actual value returned by the operating system:
$retval = ioctl(...) || -1;
printf "System returned %d\n", $retval;
The special string "0 but true" is exempt from -w complaints
about improper numeric conversions.
$rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
Beware that unlike split, join doesn't take a pattern as its
first argument. Compare split.
The keys are returned in an apparently random order. The actual
random order is subject to change in future versions of perl, but it
is guaranteed to be the same order as either the values or each
function produces (given that the hash has not been modified). Since
Perl 5.8.1 the ordering is different even between different runs of
Perl for security reasons (see Algorithmic Complexity Attacks in the perlsec manpage).
As a side effect, calling keys() resets the HASH's internal iterator,
see each. (In particular, calling keys() in void context resets
the iterator with no other overhead.)
Here is yet another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
while (@keys) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
The returned values are copies of the original keys in the hash, so modifying them will not affect the original hash. Compare values.
To sort a hash by value, you'll need to use a sort function.
Here's a descending numeric sort of a hash by its values:
foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
printf "%4d %s\n", $hash{$key}, $key;
}
As an lvalue keys allows you to increase the number of hash buckets
allocated for the given hash. This can gain you a measure of efficiency if
you know the hash is going to get big. (This is similar to pre-extending
an array by assigning a larger number to $#array.) If you say
keys %hash = 200;
then %hash will have at least 200 buckets allocated for it--256 of them,
in fact, since it rounds up to the next power of two. These
buckets will be retained even if you do %hash = (), use undef
%hash if you want to free the storage while %hash is still in scope.
You can't shrink the number of buckets allocated for the hash using
keys in this way (but you needn't worry about doing this by accident,
as trying has no effect).
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
If SIGNAL is zero, no signal is sent to the process. This is a useful way to check that a child process is alive and hasn't changed its UID. See the perlport manpage for notes on the portability of this construct.
Unlike in the shell, if SIGNAL is negative, it kills process groups instead of processes. (On System V, a negative PROCESS number will also kill process groups, but that's not portable.) That means you usually want to use positive not negative signals. You may also use a signal name in quotes.
See Signals in the perlipc manpage for more details.
last command is like the break statement in C (as used in
loops); it immediately exits the loop in question. If the LABEL is
omitted, the command refers to the innermost enclosing loop. The
continue block, if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
#...
}
last cannot be used to exit a block which returns a value such as
eval {}, sub {} or do {}, and should not be used to exit
a grep() or map() operation.
Note that a block by itself is semantically identical to a loop
that executes once. Thus last can be used to effect an early
exit out of such a block.
See also continue for an illustration of how last, next, and
redo work.
\L escape in double-quoted strings. Respects
current LC_CTYPE locale if use locale in force. See the perllocale manpage
and the perlunicode manpage for more details about locale and Unicode support.
If EXPR is omitted, uses $_.
\l escape in
double-quoted strings. Respects current LC_CTYPE locale if use
locale in force. See the perllocale manpage and the perlunicode manpage for more
details about locale and Unicode support.
If EXPR is omitted, uses $_.
$_. Note that this cannot be used on
an entire array or hash to find out how many elements these have.
For that, use scalar @array and scalar keys %hash respectively.
Note the characters: if the EXPR is in Unicode, you will get the
number of characters, not the number of bytes. To get the length
in bytes, use do { use bytes; length(EXPR) }, see the bytes manpage.
my instead, because local isn't
what most people think of as ``local''. See
Private Variables via my() in the perlsub manpage for details.
A local modifies the listed variables to be local to the enclosing block, file, or eval. If more than one value is listed, the list must be placed in parentheses. See Temporary Values via local() in the perlsub manpage for details, including issues with tied arrays and hashes.
# 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(time);
All list elements are numeric, and come straight out of the C `struct
tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
specified time. $mday is the day of the month, and $mon is the month
itself, in the range 0..11 with 0 indicating January and 11
indicating December. $year is the number of years since 1900. That
is, $year is 123 in year 2023. $wday is the day of the week, with
0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
the year, in the range 0..364 (or 0..365 in leap years.) $isdst
is true if the specified time occurs during daylight savings time,
false otherwise.
Note that the $year element is not simply the last two digits of the year. If you assume it is, then you create non-Y2K-compliant programs--and you wouldn't want to do that, would you?
The proper way to get a complete 4-digit year is simply:
$year += 1900;
And to get the last two digits of the year (e.g., '01' in 2001) do:
$year = sprintf("%02d", $year % 100);
If EXPR is omitted, localtime() uses the current time (localtime(time)).
In scalar context, localtime() returns the ctime(3) value:
$now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
This scalar value is not locale dependent but is a Perl builtin. For GMT
instead of local time use the gmtime builtin. See also the
Time::Local module (to convert the second, minutes, hours, ... back to
the integer value returned by time()), and the the POSIX manpage module's strftime(3)
and mktime(3) functions.
To get somewhat similar but locale dependent date strings, set up your locale environment variables appropriately (please see the perllocale manpage) and try for example:
use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
# or for GMT formatted appropriately for your locale:
$now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
Note that the %a and %b, the short forms of the day of the week
and the month of the year, may not necessarily be three characters wide.
lock() is a ``weak keyword'' : this means that if you've defined a function
by this name (before any calls to it), that function will be called
instead. (However, if you've said use threads, lock() is always a
keyword.) See the threads manpage.
$_. To get the log of another base, use basic algebra:
The base-N log of a number is equal to the natural log of that number
divided by the natural log of N. For example:
sub log10 {
my $n = shift;
return log($n)/log(10);
}
See also exp for the inverse operation.
stat function (including setting the
special _ filehandle) but stats a symbolic link instead of the file
the symbolic link points to. If symbolic links are unimplemented on
your system, a normal stat is done. For much more detailed
information, please see the documentation for stat.
If EXPR is omitted, stats $_.
$_ to each element) and returns the list value composed of the
results of each such evaluation. In scalar context, returns the
total number of elements so generated. Evaluates BLOCK or EXPR in
list context, so each element of LIST may produce zero, one, or
more elements in the returned value.
@chars = map(chr, @nums);
translates a list of numbers to the corresponding characters. And
%hash = map { getkey($_) => $_ } @array;
is just a funny way to write
%hash = ();
foreach $_ (@array) {
$hash{getkey($_)} = $_;
}
Note that $_ is an alias to the list value, so it can be used to
modify the elements of the LIST. While this is useful and supported,
it can cause bizarre results if the elements of LIST are not variables.
Using a regular foreach loop for this purpose would be clearer in
most cases. See also grep for an array composed of those items of
the original list for which the BLOCK or EXPR evaluates to true.
{ starts both hash references and blocks, so map { ... could be either
the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
ahead for the closing } it has to take a guess at which its dealing with
based what it finds just after the {. Usually it gets it right, but if it
doesn't it won't realize something is wrong until it gets to the } and
encounters the missing (or unexpected) comma. The syntax error will be
reported close to the } but you'll need to change something near the {
such as using a unary + to give perl some help:
%hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
%hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
%hash = map { ("\L$_", 1) } @array # this also works
%hash = map { lc($_), 1 } @array # as does this.
%hash = map +( lc($_), 1 ), @array # this is EXPR and works!
%hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
or to force an anon hash constructor use +{
@hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
and you get list of anonymous hashes each with only 1 entry.
umask). If it succeeds it
returns true, otherwise it returns false and sets $! (errno).
If omitted, MASK defaults to 0777.
In general, it is better to create directories with permissive MASK,
and let the user modify that with their umask, than it is to supply
a restrictive MASK and give the user no way to be more permissive.
The exceptions to this rule are when the file or directory should be
kept private (mail files, for instance). The perlfunc(1) entry on
umask discusses the choice of MASK in more detail.
Note that according to the POSIX 1003.1-1996 the FILENAME may have any number of trailing slashes. Some operating and filesystems do not get this right, so Perl automatically removes all trailing slashes to keep everyone happy.
use IPC::SysV;
first to get the correct constant definitions. If CMD is IPC_STAT,
then ARG must be a variable which will hold the returned msqid_ds
structure. Returns like ioctl: the undefined value for error,
"0 but true" for zero, or the actual return value otherwise. See also
SysV IPC in the perlipc manpage, IPC::SysV, and IPC::Semaphore documentation.
IPC::SysV and IPC::Msg documentation.
unpack("l! a*").
Taints the variable. Returns true if successful, or false if there is
an error. See also SysV IPC in the perlipc manpage, IPC::SysV, and
IPC::SysV::Msg documentation.
pack("l! a*", $type, $message). Returns true if successful,
or false if there is an error. See also IPC::SysV
and IPC::SysV::Msg documentation.
my declares the listed variables to be local (lexically) to the
enclosing block, file, or eval. If more than one value is listed,
the list must be placed in parentheses.
The exact semantics and interface of TYPE and ATTRS are still
evolving. TYPE is currently bound to the use of fields pragma,
and attributes are handled using the attributes pragma, or starting
from Perl 5.8.0 also via the Attribute::Handlers module. See
Private Variables via my() in the perlsub manpage for details, and the fields manpage,
the attributes manpage, and the Attribute::Handlers manpage.
next command is like the continue statement in C; it starts
the next iteration of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
#...
}
Note that if there were a continue block on the above, it would get
executed even on discarded lines. If the LABEL is omitted, the command
refers to the innermost enclosing loop.
next cannot be used to exit a block which returns a value such as
eval {}, sub {} or do {}, and should not be used to exit
a grep() or map() operation.
Note that a block by itself is semantically identical to a loop
that executes once. Thus next will exit such a block early.
See also continue for an illustration of how last, next, and
redo work.
use function, which no is the opposite of.
0x, interprets it as a
hex string. If EXPR starts off with 0b, it is interpreted as a
binary string. Leading whitespace is ignored in all three cases.)
The following will handle decimal, binary, octal, and hex in the standard
Perl or C notation:
$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses $_. To go the other way (produce a number
in octal), use sprintf() or printf():
$perms = (stat("filename"))[2] & 07777;
$oct_perms = sprintf "%lo", $perms;
The oct() function is commonly used when a string such as 644 needs
to be converted into a file mode, for example. (Although perl will
automatically convert strings into numbers as needed, this automatic
conversion assumes base 10.)
(The following is a comprehensive reference to open(): for a gentler introduction you may consider the perlopentut manpage.)
If FILEHANDLE is an undefined scalar variable (or array or hash element)
the variable is assigned a reference to a new anonymous filehandle,
otherwise if FILEHANDLE is an expression, its value is used as the name of
the real filehandle wanted. (This is considered a symbolic reference, so
use strict 'refs' should not be in effect.)
If EXPR is omitted, the scalar variable of the same name as the
FILEHANDLE contains the filename. (Note that lexical variables--those
declared with my--will not work for this purpose; so if you're
using my, specify EXPR in your call to open.)
If three or more arguments are specified then the mode of opening and
the file name are separate. If MODE is '<' or nothing, the file
is opened for input. If MODE is '>', the file is truncated and
opened for output, being created if necessary. If MODE is '>>',
the file is opened for appending, again being created if necessary.
You can put a '+' in front of the '>' or '<' to
indicate that you want both read and write access to the file; thus
'+<' is almost always preferred for read/write updates--the <
'+' >> mode would clobber the file first. You can't usually use
either read-write mode for updating textfiles, since they have
variable length records. See the -i switch in the perlrun manpage for a
better approach. The file is created with permissions of 0666
modified by the process' umask value.
These various prefixes correspond to the fopen(3) modes of 'r',
'r+', 'w', 'w+', 'a', and 'a+'.
In the 2-arguments (and 1-argument) form of the call the mode and
filename should be concatenated (in this order), possibly separated by
spaces. It is possible to omit the mode in these forms if the mode is
'<'.
If the filename begins with '|', the filename is interpreted as a
command to which output is to be piped, and if the filename ends with a
'|', the filename is interpreted as a command which pipes output to
us. See Using open() for IPC in the perlipc manpage
for more examples of this. (You are not allowed to open to a command
that pipes both in and out, but see the IPC::Open2 manpage, the IPC::Open3 manpage,
and Bidirectional Communication with Another Process in the perlipc manpage
for alternatives.)
For three or more arguments if MODE is '|-', the filename is
interpreted as a command to which output is to be piped, and if MODE
is '-|', the filename is interpreted as a command which pipes
output to us. In the 2-arguments (and 1-argument) form one should
replace dash ('-') with the command.
See Using open() for IPC in the perlipc manpage for more examples of this.
(You are not allowed to open to a command that pipes both in and
out, but see the IPC::Open2 manpage, the IPC::Open3 manpage, and
Bidirectional Communication in the perlipc manpage for alternatives.)
In the three-or-more argument form of pipe opens, if LIST is specified
(extra arguments after the command name) then LIST becomes arguments
to the command invoked if the platform supports it. The meaning of
open with more than three arguments for non-pipe modes is not yet
specified. Experimental ``layers'' may give extra LIST arguments
meaning.
In the 2-arguments (and 1-argument) form opening '-' opens STDIN
and opening '>-' opens STDOUT.
You may use the three-argument form of open to specify IO ``layers'' (sometimes also referred to as ``disciplines'') to be applied to the handle that affect how the input and output are processed (see the open manpage and the PerlIO manpage for more details). For example
open(FH, "<:utf8", "file")
will open the UTF-8 encoded file containing Unicode characters,
see the perluniintro manpage. (Note that if layers are specified in the
three-arg form then default layers set by the open pragma are
ignored.)
Open returns nonzero upon success, the undefined value otherwise. If
the open involved a pipe, the return value happens to be the pid of
the subprocess.
If you're running Perl on a system that distinguishes between text
files and binary files, then you should check out binmode for tips
for dealing with this. The key distinction between systems that need
binmode and those that don't is their text file formats. Systems
like Unix, Mac OS, and Plan 9, which delimit lines with a single
character, and which encode that character in C as "\n", do not
need binmode. The rest need it.
When opening a file, it's usually a bad idea to continue normal execution
if the request failed, so open is frequently used in connection with
die. Even if die won't do what you want (say, in a CGI script,
where you want to make a nicely formatted error message (but there are
modules that can help with that problem)) you should always check
the return value from opening a file. The infrequent exception is when
working with an unopened filehandle is actually what you want to do.
As a special case the 3 arg form with a read/write mode and the third
argument being undef:
open(TMP, "+>", undef) or die ...
opens a filehandle to an anonymous temporary file. Also using ``+<''
works for symmetry, but you really should consider writing something
to the temporary file first. You will need to seek() to do the
reading.
File handles can be opened to ``in memory'' files held in Perl scalars via:
open($fh, '>', \$variable) || ..
Though if you try to re-open STDOUT or STDERR as an ``in memory''
file, you have to close it first:
close STDOUT;
open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
Examples:
$ARTICLE = 100;
open ARTICLE or die "Can't find article $ARTICLE: $!\n";
while (<ARTICLE>) {...
open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
# if the open fails, output is discarded
open(DBASE, '+<', 'dbase.mine') # open for update
or die "Can't open 'dbase.mine' for update: $!";
open(DBASE, '+<dbase.mine') # ditto
or die "Can't open 'dbase.mine' for update: $!";
open(ARTICLE, '-|', "caesar <$article") # decrypt article
or die "Can't start caesar: $!";
open(ARTICLE, "caesar <$article |") # ditto
or die "Can't start caesar: $!";
open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
or die "Can't start sort: $!";
# in memory files
open(MEMORY,'>', \$var)
or die "Can't open memory file: $!";
print MEMORY "foo!\n"; # output will end up in $var
# process argument list of files along with any includes
foreach $file (@ARGV) {
process($file, 'fh00');
}
sub process {
my($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
local $_;
while (<$input>) { # note use of indirection
if (/^#include "(.*)"/) {
process($1, $input);
next;
}
#... # whatever
}
}
You may also, in the Bourne shell tradition, specify an EXPR beginning
with '>&', in which case the rest of the string is interpreted
as the name of a filehandle (or file descriptor, if numeric) to be
duped (as dup(2)) and opened. You may use & after >,
>>, <, +>, +>>, and +<.
The mode you specify should match the mode of the original filehandle.
(Duping a filehandle does not take into account any existing contents
of IO buffers.) If you use the 3 arg form then you can pass either a
number, the name of a filehandle or the normal ``reference to a glob''.
Here is a script that saves, redirects, and restores STDOUT and
STDERR using various methods:
#!/usr/bin/perl
open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
select STDERR; $| = 1; # make unbuffered
select STDOUT; $| = 1; # make unbuffered
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
If you specify '<&=X', where X is a file descriptor number
or a filehandle, then Perl will do an equivalent of C's fdopen of
that file descriptor (and not call dup(2)); this is more
parsimonious of file descriptors. For example:
# open for input, reusing the fileno of $fd
open(FILEHANDLE, "<&=$fd")
or
open(FILEHANDLE, "<&=", $fd)
or
# open for append, using the fileno of OLDFH
open(FH, ">>&=", OLDFH)
or
open(FH, ">>&=OLDFH")
Being parsimonious on filehandles is also useful (besides being
parsimonious) for example when something is dependent on file
descriptors, like for example locking using flock(). If you do just
open(A, '>>&B'), the filehandle A will not have the same file
descriptor as B, and therefore flock(A) will not flock(B), and vice
versa. But with open(A, '>>&=B') the filehandles will share
the same file descriptor.
Note that if you are using Perls older than 5.8.0, Perl will be using
the standard C libraries' fdopen() to implement the ``='' functionality.
On many UNIX systems fdopen() fails when file descriptors exceed a
certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
most often the default.
You can see whether Perl has been compiled with PerlIO or not by
running perl -V and looking for useperlio= line. If useperlio
is define, you have PerlIO, otherwise you don't.
If you open a pipe on the command '-', i.e., either '|-' or '-|'
with 2-arguments (or 1-argument) form of open(), then
there is an implicit fork done, and the return value of open is the pid
of the child within the parent process, and 0 within the child
process. (Use defined($pid) to determine whether the open was successful.)
The filehandle behaves normally for the parent, but i/o to that
filehandle is piped from/to the STDOUT/STDIN of the child process.
In the child process the filehandle isn't opened--i/o happens from/to
the new STDOUT or STDIN. Typically this is used like the normal
piped open when you want to exercise more control over just how the
pipe command gets executed, such as when you are running setuid, and
don't want to have to scan shell commands for metacharacters.
The following triples are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
open(FOO, "cat -n '$file'|");
open(FOO, '-|', "cat -n '$file'");
open(FOO, '-|') || exec 'cat', '-n', $file;
open(FOO, '-|', "cat", '-n', $file);
The last example in each block shows the pipe as ``list form'', which is
not yet supported on all platforms. A good rule of thumb is that if
your platform has true fork() (in other words, if your platform is
UNIX) you can use the list form.
See Safe Pipe Opens in the perlipc manpage for more examples of this.
Beginning with v5.6.0, Perl will attempt to flush all files opened for
output before any operation that may do a fork, but this may not be
supported on some platforms (see the perlport manpage). To be safe, you may need
to set $| ($AUTOFLUSH in English) or call the autoflush() method
of IO::Handle on any open handles.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor as determined by the value of $^F. See $^F in the perlvar manpage.
Closing any piped filehandle causes the parent process to wait for the
child to finish, and returns the status value in $?.
The filename passed to 2-argument (or 1-argument) form of open() will
have leading and trailing whitespace deleted, and the normal
redirection characters honored. This property, known as ``magic open'',
can often be used to good effect. A user could specify a filename of
``rsh cat file |'', or you could change certain filenames as needed:
$filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
open(FH, $filename) or die "Can't open $filename: $!";
Use 3-argument form to open a file with arbitrary weird characters in it,
open(FOO, '<', $file);
otherwise it's necessary to protect any leading and trailing whitespace:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
(this may not work on some bizarre filesystems). One should conscientiously choose between the magic and 3-arguments form of open():
open IN, $ARGV[0];
will allow the user to specify an argument of the form "rsh cat file |",
but will not work on a filename which happens to have a trailing space, while
open IN, '<', $ARGV[0];
will have exactly the opposite restrictions.
If you want a ``real'' C open (see open(2) on your system), then you
should use the sysopen function, which involves no such magic (but
may use subtly different filemodes than Perl open(), which is mapped
to C fopen()). This is
another way to protect your filenames from interpretation. For example:
use IO::Handle;
sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
or die "sysopen $path: $!";
$oldfh = select(HANDLE); $| = 1; select($oldfh);
print HANDLE "stuff $$\n";
seek(HANDLE, 0, 0);
print "File contains: ", <HANDLE>;
Using the constructor from the IO::Handle package (or one of its
subclasses, such as IO::File or IO::Socket), you can generate anonymous
filehandles that have the scope of whatever variables hold references to
them, and automatically close whenever and however you leave that scope:
use IO::File;
#...
sub read_myfile_munged {
my $ALL = shift;
my $handle = new IO::File;
open($handle, "myfile") or die "myfile: $!";
$first = <$handle>
or return (); # Automatically closed here.
mung $first or die "mung failed"; # Or here.
return $first, <$handle> if $ALL; # Or here.
$first; # Or here.
}
See seek for some details about mixing reading and writing.
readdir, telldir,
seekdir, rewinddir, and closedir. Returns true if successful.
DIRHANDLE may be an expression whose value can be used as an indirect
dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
scalar variable (or array or hash element), the variable is assigned a
reference to a new anonymous dirhandle.
DIRHANDLEs have their own namespace separate from FILEHANDLEs.
$_.
For the reverse, see chr. See the perlunicode manpage and the encoding manpage for more about Unicode.
our declares the listed variables to be valid globals within
the enclosing block, file, or eval. That is, it has the same
scoping rules as a ``my'' declaration, but does not create a local
variable. If more than one value is listed, the list must be placed
in parentheses. The our declaration has no semantic effect unless
``use strict vars'' is in effect, in which case it lets you use the
declared global variable without qualifying it with a package name.
(But only within the lexical scope of the our declaration. In this
it differs from ``use vars'', which is package scoped.)
An our declaration declares a global variable that will be visible
across its entire lexical scope, even across package boundaries. The
package in which the variable is entered is determined at the point
of the declaration, not at the point of use. This means the following
behavior holds:
package Foo;
our $bar; # declares $Foo::bar for rest of lexical scope
$bar = 20;
package Bar;
print $bar; # prints 20
Multiple our declarations in the same lexical scope are allowed
if they are in different packages. If they happened to be in the same
package, Perl will emit warnings if you have asked for them.
use warnings;
package Foo;
our $bar; # declares $Foo::bar for rest of lexical scope
$bar = 20;
package Bar;
our $bar = 30; # declares $Bar::bar for rest of lexical scope
print $bar; # prints 30
our $bar; # emits warning
An our declaration may also have a list of attributes associated
with it.
The exact semantics and interface of TYPE and ATTRS are still
evolving. TYPE is currently bound to the use of fields pragma,
and attributes are handled using the attributes pragma, or starting
from Perl 5.8.0 also via the Attribute::Handlers module. See
Private Variables via my() in the perlsub manpage for details, and the fields manpage,
the attributes manpage, and the Attribute::Handlers manpage.
The only currently recognized our() attribute is unique which
indicates that a single copy of the global is to be used by all
interpreters should the program happen to be running in a
multi-interpreter environment. (The default behaviour would be for
each interpreter to have its own copy of the global.) Examples:
our @EXPORT : unique = qw(foo);
our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
our $VERSION : unique = "1.00";
Note that this attribute also has the effect of making the global readonly when the first new interpreter is cloned (for example, when the first new thread is created).
Multi-interpreter environments can come to being either through the
fork() emulation on Windows platforms, or by embedding perl in a
multi-threaded application. The unique attribute does nothing in
all other environments.
Warning: the current implementation of this attribute operates on the
typeglob associated with the variable; this means that our $x : unique
also has the effect of our @x : unique; our %x : unique. This may be
subject to change.
The TEMPLATE is a sequence of characters that give the order and type of values, as follows:
a A string with arbitrary binary data, will be null padded.
A A text (ASCII) string, will be space padded.
Z A null terminated (ASCIZ) string, will be null padded.
b A bit string (ascending bit order inside each byte, like vec()).
B A bit string (descending bit order inside each byte).
h A hex string (low nybble first).
H A hex string (high nybble first).
c A signed char value.
C An unsigned char value. Only does bytes. See U for Unicode.
s A signed short value.
S An unsigned short value.
(This 'short' is _exactly_ 16 bits, which may differ from
what a local C compiler calls 'short'. If you want
native-length shorts, use the '!' suffix.)
i A signed integer value.
I An unsigned integer value.
(This 'integer' is _at_least_ 32 bits wide. Its exact
size depends on what a local C compiler calls 'int',
and may even be larger than the 'long' described in
the next item.)
l A signed long value.
L An unsigned long value.
(This 'long' is _exactly_ 32 bits, which may differ from
what a local C compiler calls 'long'. If you want
native-length longs, use the '!' suffix.)
n An unsigned short in "network" (big-endian) order.
N An unsigned long in "network" (big-endian) order.
v An unsigned short in "VAX" (little-endian) order.
V An unsigned long in "VAX" (little-endian) order.
(These 'shorts' and 'longs' are _exactly_ 16 bits and
_exactly_ 32 bits, respectively.)
q A signed quad (64-bit) value.
Q An unsigned quad value.
(Quads are available only if your system supports 64-bit
integer values _and_ if Perl has been compiled to support those.
Causes a fatal error otherwise.)
j A signed integer value (a Perl internal integer, IV).
J An unsigned integer value (a Perl internal unsigned integer, UV).
f A single-precision float in the native format.
d A double-precision float in the native format.
F A floating point value in the native native format
(a Perl internal floating point value, NV).
D A long double-precision float in the native format.
(Long doubles are available only if your system supports long
double values _and_ if Perl has been compiled to support those.
Causes a fatal error otherwise.)
p A pointer to a null-terminated string.
P A pointer to a structure (fixed-length string).
u A uuencoded string.
U A Unicode character number. Encodes to UTF-8 internally
(or UTF-EBCDIC in EBCDIC platforms).
w A BER compressed integer. Its bytes represent an unsigned
integer in base 128, most significant digit first, with as
few digits as possible. Bit eight (the high bit) is set
on each byte except the last.
x A null byte.
X Back up a byte.
@ Null fill to absolute position, counted from the start of
the innermost ()-group.
( Start of a ()-group.
The following rules apply:
a, A, Z, b, B, h,
H, @, x, X and P the pack function will gobble up that
many values from the LIST. A * for the repeat count means to use
however many items are left, except for @, x, X, where it is
equivalent to 0, and u, where it is equivalent to 1 (or 45, what
is the same). A numeric repeat count may optionally be enclosed in
brackets, as in pack 'C[80]', @arr.
One can replace the numeric repeat count by a template enclosed in brackets;
then the packed length of this template in bytes is used as a count.
For example, x[L] skips a long (it skips the number of bytes in a long);
the template $t X[$t] $t unpack()s twice what $t unpacks.
If the template in brackets contains alignment commands (such as x![d]),
its packed length is calculated as if the start of the template has the maximal
possible alignment.
When used with Z, * results in the addition of a trailing null
byte (so the packed result will be one longer than the byte length
of the item).
The repeat count for u is interpreted as the maximal number of bytes
to encode per line of output, with 0 and 1 replaced by 45.
a, A, and Z types gobble just one value, but pack it as a
string of length count, padding with nulls or spaces as necessary. When
unpacking, A strips trailing spaces and nulls, Z strips everything
after the first null, and a returns data verbatim. When packing,
a, and Z are equivalent.
If the value-to-pack is too long, it is truncated. If too long and an
explicit count is provided, Z packs only $count-1 bytes, followed
by a null byte. Thus Z always packs a trailing null byte under
all circumstances.
b and B fields pack a string that many bits long.
Each byte of the input field of pack() generates 1 bit of the result.
Each result bit is based on the least-significant bit of the corresponding
input byte, i.e., on ord($byte)%2. In particular, bytes "0" and
"1" generate bits 0 and 1, as do bytes "\0" and "\1".
Starting from the beginning of the input string of pack(), each 8-tuple
of bytes is converted to 1 byte of output. With format b
the first byte of the 8-tuple determines the least-significant bit of a
byte, and with format B it determines the most-significant bit of
a byte.
If the length of the input string is not exactly divisible by 8, the remainder is packed as if the input string were padded by null bytes at the end. Similarly, during unpack()ing the ``extra'' bits are ignored.
If the input string of pack() is longer than needed, extra bytes are ignored.
A * for the repeat count of pack() means to use all the bytes of
the input field. On unpack()ing the bits are converted to a string
of "0"s and "1"s.
h and H fields pack a string that many nybbles (4-bit groups,
representable as hexadecimal digits, 0-9a-f) long.
Each byte of the input field of pack() generates 4 bits of the result.
For non-alphabetical bytes the result is based on the 4 least-significant
bits of the input byte, i.e., on ord($byte)%16. In particular,
bytes "0" and "1" generate nybbles 0 and 1, as do bytes
"\0" and "\1". For bytes "a".."f" and "A".."F" the result
is compatible with the usual hexadecimal digits, so that "a" and
"A" both generate the nybble 0xa==10. The result for bytes
"g".."z" and "G".."Z" is not well-defined.
Starting from the beginning of the input string of pack(), each pair
of bytes is converted to 1 byte of output. With format h the
first byte of the pair determines the least-significant nybble of the
output byte, and with format H it determines the most-significant
nybble.
If the length of the input string is not even, it behaves as if padded by a null byte at the end. Similarly, during unpack()ing the ``extra'' nybbles are ignored.
If the input string of pack() is longer than needed, extra bytes are ignored.
A * for the repeat count of pack() means to use all the bytes of
the input field. On unpack()ing the bits are converted to a string
of hexadecimal digits.
p type packs a pointer to a null-terminated string. You are
responsible for ensuring the string is not a temporary value (which can
potentially get deallocated before you get around to using the packed result).
The P type packs a pointer to a structure of the size indicated by the
length. A NULL pointer is created if the corresponding value for p or
P is undef, similarly for unpack().
The / template character allows packing and unpacking of strings where
the packed structure contains a byte count followed by the string itself.
You write length-item/string-item.
The length-item can be any pack template letter, and describes
how the length value is packed. The ones likely to be of most use are
integer-packing ones like n (for Java strings), w (for ASN.1 or
SNMP) and N (for Sun XDR).
For pack, the string-item must, at present, be "A*", "a*" or
"Z*". For unpack the length of the string is obtained from the
length-item, but if you put in the '*' it will be ignored. For all other
codes, unpack applies the length value to the next item, which must not
have a repeat count.
unpack 'C/a', "\04Gurusamy"; gives 'Guru'
unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
The length-item is not returned explicitly from unpack.
Adding a count to the length-item letter is unlikely to do anything
useful, unless that letter is A, a or Z. Packing with a
length-item of a or Z may introduce "\000" characters,
which Perl does not regard as legal in numeric strings.
s, S, l, and L may be
immediately followed by a ! suffix to signify native shorts or
longs--as you can see from above for example a bare l does mean
exactly 32 bits, the native long (as seen by the local C compiler)
may be larger. This is an issue mainly in 64-bit platforms. You can
see whether using ! makes any difference by
print length(pack("s")), " ", length(pack("s!")), "\n";
print length(pack("l")), " ", length(pack("l!")), "\n";
i! and I! also work but only because of completeness;
they are identical to i and I.
The actual sizes (in bytes) of native shorts, ints, longs, and long longs on the platform where Perl was built are also available via the Config manpage:
use Config;
print $Config{shortsize}, "\n";
print $Config{intsize}, "\n";
print $Config{longsize}, "\n";
print $Config{longlongsize}, "\n";
(The $Config{longlongsize} will be undefined if your system does
not support long longs.)
s, S, i, I, l, L, j, and J
are inherently non-portable between processors and operating systems
because they obey the native byteorder and endianness. For example a
4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
(arranged in and handled by the CPU registers) into bytes as
0x12 0x34 0x56 0x78 # big-endian
0x78 0x56 0x34 0x12 # little-endian
Basically, the Intel and VAX CPUs are little-endian, while everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq used/uses them in little-endian mode; SGI/Cray uses them in big-endian mode.
The names `big-endian' and `little-endian' are comic references to the classic ``Gulliver's Travels'' (via the paper ``On Holy Wars and a Plea for Peace'' by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and the egg-eating habits of the Lilliputians.
Some systems may have even weirder byte orders such as
0x56 0x78 0x12 0x34
0x34 0x12 0x78 0x56
You can see your system's preference with
print join(" ", map { sprintf "%#02x", $_ }
unpack("C*",pack("L",0x12345678))), "\n";
The byteorder on the platform where Perl was built is also available via the Config manpage:
use Config;
print $Config{byteorder}, "\n";
Byteorders '1234' and '12345678' are little-endian, '4321'
and '87654321' are big-endian.
If you want portable packed integers use the formats n, N,
v, and V, their byte endianness and size are known.
See also the perlport manpage.
Note that Perl uses doubles internally for all numeric calculation, and
converting from double into float and thence back to double again will
lose precision (i.e., unpack("f", pack("f", $foo)) will not in general
equal $foo).
U, the resulting string will be
treated as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a
string with an initial U0, and the bytes that follow will be
interpreted as Unicode characters. If you don't want this to happen,
you can begin your pattern with C0 (or anything else) to force Perl
not to UTF-8 encode your string, and then follow this with a U*
somewhere in your pattern.
You must yourself do any alignment or padding by inserting for example
enough 'x'es while packing. There is no way to pack() and unpack()
could know where the bytes are going to or coming from. Therefore
pack (and unpack) handle their output and input as flat
sequences of bytes.
A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
take a repeat count, both as postfix, and for unpack() also via the /
template character. Within each repetition of a group, positioning with
@ starts again at 0. Therefore, the result of
pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
is the string ``\0a\0\0bc''.
x and X accept ! modifier. In this case they act as
alignment commands: they jump forward/back to the closest position
aligned at a multiple of count bytes. For example, to pack() or
unpack() C's struct {char c; double d; char cc[2]} one may need to
use the template C x![d] d C[2]; this assumes that doubles must be
aligned on the double's size.
For alignment commands count of 0 is equivalent to count of 1;
both result in no-ops.
# and goes to the end of line.
White space may be used to separate pack codes from each other, but
a ! modifier and a repeat count must follow immediately.
If TEMPLATE requires more arguments to pack() than actually given, pack()
assumes additional "" arguments. If TEMPLATE requires less arguments
to pack() than actually given, extra arguments are ignored.
Examples:
$foo = pack("CCCC",65,66,67,68);
# foo eq "ABCD"
$foo = pack("C4",65,66,67,68);
# same thing
$foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
# note: the above examples featuring "C" and "c" are true
# only on ASCII and ASCII-derived systems such as ISO Latin 1
# and UTF-8. In EBCDIC the first example would be
# $foo = pack("CCCC",193,194,195,196);
$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
$foo = pack("aaaa","abcd","x","y","z");
# "axyz"
$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
$utmp_template = "Z8 Z8 Z16 L";
$utmp = pack($utmp_template, @utmp1);
# a struct utmp (BSDish)
@utmp2 = unpack($utmp_template, $utmp);
# "@utmp1" eq "@utmp2"
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
$foo = pack('sx2l', 12, 34);
# short 12, two zero bytes padding, long 34
$bar = pack('s@4l', 12, 34);
# short 12, zero fill to position 4, long 34
# $foo eq $bar
The same template may generally also be used in unpack().
my operator).
All further unqualified dynamic identifiers will be in this namespace.
A package statement affects only dynamic variables--including those
you've used local on--but not lexical variables, which are created
with my. Typically it would be the first declaration in a file to
be included by the require or use operator. You can switch into a
package in more than one place; it merely influences which symbol table
is used by the compiler for the rest of that block. You can refer to
variables and filehandles in other packages by prefixing the identifier
with the package name and a double colon: $Package::Variable.
If the package name is null, the main package as assumed. That is,
$::sail is equivalent to $main::sail (as well as to $main'sail,
still seen in older code).
If NAMESPACE is omitted, then there is no current package, and all identifiers must be fully qualified or lexicals. However, you are strongly advised not to make use of this feature. Its use can cause unexpected behaviour, even crashing some versions of Perl. It is deprecated, and will be removed from a future release.
See Packages in the perlmod manpage for more information about packages, modules, and classes. See the perlsub manpage for other scoping issues.
$| to flush your WRITEHANDLE
after each command, depending on the application.
See the IPC::Open2 manpage, the IPC::Open3 manpage, and Bidirectional Communication in the perlipc manpage for examples of such things.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptors as determined by the value of $^F. See $^F in the perlvar manpage.
$ARRAY[$#ARRAY--]
If there are no elements in the array, returns the undefined value
(although this may happen at other times as well). If ARRAY is
omitted, pops the @ARGV array in the main program, and the @_
array in subroutines, just like shift.
m//g search left off for the variable
in question ($_ is used when the variable is not specified). May be
modified to change that offset. Such modification will also influence
the \G zero-width assertion in regular expressions. See the perlre manpage and
the perlop manpage.
+ or put parentheses around the arguments.)
If FILEHANDLE is omitted, prints by default to standard output (or
to the last selected output channel--see select). If LIST is
also omitted, prints $_ to the currently selected output channel.
To set the default output channel to something other than STDOUT
use the select operation. The current value of $, (if any) is
printed between each LIST item. The current value of $\ (if
any) is printed after the entire LIST has been printed. Because
print takes a LIST, anything in the LIST is evaluated in list
context, and any subroutine that you call will have one or more of
its expressions evaluated in list context. Also be careful not to
follow the print keyword with a left parenthesis unless you want
the corresponding right parenthesis to terminate the arguments to
the print--interpose a + or put parentheses around all the
arguments.
Note that if you're storing FILEHANDLES in an array or other expression, you will have to use a block returning its value instead:
print { $files[$i] } "stuff\n";
print { $OK ? STDOUT : STDERR } "stuff\n";
print FILEHANDLE sprintf(FORMAT, LIST), except that $\
(the output record separator) is not appended. The first argument
of the list will be interpreted as the printf format. See sprintf
for an explanation of the format argument. If use locale is in effect,
the character used for the decimal point in formatted real numbers is
affected by the LC_NUMERIC locale. See the perllocale manpage.
Don't fall into the trap of using a printf when a simple
print would do. The print is more efficient and less
error prone.
undef if the
function has no prototype). FUNCTION is a reference to, or the name of,
the function whose prototype you want to retrieve.
If FUNCTION is a string starting with CORE::, the rest is taken as a
name for Perl builtin. If the builtin is not overridable (such as
qw//) or its arguments cannot be expressed by a prototype (such as
system) returns undef because the builtin does not really behave
like a Perl function. Otherwise, the string describing the equivalent
prototype is returned.
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
but is more efficient. Returns the new number of elements in the array.
/[A-Za-z_0-9]/ will be preceded by a backslash in the
returned string, regardless of any locale settings.)
This is the internal function implementing
the \Q escape in double-quoted strings.
If EXPR is omitted, uses $_.
0 and less
than the value of EXPR. (EXPR should be positive.) If EXPR is
omitted, the value 1 is used. Currently EXPR with the value 0 is
also special-cased as 1 - this has not been documented before perl 5.8.0
and is subject to change in future versions of perl. Automatically calls
srand unless srand has already been called. See also srand.
Apply int() to the value returned by rand() if you want random
integers instead of random fractional numbers. For example,
int(rand(10))
returns a random integer between 0 and 9, inclusive.
(Note: If your rand function consistently returns numbers that are too large or too small, then your version of Perl was probably compiled with the wrong number of RANDBITS.)
0 at end of file, or undef if there was an error (in
the latter case $! is also set). SCALAR will be grown or shrunk
so that the last character actually read is the last character of the
scalar after the read.
An OFFSET may be specified to place the read data at some place in the
string other than the beginning. A negative OFFSET specifies
placement at that many characters counting backwards from the end of
the string. A positive OFFSET greater than the length of SCALAR
results in the string being padded to the required size with "\0"
bytes before the result of the read is appended.
The call is actually implemented in terms of either Perl's or system's
fread() call. To get a true read(2) system call, see sysread.
Note the characters: depending on the status of the filehandle,
either (8-bit) bytes or characters are read. By default all
filehandles operate on bytes, but for example if the filehandle has
been opened with the :utf8 I/O layer (see open, and the open
pragma, the open manpage), the I/O will operate on UTF-8 encoded Unicode
characters, not bytes. Similarly for the :encoding pragma:
in that case pretty much any characters can be read.
opendir.
If used in list context, returns all the rest of the entries in the
directory. If there are no more entries, returns an undefined value in
scalar context or a null list in list context.
If you're planning to filetest the return values out of a readdir, you'd
better prepend the directory in question. Otherwise, because we didn't
chdir there, it would have been testing the wrong file.
opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
@dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
closedir DIR;
$/ or $INPUT_RECORD_SEPARATOR). See $/ in the perlvar manpage.
When $/ is set to undef, when readline() is in scalar
context (i.e. file slurp mode), and when an empty file is read, it
returns '' the first time, followed by undef subsequently.
This is the internal function implementing the <EXPR>
operator, but you can use it directly. The <EXPR>
operator is discussed in more detail in I/O Operators in the perlop manpage.
$line = <STDIN>;
$line = readline(*STDIN); # same thing
If readline encounters an operating system error, $! will be set with the
corresponding error message. It can be helpful to check $! when you are
reading from filehandles you don't trust, such as a tty or a socket. The
following example uses the operator form of readline, and takes the necessary
steps to ensure that readline was successful.
for (;;) {
undef $!;
unless (defined( $line = <> )) {
die $! if $!;
last; # reached EOF
}
# ...
}
$! (errno). If EXPR is
omitted, uses $_.
$/ or $INPUT_RECORD_SEPARATOR).
This is the internal function implementing the qx/EXPR/
operator, but you can use it directly. The qx/EXPR/
operator is discussed in more detail in I/O Operators in the perlop manpage.
recvfrom(2) system call.
See UDP: Message Passing in the perlipc manpage for examples.
Note the characters: depending on the status of the socket, either
(8-bit) bytes or characters are received. By default all sockets
operate on bytes, but for example if the socket has been changed using
binmode() to operate with the :utf8 I/O layer (see the open
pragma, the open manpage), the I/O will operate on UTF-8 encoded Unicode
characters, not bytes. Similarly for the :encoding pragma:
in that case pretty much any characters can be read.
redo command restarts the loop block without evaluating the
conditional again. The continue block, if any, is not executed. If
the LABEL is omitted, the command refers to the innermost enclosing
loop. This command is normally used by programs that want to lie to
themselves about what was just input:
# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
LINE: while (<STDIN>) {
while (s|({.*}.*){.*}|$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s|^|$front\{|;
redo LINE;
}
}
}
print;
}
redo cannot be used to retry a block which returns a value such as
eval {}, sub {} or do {}, and should not be used to exit
a grep() or map() operation.
Note that a block by itself is semantically identical to a loop
that executes once. Thus redo inside such a block will effectively
turn it into a looping construct.
See also continue for an illustration of how last, next, and
redo work.
$_ will be used. The value returned depends on the
type of thing the reference is a reference to.
Builtin types include:
SCALAR
ARRAY
HASH
CODE
REF
GLOB
LVALUE
If the referenced object has been blessed into a package, then that package
name is returned instead. You can think of ref as a typeof operator.
if (ref($r) eq "HASH") {
print "r is a reference to a hash.\n";
}
unless (ref($r)) {
print "r is not a reference at all.\n";
}
if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
print "r is a reference to something that isa hash.\n";
}
See also the perlref manpage.
Behavior of this function varies wildly depending on your system
implementation. For example, it will usually not work across file system
boundaries, even though the system mv command sometimes compensates
for this. Other restrictions include whether it works on directories,
open files, or pre-existing files. Check the perlport manpage and either the
rename(2) manpage or equivalent system documentation for details.
$_ if EXPR is not supplied.
VERSION may be either a numeric argument such as 5.006, which will be
compared to $], or a literal of the form v5.6.1, which will be compared
to $^V (aka $PERL_VERSION). A fatal error is produced at run time if
VERSION is greater than the version of the current Perl interpreter.
Compare with use, which can do a similar check at compile time.
Specifying VERSION as a literal of the form v5.6.1 should generally be avoided, because it leads to misleading error messages under earlier versions of Perl which do not support this syntax. The equivalent numeric version should be used instead.
require v5.6.1; # run time version check
require 5.6.1; # ditto
require 5.006_001; # ditto; preferred for backwards compatibility
Otherwise, demands that a library file be included if it hasn't already
been included. The file is included via the do-FILE mechanism, which is
essentially just a variety of eval. Has semantics similar to the
following subroutine:
sub require {
my ($filename) = @_;
if (exists $INC{$filename}) {
return 1 if $INC{$filename};
die "Compilation failed in require";
}
my ($realfilename,$result);
ITER: {
foreach $prefix (@INC) {
$realfilename = "$prefix/$filename";
if (-f $realfilename) {
$INC{$filename} = $realfilename;
$result = do $realfilename;
last ITER;
}
}
die "Can't find $filename in \@INC";
}
if ($@) {
$INC{$filename} = undef;
die $@;
} elsif (!$result) {
delete $INC{$filename};
die "$filename did not return true value";
} else {
return $result;
}
}
Note that the file will not be included twice under the same specified name.
The file must return true as the last statement to indicate
successful execution of any initialization code, so it's customary to
end such a file with 1; unless you're sure it'll return true
otherwise. But it's better just to put the 1;, in case you add more
statements.
If EXPR is a bareword, the require assumes a ``.pm'' extension and replaces ``::'' with ``/'' in the filename for you, to make it easy to load standard modules. This form of loading of modules does not risk altering your namespace.
In other words, if you try this:
require Foo::Bar; # a splendid bareword
The require function will actually look for the ``Foo/Bar.pm'' file in the
directories specified in the @INC array.
But if you try this:
$class = 'Foo::Bar';
require $class; # $class is not a bareword
#or
require "Foo::Bar"; # not a bareword because of the ""
The require function will look for the ``Foo::Bar'' file in the @INC array and will complain about not finding ``Foo::Bar'' there. In this case you can do:
eval "require $class";
Now that you understand how require looks for files in the case of
a bareword argument, there is a little extra functionality going on
behind the scenes. Before require looks for a ``.pm'' extension,
it will first look for a filename with a ``.pmc'' extension. A file
with this extension is assumed to be Perl bytecode generated by
B::Bytecode. If this file is found, and it's modification
time is newer than a coinciding ``.pm'' non-compiled file, it will be
loaded in place of that non-compiled file ending in a ``.pm'' extension.
You can also insert hooks into the import facility, by putting directly Perl code into the @INC array. There are three forms of hooks: subroutine references, array references and blessed objects.
Subroutine references are the simplest case. When the inclusion system
walks through @INC and encounters a subroutine, this subroutine gets
called with two parameters, the first being a reference to itself, and the
second the name of the file to be included (e.g. ``Foo/Bar.pm''). The
subroutine should return undef or a filehandle, from which the file to
include will be read. If undef is returned, require will look at
the remaining elements of @INC.
If the hook is an array reference, its first element must be a subroutine reference. This subroutine is called as above, but the first parameter is the array reference. This enables to pass indirectly some arguments to the subroutine.
In other words, you can write:
push @INC, \&my_sub;
sub my_sub {
my ($coderef, $filename) = @_; # $coderef is \&my_sub
...
}
or:
push @INC, [ \&my_sub, $x, $y, ... ];
sub my_sub {
my ($arrayref, $filename) = @_;
# Retrieve $x, $y, ...
my @parameters = @$arrayref[1..$#$arrayref];
...
}
If the hook is an object, it must provide an INC method, that will be
called as above, the first parameter being the object itself. (Note that
you must fully qualify the sub's name, as it is always forced into package
main.) Here is a typical code layout:
# In Foo.pm
package Foo;
sub new { ... }
sub Foo::INC {
my ($self, $filename) = @_;
...
}
# In the main program
push @INC, new Foo(...);
Note that these hooks are also permitted to set the %INC entry corresponding to the files they have loaded. See %INC in the perlvar manpage.
For a yet-more-powerful import facility, see use and the perlmod manpage.
continue block at the end of a loop to clear
variables and reset ?? searches so that they work again. The
expression is interpreted as a list of single characters (hyphens
allowed for ranges). All variables and arrays beginning with one of
those letters are reset to their pristine state. If the expression is
omitted, one-match searches (?pattern?) are reset to match again. Resets
only variables or searches in the current package. Always returns
1. Examples:
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
reset; # just reset ?one-time? searches
Resetting "A-Z" is not recommended because you'll wipe out your
@ARGV and @INC arrays and your %ENV hash. Resets only package
variables--lexical variables are unaffected, but they clean themselves
up on scope exit anyway, so you'll probably want to use them instead.
See my.
eval, or do FILE with the value
given in EXPR. Evaluation of EXPR may be in list, scalar, or void
context, depending on how the return value will be used, and the context
may vary from one execution to the next (see wantarray). If no EXPR
is given, returns an empty list in list context, the undefined value in
scalar context, and (of course) nothing at all in a void context.
(Note that in the absence of an explicit return, a subroutine, eval,
or do FILE will automatically return the value of the last expression
evaluated.)
print reverse <>; # line tac, last line first
undef $/; # for efficiency of <>
print scalar reverse <>; # character tac, last line tsrif
Used without arguments in scalar context, reverse() reverses $_.
This operator is also handy for inverting a hash, although there are some caveats. If a value is duplicated in the original hash, only one of those can be represented as a key in the inverted hash. Also, this has to unwind one hash and build a whole new one, which may take some time on a large hash, such as from a DBM file.
%by_name = reverse %by_address; # Invert the hash
readdir routine on DIRHANDLE.
index() except that it returns the position of the LAST
occurrence of SUBSTR in STR. If POSITION is specified, returns the
last occurrence at or before that position.
$! (errno). If FILENAME is omitted, uses $_.
@counts = ( scalar @a, scalar @b, scalar @c );
There is no equivalent operator to force an expression to
be interpolated in list context because in practice, this is never
needed. If you really wanted to do so, however, you could use
the construction @{[ (some expression) ]}, but usually a simple
(some expression) suffices.
Because scalar is unary operator, if you accidentally use for EXPR a
parenthesized list, this behaves as a scalar comma expression, evaluating
all but the last element in void context and returning the final element
evaluated in scalar context. This is seldom what you want.
The following single statement:
print uc(scalar(&foo,$bar)),$baz;
is the moral equivalent of these two:
&foo;
print(uc($bar),$baz);
See the perlop manpage for more details on unary operators and the comma operator.
fseek call of stdio.
FILEHANDLE may be an expression whose value gives the name of the
filehandle. The values for WHENCE are 0 to set the new position
in bytes to POSITION, 1 to set it to the current position plus
POSITION, and 2 to set it to EOF plus POSITION (typically
negative). For WHENCE you may use the constants SEEK_SET,
SEEK_CUR, and SEEK_END (start of the file, current position, end
of the file) from the Fcntl module. Returns 1 upon success, 0
otherwise.
Note the in bytes: even if the filehandle has been set to
operate on characters (for example by using the :utf8 open
layer), tell() will return byte offsets, not character offsets
(because implementing that would render seek() and tell() rather slow).
If you want to position file for sysread or syswrite, don't use
seek--buffering makes its effect on the file's system position
unpredictable and non-portable. Use sysseek instead.
Due to the rules and rigors of ANSI C, on some systems you have to do a
seek whenever you switch between reading and writing. Amongst other
things, this may have the effect of calling stdio's clearerr(3).
A WHENCE of 1 (SEEK_CUR) is useful for not moving the file position:
seek(TEST,0,1);
This is also useful for applications emulating tail -f. Once you hit
EOF on your read, and then sleep for a while, you might have to stick in a
seek() to reset things. The seek doesn't change the current position,
but it does clear the end-of-file condition on the handle, so that the
next <FILE> makes Perl try again to read something. We hope.
If that doesn't work (some IO implementations are particularly cantankerous), then you may need something more like this:
for (;;) {
for ($curpos = tell(FILE); $_ = <FILE>;
$curpos = tell(FILE)) {
# search for some stuff and put it into files
}
sleep($for_a_while);
seek(FILE, $curpos, 0);
}
readdir routine on DIRHANDLE. POS
must be a value returned by telldir. Has the same caveats about
possible directory compaction as the corresponding system library
routine.
write or a print without a filehandle will
default to this FILEHANDLE. Second, references to variables related to
output will refer to this output channel. For example, if you have to
set the top of form format for more than one output channel, you might
do the following:
select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';
FILEHANDLE may be an expression whose value gives the name of the actual filehandle. Thus:
$oldfh = select(STDERR); $| = 1; select($oldfh);
Some programmers may prefer to think of filehandles as objects with methods, preferring to write the last example as:
use IO::Handle;
STDERR->autoflush(1);
select(2) system call with the bit masks specified, which
can be constructed using fileno and vec, along these lines:
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
vec($win,fileno(STDOUT),1) = 1;
$ein = $rin | $win;
If you want to select on many filehandles you might wish to write a subroutine:
sub fhbits {
my(@fhlist) = split(' ',$_[0]);
my($bits);
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$bits;
}
$rin = fhbits('STDIN TTY SOCK');
The usual idiom is:
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
or to block until something becomes ready just do this
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
Most systems do not bother to return anything useful in $timeleft, so
calling select() in scalar context just returns $nfound.
Any of the bit masks can also be undef. The timeout, if specified, is in seconds, which may be fractional. Note: not all implementations are capable of returning the $timeleft. If not, they always return $timeleft equal to the supplied $timeout.
You can effect a sleep of 250 milliseconds this way:
select(undef, undef, undef, 0.25);
Note that whether select gets restarted after signals (say, SIGALRM)
is implementation-dependent. See also the perlport manpage for notes on the
portability of select.
WARNING: One should not attempt to mix buffered I/O (like read
or <FH>) with select, except as permitted by POSIX, and even
then only on POSIX systems. You have to use sysread instead.
semctl. You'll probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is IPC_STAT or
GETALL, then ARG must be a variable which will hold the returned
semid_ds structure or semaphore value array. Returns like ioctl:
the undefined value for error, ``0 but true'' for zero, or the actual
return value otherwise. The ARG must consist of a vector of native
short integers, which may be created with pack("s!",(0)x$nsem).
See also SysV IPC in the perlipc manpage, IPC::SysV, IPC::Semaphore
documentation.
IPC::SysV, IPC::SysV::Semaphore
documentation.
pack("s!3", $semnum, $semop, $semflag). The number of semaphore
operations is implied by the length of OPSTRING. Returns true if
successful, or false if there is an error. As an example, the
following code waits on semaphore $semnum of semaphore id $semid:
$semop = pack("s!3", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
To signal the semaphore, replace -1 with 1. See also
SysV IPC in the perlipc manpage, IPC::SysV, and IPC::SysV::Semaphore
documentation.
sendto. Returns the number of
characters sent, or the undefined value if there is an error. The C
system call sendmsg(2) is currently unimplemented. See
UDP: Message Passing in the perlipc manpage for examples.
Note the characters: depending on the status of the socket, either
(8-bit) bytes or characters are sent. By default all sockets operate
on bytes, but for example if the socket has been changed using
binmode() to operate with the :utf8 I/O layer (see open, or the
open pragma, the open manpage), the I/O will operate on UTF-8 encoded
Unicode characters, not bytes. Similarly for the :encoding pragma:
in that case pretty much any characters can be sent.
0 for the current
process. Will produce a fatal error if used on a machine that doesn't
implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
it defaults to 0,0. Note that the BSD 4.2 version of setpgrp does not
accept any arguments, so only setpgrp(0,0) is portable. See also
POSIX::setsid().
setpriority(2).) Will produce a fatal error if used on a machine
that doesn't implement setpriority(2).
undef if you don't want to pass an
argument.
@_ array within the lexical scope of subroutines and formats, and the
@ARGV array at file scopes or within the lexical scopes established by
the eval '', BEGIN {}, INIT {}, CHECK {}, and END {}
constructs.
See also unshift, push, and pop. shift and unshift do the
same thing to the left end of an array that pop and push do to the
right end.
use IPC::SysV;
first to get the correct constant definitions. If CMD is IPC_STAT,
then ARG must be a variable which will hold the returned shmid_ds
structure. Returns like ioctl: the undefined value for error, ``0 but
true'' for zero, or the actual return value otherwise.
See also SysV IPC in the perlipc manpage and IPC::SysV documentation.
IPC::SysV documentation.
shmread() taints the variable. See also SysV IPC in the perlipc manpage,
IPC::SysV documentation, and the IPC::Shareable module from CPAN.
shutdown(SOCKET, 0); # I/we have stopped reading data
shutdown(SOCKET, 1); # I/we have stopped writing data
shutdown(SOCKET, 2); # I/we have stopped using this socket
This is useful with sockets when you want to tell the other side you're done writing but not done reading, or vice versa. It's also a more insistent form of close because it also disables the file descriptor in any forked copies in other processes.
$_.
For the inverse sine operation, you may use the Math::Trig::asin
function, or use this relation:
sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
SIGALRM.
Returns the number of seconds actually slept. You probably cannot
mix alarm and sleep calls, because sleep is often implemented
using alarm.
On some older systems, it may sleep up to a full second less than what you requested, depending on how it counts seconds. Most modern systems always sleep the full amount. They may appear to sleep longer than that, however, because your process might not be scheduled right away in a busy multitasking system.
For delays of finer granularity than one second, you may use Perl's
syscall interface to access setitimer(2) if your system supports
it, or else see select above. The Time::HiRes module (from CPAN,
and starting from Perl 5.8 part of the standard distribution) may also
help.
See also the POSIX module's pause function.
use Socket first
to get the proper definitions imported. See the examples in
Sockets: Client/Server Communication in the perlipc manpage.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor, as determined by the value of $^F. See $^F in the perlvar manpage.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptors, as determined by the value of $^F. See $^F in the perlvar manpage.
Some systems defined pipe in terms of socketpair, in which a call
to pipe(Rdr, Wtr) is essentially:
use Socket;
socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
shutdown(Rdr, 1); # no more writing for reader
shutdown(Wtr, 0); # no more reading for writer
See the perlipc manpage for an example of socketpair use. Perl 5.8 and later will emulate socketpair using IP sockets to localhost if your system implements sockets but not socketpair.
sort() is undefined.
If SUBNAME or BLOCK is omitted, sorts in standard string comparison
order. If SUBNAME is specified, it gives the name of a subroutine
that returns an integer less than, equal to, or greater than 0,
depending on how the elements of the list are to be ordered. (The <
<= >> and cmp operators are extremely useful in such routines.)
SUBNAME may be a scalar variable name (unsubscripted), in which case
the value provides the name of (or a reference to) the actual
subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
an anonymous, in-line sort subroutine.
If the subroutine's prototype is ($$), the elements to be compared
are passed by reference in @_, as for a normal subroutine. This is
slower than unprototyped subroutines, where the elements to be
compared are passed into the subroutine
as the package global variables $a and $b (see example below). Note that
in the latter case, it is usually counter-productive to declare $a and
$b as lexicals.
In either case, the subroutine may not be recursive. The values to be compared are always passed by reference, so don't modify them.
You also cannot exit out of the sort block or subroutine using any of the
loop control operators described in the perlsyn manpage or with goto.
When use locale is in effect, sort LIST sorts LIST according to the
current collation locale. See the perllocale manpage.
Perl 5.6 and earlier used a quicksort algorithm to implement sort.
That algorithm was not stable, and could go quadratic. (A stable sort
preserves the input order of elements that compare equal. Although
quicksort's run time is O(NlogN) when averaged over all arrays of
length N, the time can be O(N**2), quadratic behavior, for some
inputs.) In 5.7, the quicksort implementation was replaced with
a stable mergesort algorithm whose worst case behavior is O(NlogN).
But benchmarks indicated that for some inputs, on some platforms,
the original quicksort was faster. 5.8 has a sort pragma for
limited control of the sort. Its rather blunt control of the
underlying algorithm may not persist into future perls, but the
ability to characterize the input or output in implementation
independent ways quite probably will. See the sort manpage.
Examples:
# sort lexically
@articles = sort @files;
# same thing, but with explicit sort routine
@articles = sort {$a cmp $b} @files;
# now case-insensitively
@articles = sort {uc($a) cmp uc($b)} @files;
# same thing in reversed order
@articles = sort {$b cmp $a} @files;
# sort numerically ascending
@articles = sort {$a <=> $b} @files;
# sort numerically descending
@articles = sort {$b <=> $a} @files;
# this sorts the %age hash by value instead of key
# using an in-line function
@eldest = sort { $age{$b} <=> $age{$a} } keys %age;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming numeric
}
@sortedclass = sort byage @class;
sub backwards { $b cmp $a }
@harry = qw(dog cat x Cain Abel);
@george = qw(gone chased yz Punished Axed);
print sort @harry;
# prints AbelCaincatdogx
print sort backwards @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
# inefficiently sort by descending numeric compare using
# the first integer after the first = sign, or the
# whole record case-insensitively otherwise
@new = sort {
($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
||
uc($a) cmp uc($b)
} @old;
# same thing, but much more efficiently;
# we'll build auxiliary indices instead
# for speed
@nums = @caps = ();
for (@old) {
push @nums, /=(\d+)/;
push @caps, uc($_);
}
@new = @old[ sort {
$nums[$b] <=> $nums[$a]
||
$caps[$a] cmp $caps[$b]
} 0..$#old
];
# same thing, but without any temps
@new = map { $_->[0] }
sort { $b->[1] <=> $a->[1]
||
$a->[2] cmp $b->[2]
} map { [$_, /=(\d+)/, uc($_)] } @old;
# using a prototype allows you to use any comparison subroutine
# as a sort subroutine (including other package's subroutines)
package other;
sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
package main;
@new = sort other::backwards @old;
# guarantee stability, regardless of algorithm
use sort 'stable';
@new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
# force use of mergesort (not portable outside Perl 5.8)
use sort '_mergesort'; # note discouraging _
@new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
If you're using strict, you must not declare $a
and $b as lexicals. They are package globals. That means
if you're in the main package and type
@articles = sort {$b <=> $a} @files;
then $a and $b are $main::a and $main::b (or $::a and $::b),
but if you're in the FooPack package, it's the same as typing
@articles = sort {$FooPack::b <=> $FooPack::a} @files;
The comparison function is required to behave. If it returns
inconsistent results (sometimes saying $x[1] is less than $x[2] and
sometimes saying the opposite, for example) the results are not
well-defined.
Because <=> returns undef when either operand is NaN
(not-a-number), and because sort will trigger a fatal error unless the
result of a comparison is defined, when sorting with a comparison function
like $a <=> $b, be careful about lists that might contain a NaN.
The following example takes advantage of the fact that NaN != NaN to
eliminate any NaNs from the input.
@result = sort { $a <=> $b } grep { $_ == $_ } @input;
undef if no elements are
removed. The array grows or shrinks as necessary.
If OFFSET is negative then it starts that far from the end of the array.
If LENGTH is omitted, removes everything from OFFSET onward.
If LENGTH is negative, removes the elements from OFFSET onward
except for -LENGTH elements at the end of the array.
If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
past the end of the array, perl issues a warning, and splices at the
end of the array.
The following equivalences hold (assuming $[ == 0 and $#a >= $i )
push(@a,$x,$y) splice(@a,@a,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
$a[$i] = $y splice(@a,$i,1,$y)
Example, assuming array lengths are passed before arrays:
sub aeq { # compare two list values
my(@a) = splice(@_,0,shift);
my(@b) = splice(@_,0,shift);
return 0 unless @a == @b; # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
}
return 1;
}
if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
In scalar context, returns the number of fields found and splits into
the @_ array. Use of split in scalar context is deprecated, however,
because it clobbers your subroutine arguments.
If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
splits on whitespace (after skipping any leading whitespace). Anything
matching PATTERN is taken to be a delimiter separating the fields. (Note
that the delimiter may be longer than one character.)
If LIMIT is specified and positive, it represents the maximum number
of fields the EXPR will be split into, though the actual number of
fields returned depends on the number of times PATTERN matches within
EXPR. If LIMIT is unspecified or zero, trailing null fields are
stripped (which potential users of pop would do well to remember).
If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
had been specified. Note that splitting an EXPR that evaluates to the
empty string always returns the empty list, regardless of the LIMIT
specified.
A pattern matching the null string (not to be confused with
a null pattern //, which is just one member of the set of patterns
matching a null string) will split the value of EXPR into separate
characters at each point it matches that way. For example:
print join(':', split(/ */, 'hi there'));
produces the output 'h:i:t:h:e:r:e'.
Using the empty pattern // specifically matches the null string, and is
not be confused with the use of // to mean ``the last successful pattern
match''.
Empty leading (or trailing) fields are produced when there are positive width matches at the beginning (or end) of the string; a zero-width match at the beginning (or end) of the string does not produce an empty field. For example:
print join(':', split(/(?=\w)/, 'hi there!'));
produces the output 'h:i :t:h:e:r:e!'.
The LIMIT parameter can be used to split a line partially
($login, $passwd, $remainder) = split(/:/, $_, 3);
When assigning to a list, if LIMIT is omitted, or zero, Perl supplies a LIMIT one larger than the number of variables in the list, to avoid unnecessary work. For the list above LIMIT would have been 4 by default. In time critical applications it behooves you not to split into more fields than you really need.
If the PATTERN contains parentheses, additional list elements are created from each matching substring in the delimiter.
split(/([,-])/, "1-10,20", 3);
produces the list value
(1, '-', 10, ',', 20)
If you had the entire header of a normal Unix email message in $header, you could split it up into fields and their values this way:
$header =~ s/\n\s+/ /g; # fix continuation lines
%hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
The pattern /PATTERN/ may be replaced with an expression to specify
patterns that vary at runtime. (To do runtime compilation only once,
use /$variable/o.)
As a special case, specifying a PATTERN of space (' ') will split on
white space just as split with no arguments does. Thus, split(' ') can
be used to emulate awk's default behavior, whereas split(/ /)
will give you as many null initial fields as there are leading spaces.
A split on /\s+/ is like a split(' ') except that any leading
whitespace produces a null first field. A split with no arguments
really does a split(' ', $_) internally.
A PATTERN of /^/ is treated as if it were /^/m, since it isn't
much use otherwise.
Example:
open(PASSWD, '/etc/passwd');
while (<PASSWD>) {
chomp;
($login, $passwd, $uid, $gid,
$gcos, $home, $shell) = split(/:/);
#...
}
As with regular pattern matching, any capturing parentheses that are not
matched in a split() will be set to undef when returned:
@fields = split /(A)|B/, "1A2B3";
# @fields is (1, 'A', 2, undef, 3)
printf conventions of the C
library function sprintf. See below for more details
and see sprintf(3) or printf(3) on your system for an explanation of
the general principles.
For example:
# Format number with up to 8 leading zeroes
$result = sprintf("%08d", $number);
# Round number to 3 digits after decimal point
$rounded = sprintf("%.3f", $number);
Perl does its own sprintf formatting--it emulates the C
function sprintf, but it doesn't use it (except for floating-point
numbers, and even then only the standard modifiers are allowed). As a
result, any non-standard extensions in your local sprintf are not
available from Perl.
Unlike printf, sprintf does not do what you probably mean when you
pass it an array as your first argument. The array is given scalar context,
and instead of using the 0th element of the array as the format, Perl will
use the count of elements in the array as the format, which is almost never
useful.
Perl's sprintf permits the following universally-known conversions:
%% a percent sign %c a character with the given number %s a string %d a signed integer, in decimal %u an unsigned integer, in decimal %o an unsigned integer, in octal %x an unsigned integer, in hexadecimal %e a floating-point number, in scientific notation %f a floating-point number, in fixed decimal notation %g a floating-point number, in %e or %f notation
In addition, Perl permits the following widely-supported conversions:
%X like %x, but using upper-case letters
%E like %e, but using an upper-case "E"
%G like %g, but with an upper-case "E" (if applicable)
%b an unsigned integer, in binary
%p a pointer (outputs the Perl value's address in hexadecimal)
%n special: *stores* the number of characters output so far
into the next variable in the parameter list
Finally, for backward (and we do mean ``backward'') compatibility, Perl permits these unnecessary but widely-supported conversions:
%i a synonym for %d %D a synonym for %ld %U a synonym for %lu %O a synonym for %lo %F a synonym for %f
Note that the number of exponent digits in the scientific notation produced
by %e, %E, %g and %G for numbers with the modulus of the
exponent less than 100 is system-dependent: it may be three or less
(zero-padded as necessary). In other words, 1.23 times ten to the
99th may be either ``1.23e99'' or ``1.23e099''.
Between the % and the format letter, you may specify a number of
additional attributes controlling the interpretation of the format.
In order, these are:
2$. By default sprintf
will format the next unused argument in the list, but this allows you
to take the arguments out of order. Eg:
printf '%2$d %1$d', 12, 34; # prints "34 12" printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
For example:
printf '<% d>', 12; # prints "< 12>" printf '<%+d>', 12; # prints "<+12>" printf '<%6s>', 12; # prints "< 12>" printf '<%-6s>', 12; # prints "<12 >" printf '<%06s>', 12; # prints "<000012>" printf '<%#x>', 12; # prints "<0xc>"
v, optionally specifying the join string to use.
This flag tells perl to interpret the supplied string as a vector
of integers, one for each character in the string, separated by
a given string (a dot . by default). This can be useful for
displaying ordinal values of characters in arbitrary strings:
printf "version is v%vd\n", $^V; # Perl's version
Put an asterisk * before the v to override the string to
use to separate the numbers:
printf "address is %*vX\n", ":", $addr; # IPv6 address printf "bits are %0*v8b\n", " ", $bits; # random bitstring
You can also explicitly specify the argument number to use for
the join string using eg *2$v:
printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
*)
or from a specified argument (with eg *2$):
printf '<%s>', "a"; # prints "<a>" printf '<%6s>', "a"; # prints "< a>" printf '<%*s>', 6, "a"; # prints "< a>" printf '<%*2$s>', "a", 6; # prints "< a>" printf '<%2s>', "long"; # prints "<long>" (does not truncate)
If a field width obtained through * is negative, it has the same
effect as the - flag: left-justification.
. followed by a number.
For floating point formats, with the exception of 'g' and 'G', this specifies
the number of decimal places to show (the default being 6), eg:
# these examples are subject to system-specific variation printf '<%f>', 1; # prints "<1.000000>" printf '<%.1f>', 1; # prints "<1.0>" printf '<%.0f>', 1; # prints "<1>" printf '<%e>', 10; # prints "<1.000000e+01>" printf '<%.1e>', 10; # prints "<1.0e+01>"
For 'g' and 'G', this specifies the maximum number of digits to show, including prior to the decimal point as well as after it, eg:
# these examples are subject to system-specific variation printf '<%g>', 1; # prints "<1>" printf '<%.10g>', 1; # prints "<1>" printf '<%g>', 100; # prints "<100>" printf '<%.1g>', 100; # prints "<1e+02>" printf '<%.2g>', 100.01; # prints "<1e+02>" printf '<%.5g>', 100.01; # prints "<100.01>" printf '<%.4g>', 100.01; # prints "<100>"
For integer conversions, specifying a precision implies that the output of the number itself should be zero-padded to this width:
printf '<%.6x>', 1; # prints "<000001>" printf '<%#.6x>', 1; # prints "<0x000001>" printf '<%-10.6x>', 1; # prints "<000001 >"
For string conversions, specifying a precision truncates the string to fit in the specified width:
printf '<%.5s>', "truncated"; # prints "<trunc>" printf '<%10.5s>', "truncated"; # prints "< trunc>"
You can also get the precision from the next argument using .*:
printf '<%.6x>', 1; # prints "<000001>" printf '<%.*x>', 6, 1; # prints "<000001>"
You cannot currently get the precision from a specified number,
but it is intended that this will be possible in the future using
eg .*2$:
printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
l, h, V, q, L, or ll. For integer
conversions (d u o x X b i D U O), numbers are usually assumed to be
whatever the default integer size is on your platform (usually 32 or 64
bits), but you can override this to use instead one of the standard C types,
as supported by the compiler used to build Perl:
l interpret integer as C type "long" or "unsigned long"
h interpret integer as C type "short" or "unsigned short"
q, L or ll interpret integer as C type "long long", "unsigned long long".
or "quads" (typically 64-bit integers)
The last will produce errors if Perl does not understand ``quads'' in your installation. (This requires that either the platform natively supports quads or Perl was specifically compiled to support quads.) You can find out whether your Perl supports quads via the Config manpage:
use Config;
($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
print "quads\n";
For floating point conversions (e f g E F G), numbers are usually assumed
to be the default floating point size on your platform (double or long double),
but you can force 'long double' with q, L, or ll if your
platform supports them. You can find out whether your Perl supports long
doubles via the Config manpage:
use Config;
$Config{d_longdbl} eq 'define' && print "long doubles\n";
You can find out whether Perl considers 'long double' to be the default floating point size to use on your platform via the Config manpage:
use Config;
($Config{uselongdouble} eq 'define') &&
print "long doubles by default\n";
It can also be the case that long doubles and doubles are the same thing:
use Config;
($Config{doublesize} == $Config{longdblsize}) &&
print "doubles are long doubles\n";
The size specifier V has no effect for Perl code, but it is supported
for compatibility with XS code; it means 'use the standard size for
a Perl integer (or floating-point number)', which is already the
default for Perl code.
* to require additional arguments, these are consumed from
the argument list in the order in which they appear in the format
specification before the value to format. Where an argument is
specified using an explicit index, this does not affect the normal
order for the arguments (even when the explicitly specified index
would have been the next argument in any case).
So:
printf '<%*.*s>', $a, $b, $c;
would use $a for the width, $b for the precision and $c
as the value to format, while:
print '<%*1$.*s>', $a, $b;
would use $a for the width and the precision, and $b as the
value to format.
Here are some more examples - beware that when using an explicit
index, the $ may need to be escaped:
printf "%2\$d %d\n", 12, 34; # will print "34 12\n" printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n" printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n" printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
If use locale is in effect, the character used for the decimal
point in formatted real numbers is affected by the LC_NUMERIC locale.
See the perllocale manpage.
$_. Only works on non-negative operands, unless you've
loaded the standard Math::Complex module.
use Math::Complex;
print sqrt(-2); # prints 1.4142135623731i
rand operator.
The point of the function is to ``seed'' the rand function so that
rand can produce a different sequence each time you run your
program.
If srand() is not called explicitly, it is called implicitly at the
first use of the rand operator. However, this was not the case in
versions of Perl before 5.004, so if your script will run under older
Perl versions, it should call srand.
Most programs won't even call srand() at all, except those that
need a cryptographically-strong starting point rather than the
generally acceptable default, which is based on time of day,
process ID, and memory allocation, or the /dev/urandom device,
if available.
You can call srand($seed) with the same $seed to reproduce the
same sequence from rand(), but this is usually reserved for
generating predictable results for testing or debugging.
Otherwise, don't call srand() more than once in your program.
Do not call srand() (i.e. without an argument) more than once in
a script. The internal state of the random number generator should
contain more entropy than can be provided by any seed, so calling
srand() again actually loses randomness.
Most implementations of srand take an integer and will silently
truncate decimal numbers. This means srand(42) will usually
produce the same results as srand(42.1). To be safe, always pass
srand an integer.
In versions of Perl prior to 5.004 the default seed was just the
current time. This isn't a particularly good seed, so many old
programs supply their own seed value (often time ^ $$ or time ^
($$ + ($$ << 15))), but that isn't necessary any more.
Note that you need something much more random than the default seed for cryptographic purposes. Checksumming the compressed output of one or more rapidly changing operating system status programs is the usual method. For example:
srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
If you're particularly concerned with this, see the Math::TrulyRandom
module in CPAN.
Frequently called programs (like CGI scripts) that simply use
time ^ $$
for a seed can fall prey to the mathematical property that
a^b == (a+1)^(b+1)
one-third of the time. So don't do that.
$_. Returns a null list if the stat fails. Typically used
as follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
Not all fields are supported on all filesystem types. Here are the meanings of the fields:
0 dev device number of filesystem 1 ino inode number 2 mode file mode (type and permissions) 3 nlink number of (hard) links to the file 4 uid numeric user ID of file's owner 5 gid numeric group ID of file's owner 6 rdev the device identifier (special files only) 7 size total size of file, in bytes 8 atime last access time in seconds since the epoch 9 mtime last modify time in seconds since the epoch 10 ctime inode change time in seconds since the epoch (*) 11 blksize preferred block size for file system I/O 12 blocks actual number of blocks allocated
(The epoch was at 00:00 January 1, 1970 GMT.)
(*) The ctime field is non-portable. In particular, you cannot expect it to be a ``creation time'', see Files and Filesystems in the perlport manpage for details.
If stat is passed the special filehandle consisting of an underline, no
stat is done, but the current contents of the stat structure from the
last stat, lstat, or filetest are returned. Example:
if (-x $file && (($d) = stat(_)) && $d < 0) {
print "$file is executable NFS file\n";
}
(This works on machines only for which the device number is negative under NFS.)
Because the mode contains both the file type and its permissions, you
should mask off the file type portion and (s)printf using a "%o"
if you want to see the real permissions.
$mode = (stat($filename))[2];
printf "Permissions are %04o\n", $mode & 07777;
In scalar context, stat returns a boolean value indicating success
or failure, and, if successful, sets the information associated with
the special filehandle _.
The File::stat module provides a convenient, by-name access mechanism:
use File::stat;
$sb = stat($filename);
printf "File is %s, size is %s, perm %04o, mtime %s\n",
$filename, $sb->size, $sb->mode & 07777,
scalar localtime $sb->mtime;
You can import symbolic mode constants (S_IF*) and functions
(S_IS*) from the Fcntl module:
use Fcntl ':mode';
$mode = (stat($filename))[2];
$user_rwx = ($mode & S_IRWXU) >> 6;
$group_read = ($mode & S_IRGRP) >> 3;
$other_execute = $mode & S_IXOTH;
printf "Permissions are %04o\n", S_IMODE($mode), "\n";
$is_setuid = $mode & S_ISUID;
$is_setgid = S_ISDIR($mode);
You could write the last two using the -u and -d operators.
The commonly available S_IF* constants are
# Permissions: read, write, execute, for user, group, others.
S_IRWXU S_IRUSR S_IWUSR S_IXUSR
S_IRWXG S_IRGRP S_IWGRP S_IXGRP
S_IRWXO S_IROTH S_IWOTH S_IXOTH
# Setuid/Setgid/Stickiness/SaveText.
# Note that the exact meaning of these is system dependent.
S_ISUID S_ISGID S_ISVTX S_ISTXT
# File types. Not necessarily all are available on your system.
S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
# The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
S_IREAD S_IWRITE S_IEXEC
and the S_IF* functions are
S_IMODE($mode) the part of $mode containing the permission bits
and the setuid/setgid/sticky bits
S_IFMT($mode) the part of $mode containing the file type
which can be bit-anded with e.g. S_IFREG
or with the following functions
# The operators -f, -d, -l, -b, -c, -p, and -S.
S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
# No direct -X operator counterpart, but for the first one
# the -g operator is often equivalent. The ENFMT stands for
# record flocking enforcement, a platform-dependent feature.
S_ISENFMT($mode) S_ISWHT($mode)
See your native chmod(2) and stat(2) documentation for more details
about the S_* constants. To get status info for a symbolic link
instead of the target file behind the link, use the lstat function.
$_ if unspecified) in anticipation of
doing many pattern matches on the string before it is next modified.
This may or may not save time, depending on the nature and number of
patterns you are searching on, and on the distribution of character
frequencies in the string to be searched--you probably want to compare
run times with and without it to see which runs faster. Those loops
which scan for many short constant strings (including the constant
parts of more complex patterns) will benefit most. You may have only
one study active at a time--if you study a different scalar the first
is ``unstudied''. (The way study works is this: a linked list of every
character in the string to be searched is made, so we know, for
example, where all the 'k' characters are. From each search string,
the rarest character is selected, based on some static frequency tables
constructed from some C programs and English text. Only those places
that contain this ``rarest'' character are examined.)
For example, here is a loop that inserts index producing entries before any line containing a certain pattern:
while (<>) {
study;
print ".IX foo\n" if /\bfoo\b/;
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
# ...
print;
}
In searching for /\bfoo\b/, only those locations in $_ that contain f
will be looked at, because f is rarer than o. In general, this is
a big win except in pathological cases. The only question is whether
it saves you more time than it took to build the linked list in the
first place.
Note that if you have to look for strings that you don't know till
runtime, you can build an entire loop as a string and eval that to
avoid recompiling all your patterns all the time. Together with
undefining $/ to input entire files as one record, this can be very
fast, often faster than specialized programs like fgrep(1). The following
scans a list of files (@files) for a list of words (@words), and prints
out the names of those files that contain a match:
$search = 'while (<>) { study;';
foreach $word (@words) {
$search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
}
$search .= "}";
@ARGV = @files;
undef $/;
eval $search; # this screams
$/ = "\n"; # put back to normal input delimiter
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
See the perlsub manpage and the perlref manpage for details about subroutines and references, and the attributes manpage and the Attribute::Handlers manpage for more information about attributes.
0, or whatever you've set $[ to (but don't do that).
If OFFSET is negative (or more precisely, less than $[), starts
that far from the end of the string. If LENGTH is omitted, returns
everything to the end of the string. If LENGTH is negative, leaves that
many characters off the end of the string.
You can use the substr() function as an lvalue, in which case EXPR
must itself be an lvalue. If you assign something shorter than LENGTH,
the string will shrink, and if you assign something longer than LENGTH,
the string will grow to accommodate it. To keep the string the same
length you may need to pad or chop your value using sprintf.
If OFFSET and LENGTH specify a substring that is partly outside the
string, only the part within the string is returned. If the substring
is beyond either end of the string, substr() returns the undefined
value and produces a warning. When used as an lvalue, specifying a
substring that is entirely outside the string is a fatal error.
Here's an example showing the behavior for boundary cases:
my $name = 'fred';
substr($name, 4) = 'dy'; # $name is now 'freddy'
my $null = substr $name, 6, 2; # returns '' (no warning)
my $oops = substr $name, 7; # returns undef, with warning
substr($name, 7) = 'gap'; # fatal error
An alternative to using substr() as an lvalue is to specify the
replacement string as the 4th argument. This allows you to replace
parts of the EXPR and return what was there before in one operation,
just as you can with splice().
If the lvalue returned by substr is used after the EXPR is changed in
any way, the behaviour may not be as expected and is subject to change.
This caveat includes code such as print(substr($foo,$a,$b)=$bar) or
(substr($foo,$a,$b)=$bar)=$fud (where $foo is changed via the
substring assignment, and then the substr is used again), or where a
substr() is aliased via a foreach loop or passed as a parameter or
a reference to it is taken and then the alias, parameter, or deref'd
reference either is used after the original EXPR has been changed or
is assigned to and then used a second time.
1 for success, 0 otherwise. On systems that don't support
symbolic links, produces a fatal error at run time. To check for that,
use eval:
$symlink_exists = eval { symlink("",""); 1 };
syscall
because Perl has to assume that any string pointer might be written
through. If your
integer arguments are not literals and have never been interpreted in a
numeric context, you may need to add 0 to them to force them to look
like numbers. This emulates the syswrite function (or vice versa):
require 'syscall.ph'; # may need to run h2ph
$s = "hi there\n";
syscall(&SYS_write, fileno(STDOUT), $s, length $s);
Note that Perl supports passing of up to only 14 arguments to your system call, which in practice should usually suffice.
Syscall returns whatever value returned by the system call it calls.
If the system call fails, syscall returns -1 and sets $! (errno).
Note that some system calls can legitimately return -1. The proper
way to handle such calls is to assign $!=0; before the call and
check the value of $! if syscall returns -1.
There's a problem with syscall(&SYS_pipe): it returns the file
number of the read end of the pipe it creates. There is no way
to retrieve the file number of the other end. You can avoid this
problem by using pipe instead.
open function with the parameters
FILENAME, MODE, PERMS.
The possible values and flag bits of the MODE parameter are
system-dependent; they are available via the standard module Fcntl.
See the documentation of your operating system's open to see which
values and flag bits are available. You may combine several flags
using the |-operator.
Some of the most common values are O_RDONLY for opening the file in
read-only mode, O_WRONLY for opening the file in write-only mode,
and O_RDWR for opening the file in read-write mode.
For historical reasons, some values work on almost every system supported by perl: zero means read-only, one means write-only, and two means read/write. We know that these values do not work under OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to use them in new code.
If the file named by FILENAME does not exist and the open call creates
it (typically because MODE includes the O_CREAT flag), then the value of
PERMS specifies the permissions of the newly created file. If you omit
the PERMS argument to sysopen, Perl uses the octal value 0666.
These permission values need to be in octal, and are modified by your
process's current umask.
In many systems the O_EXCL flag is available for opening files in
exclusive mode. This is not locking: exclusiveness means here that
if the file already exists, sysopen() fails. O_EXCL may not work
on network filesystems, and has no effect unless the O_CREAT flag
is set as well. Setting O_CREAT|O_EXCL prevents the file from
being opened if it is a symbolic link. It does not protect against
symbolic links in the file's path.
Sometimes you may want to truncate an already-existing file. This
can be done using the O_TRUNC flag. The behavior of
O_TRUNC with O_RDONLY is undefined.
You should seldom if ever use 0644 as argument to sysopen, because
that takes away the user's option to have a more permissive umask.
Better to omit it. See the perlfunc(1) entry on umask for more
on this.
Note that sysopen depends on the fdopen() C library function.
On many UNIX systems, fdopen() is known to fail when file descriptors
exceed a certain value, typically 255. If you need more file
descriptors than that, consider rebuilding Perl to use the sfio
library, or perhaps using the POSIX::open() function.
See the perlopentut manpage for a kinder, gentler explanation of opening files.
print,
write, seek, tell, or eof can cause confusion because the
perlio or stdio layers usually buffers data. Returns the number of
bytes actually read, 0 at end of file, or undef if there was an
error (in the latter case $! is also set). SCALAR will be grown or
shrunk so that the last byte actually read is the last byte of the
scalar after the read.
An OFFSET may be specified to place the read data at some place in the
string other than the beginning. A negative OFFSET specifies
placement at that many characters counting backwards from the end of
the string. A positive OFFSET greater than the length of SCALAR
results in the string being padded to the required size with "\0"
bytes before the result of the read is appended.
There is no syseof() function, which is ok, since eof() doesn't work
very well on device files (like ttys) anyway. Use sysread() and check
for a return value for 0 to decide whether you're done.
Note that if the filehandle has been marked as :utf8 Unicode
characters are read instead of bytes (the LENGTH, OFFSET, and the
return value of sysread() are in Unicode characters).
The :encoding(...) layer implicitly introduces the :utf8 layer.
See binmode, open, and the open pragma, the open manpage.
0 to set the new
position to POSITION, 1 to set the it to the current position plus
POSITION, and 2 to set it to EOF plus POSITION (typically
negative).
Note the in bytes: even if the filehandle has been set to operate
on characters (for example by using the :utf8 I/O layer), tell()
will return byte offsets, not character offsets (because implementing
that would render sysseek() very slow).
sysseek() bypasses normal buffered IO, so mixing this with reads (other
than sysread, for example >< or read()) print, write,
seek, tell, or eof may cause confusion.
For WHENCE, you may also use the constants SEEK_SET, SEEK_CUR,
and SEEK_END (start of the file, current position, end of the file)
from the Fcntl module. Use of the constants is also more portable
than relying on 0, 1, and 2. For example to define a ``systell'' function:
use Fcntl 'SEEK_CUR';
sub systell { sysseek($_[0], 0, SEEK_CUR) }
Returns the new position, or the undefined value on failure. A position
of zero is returned as the string "0 but true"; thus sysseek returns
true on success and false on failure, yet you can still easily determine
the new position.
exec LIST, except that a fork is
done first, and the parent process waits for the child process to
complete. Note that argument processing varies depending on the
number of arguments. If there is more than one argument in LIST,
or if LIST is an array with more than one value, starts the program
given by the first element of the list with arguments given by the
rest of the list. If there is only one scalar argument, the argument
is checked for shell metacharacters, and if there are any, the
entire argument is passed to the system's command shell for parsing
(this is /bin/sh -c on Unix platforms, but varies on other
platforms). If there are no shell metacharacters in the argument,
it is split into words and passed directly to execvp, which is
more efficient.
Beginning with v5.6.0, Perl will attempt to flush all files opened for
output before any operation that may do a fork, but this may not be
supported on some platforms (see the perlport manpage). To be safe, you may need
to set $| ($AUTOFLUSH in English) or call the autoflush() method
of IO::Handle on any open handles.
The return value is the exit status of the program as returned by the
wait call. To get the actual exit value shift right by eight (see below).
See also exec. This is not what you want to use to capture
the output from a command, for that you should use merely backticks or
qx//, as described in `STRING` in the perlop manpage. Return value of -1
indicates a failure to start the program (inspect $! for the reason).
Like exec, system allows you to lie to a program about its name if
you use the system PROGRAM LIST syntax. Again, see exec.
Since SIGINT and SIGQUIT are ignored during the execution of
system, if you expect your program to terminate on receipt of these
signals you will need to arrange to do so yourself based on the return
value.
@args = ("command", "arg1", "arg2");
system(@args) == 0
or die "system @args failed: $?"
You can check all the failure possibilities by inspecting
$? like this:
if ($? == -1) {
print "failed to execute: $!\n";
}
elsif ($? & 127) {
printf "child died with signal %d, %s coredump\n",
($? & 127), ($? & 128) ? 'with' : 'without';
}
else {
printf "child exited with value %d\n", $? >> 8;
}
or more portably by using the W*() calls of the POSIX extension; see the perlport manpage for more information.
When the arguments get executed via the system shell, results and return codes will be subject to its quirks and capabilities. See `STRING` in the perlop manpage and exec for details.
sysread()), print, write,
seek, tell, or eof may cause confusion because the perlio and
stdio layers usually buffers data. Returns the number of bytes
actually written, or undef if there was an error (in this case the
errno variable $! is also set). If the LENGTH is greater than the
available data in the SCALAR after the OFFSET, only as much data as is
available will be written.
An OFFSET may be specified to write the data from some part of the string other than the beginning. A negative OFFSET specifies writing that many characters counting backwards from the end of the string. In the case the SCALAR is empty you can use OFFSET but only zero offset.
Note that if the filehandle has been marked as :utf8, Unicode
characters are written instead of bytes (the LENGTH, OFFSET, and the
return value of syswrite() are in UTF-8 encoded Unicode characters).
The :encoding(...) layer implicitly introduces the :utf8 layer.
See binmode, open, and the open pragma, the open manpage.
Note the in bytes: even if the filehandle has been set to
operate on characters (for example by using the :utf8 open
layer), tell() will return byte offsets, not character offsets
(because that would render seek() and tell() rather slow).
The return value of tell() for the standard streams like the STDIN
depends on the operating system: it may return -1 or something else.
tell() on pipes, fifos, and sockets usually returns -1.
There is no systell function. Use sysseek(FH, 0, 1) for that.
Do not use tell() on a filehandle that has been opened using
sysopen(), use sysseek() for that as described above. Why? Because
sysopen() creates unbuffered, ``raw'', filehandles, while open() creates
buffered filehandles. sysseek() make sense only on the first kind,
tell() only makes sense on the second kind.
readdir routines on DIRHANDLE.
Value may be given to seekdir to access a particular location in a
directory. Has the same caveats about possible directory compaction as
the corresponding system library routine.
new
method of the class (meaning TIESCALAR, TIEHANDLE, TIEARRAY,
or TIEHASH). Typically these are arguments such as might be passed
to the dbm_open() function of C. The object returned by the new
method is also returned by the tie function, which would be useful
if you want to access other methods in CLASSNAME.
Note that functions such as keys and values may return huge lists
when used on large objects, like DBM files. You may prefer to use the
each function to iterate over such. Example:
# print out history file offsets
use NDBM_File;
tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
A class implementing a hash should have the following methods:
TIEHASH classname, LIST
FETCH this, key
STORE this, key, value
DELETE this, key
CLEAR this
EXISTS this, key
FIRSTKEY this
NEXTKEY this, lastkey
SCALAR this
DESTROY this
UNTIE this
A class implementing an ordinary array should have the following methods:
TIEARRAY classname, LIST
FETCH this, key
STORE this, key, value
FETCHSIZE this
STORESIZE this, count
CLEAR this
PUSH this, LIST
POP this
SHIFT this
UNSHIFT this, LIST
SPLICE this, offset, length, LIST
EXTEND this, count
DESTROY this
UNTIE this
A class implementing a file handle should have the following methods:
TIEHANDLE classname, LIST
READ this, scalar, length, offset
READLINE this
GETC this
WRITE this, scalar, length, offset
PRINT this, LIST
PRINTF this, format, LIST
BINMODE this
EOF this
FILENO this
SEEK this, position, whence
TELL this
OPEN this, mode, LIST
CLOSE this
DESTROY this
UNTIE this
A class implementing a scalar should have the following methods:
TIESCALAR classname, LIST
FETCH this,
STORE this, value
DESTROY this
UNTIE this
Not all methods indicated above need be implemented. See the perltie manpage, the Tie::Hash manpage, the Tie::Array manpage, the Tie::Scalar manpage, and the Tie::Handle manpage.
Unlike dbmopen, the tie function will not use or require a module
for you--you need to do that explicitly yourself. See the DB_File manpage
or the Config module for interesting tie implementations.
For further details see the perltie manpage, tied VARIABLE.
tie call that bound the variable
to a package.) Returns the undefined value if VARIABLE isn't tied to a
package.
gmtime and
localtime. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
1904 in the current local time zone for its epoch.
For measuring time in better granularity than one second,
you may use either the Time::HiRes module (from CPAN, and starting from
Perl 5.8 part of the standard distribution), or if you have
gettimeofday(2), you may be able to use the syscall interface of Perl.
See the perlfaq8 manpage for details.
($user,$system,$cuser,$csystem) = times;
In scalar context, times returns $user.
y///. See the perlop manpage.
The behavior is undefined if LENGTH is greater than the length of the file.
\U escape in double-quoted strings. Respects
current LC_CTYPE locale if use locale in force. See the perllocale manpage
and the perlunicode manpage for more details about locale and Unicode support.
It does not attempt to do titlecase mapping on initial letters. See
ucfirst for that.
If EXPR is omitted, uses $_.
\u escape in double-quoted strings. Respects current LC_CTYPE
locale if use locale in force. See the perllocale manpage and the perlunicode manpage
for more details about locale and Unicode support.
If EXPR is omitted, uses $_.
The Unix permission rwxr-x--- is represented as three sets of three
bits, or three octal digits: 0750 (the leading 0 indicates octal
and isn't one of the digits). The umask value is such a number
representing disabled permissions bits. The permission (or ``mode'')
values you pass mkdir or sysopen are modified by your umask, so
even if you tell sysopen to create a file with permissions 0777,
if your umask is 0022 then the file will actually be created with
permissions 0755. If your umask were 0027 (group can't
write; others can't read, write, or execute), then passing
sysopen 0666 would create a file with mode 0640 (0666 &~
027 is 0640).
Here's some advice: supply a creation mode of 0666 for regular
files (in sysopen) and one of 0777 for directories (in
mkdir) and executable files. This gives users the freedom of
choice: if they want protected files, they might choose process umasks
of 022, 027, or even the particularly antisocial mask of 077.
Programs should rarely if ever make policy decisions better left to
the user. The exception to this is when writing files that should be
kept private: mail files, web browser cookies, .rhosts files, and
so on.
If umask(2) is not implemented on your system and you are trying to
restrict access for yourself (i.e., (EXPR & 0700) > 0), produces a
fatal error at run time. If umask(2) is not implemented and you are
not trying to restrict access for yourself, returns undef.
Remember that a umask is a number, usually given in octal; it is not a string of octal digits. See also oct, if all you have is a string.
@), a hash (using %), a subroutine
(using &), or a typeglob (using *). (Saying undef $hash{$key}
will probably not do what you expect on most predefined variables or
DBM list values, so don't do that; see delete.) Always returns the
undefined value. You can omit the EXPR, in which case nothing is
undefined, but you still get an undefined value that you could, for
instance, return from a subroutine, assign to a variable or pass as a
parameter. Examples:
undef $foo;
undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
undef @ary;
undef %hash;
undef &mysub;
undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
select undef, undef, undef, 0.25;
($a, $b, undef, $c) = &foo; # Ignore third value returned
Note that this is a unary operator, not a list operator.
$cnt = unlink 'a', 'b', 'c';
unlink @goners;
unlink <*.bak>;
Note: unlink will not delete directories unless you are superuser and
the -U flag is supplied to Perl. Even if these conditions are
met, be warned that unlinking a directory can inflict damage on your
filesystem. Use rmdir instead.
If LIST is omitted, uses $_.
unpack does the reverse of pack: it takes a string
and expands it out into a list of values.
(In scalar context, it returns merely the first value produced.)
The string is broken into chunks described by the TEMPLATE. Each chunk
is converted separately to a value. Typically, either the string is a result
of pack, or the bytes of the string represent a C structure of some
kind.
The TEMPLATE has the same format as in the pack function.
Here's a subroutine that does substring:
sub substr {
my($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there's
sub ordinal { unpack("c",$_[0]); } # same as ord()
In addition to fields allowed in pack(), you may prefix a field with
a %<number> to indicate that
you want a <number>-bit checksum of the items instead of the items
themselves. Default is a 16-bit checksum. Checksum is calculated by
summing numeric values of expanded values (for string fields the sum of
ord($char) is taken, for bit fields the sum of zeroes and ones).
For example, the following computes the same number as the System V sum program:
$checksum = do {
local $/; # slurp!
unpack("%32C*",<>) % 65535;
};
The following efficiently counts the number of set bits in a bit vector:
$setbits = unpack("%32b*", $selectmask);
The p and P formats should be used with care. Since Perl
has no way of checking whether the value passed to unpack()
corresponds to a valid memory location, passing a pointer value that's
not known to be valid is likely to have disastrous consequences.
If there are more pack codes or if the repeat count of a field or a group
is larger than what the remainder of the input string allows, the result
is not well defined: in some cases, the repeat count is decreased, or
unpack() will produce null strings or zeroes, or terminate with an
error. If the input string is longer than one described by the TEMPLATE,
the rest is ignored.
See pack for more examples and notes.
tie.)
Has no effect if the variable is not tied.
shift. Or the opposite of a push,
depending on how you look at it. Prepends list to the front of the
array, and returns the new number of elements in the array.
unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
Note the LIST is prepended whole, not one element at a time, so the
prepended elements stay in the same order. Use reverse to do the
reverse.
BEGIN { require Module; import Module LIST; }
except that Module must be a bareword.
VERSION may be either a numeric argument such as 5.006, which will be
compared to $], or a literal of the form v5.6.1, which will be compared
to $^V (aka $PERL_VERSION. A fatal error is produced if VERSION is
greater than the version of the current Perl interpreter; Perl will not
attempt to parse the rest of the file. Compare with require, which can
do a similar check at run time.
Specifying VERSION as a literal of the form v5.6.1 should generally be avoided, because it leads to misleading error messages under earlier versions of Perl which do not support this syntax. The equivalent numeric version should be used instead.
use v5.6.1; # compile time version check
use 5.6.1; # ditto
use 5.006_001; # ditto; preferred for backwards compatibility
This is often useful if you need to check the current Perl version before
useing library modules that have changed in incompatible ways from
older versions of Perl. (We try not to do this more than we have to.)
The BEGIN forces the require and import to happen at compile time. The
require makes sure the module is loaded into memory if it hasn't been
yet. The import is not a builtin--it's just an ordinary static method
call into the Module package to tell the module to import the list of
features back into the current package. The module can implement its
import method any way it likes, though most modules just choose to
derive their import method via inheritance from the Exporter class that
is defined in the Exporter module. See the Exporter manpage. If no import
method can be found then the call is skipped.
If you do not want to call the package's import method (for instance,
to stop your namespace from being altered), explicitly supply the empty list:
use Module ();
That is exactly equivalent to
BEGIN { require Module }
If the VERSION argument is present between Module and LIST, then the
use will call the VERSION method in class Module with the given
version as an argument. The default VERSION method, inherited from
the UNIVERSAL class, croaks if the given version is larger than the
value of the variable $Module::VERSION.
Again, there is a distinction between omitting LIST (import called
with no arguments) and an explicit empty LIST () (import not
called). Note that there is no comma after VERSION!
Because this is a wide-open interface, pragmas (compiler directives) are also implemented this way. Currently implemented pragmas are:
use constant;
use diagnostics;
use integer;
use sigtrap qw(SEGV BUS);
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
use warnings qw(all);
use sort qw(stable _quicksort _mergesort);
Some of these pseudo-modules import semantics into the current
block scope (like strict or integer, unlike ordinary modules,
which import symbols into the current package (which are effective
through the end of the file).
There's a corresponding no command that unimports meanings imported
by use, i.e., it calls unimport Module LIST instead of import.
no integer;
no strict 'refs';
no warnings;
See the perlmodlib manpage for a list of standard modules and pragmas. See the perlrun manpage
for the -M and -m command-line options to perl that give use
functionality from the command-line.
touch(1) command when the files already exist.
#!/usr/bin/perl
$atime = $mtime = time;
utime $atime, $mtime, @ARGV;
Since perl 5.7.2, if the first two elements of the list are undef, then
the utime(2) function in the C library will be called with a null second
argument. On most systems, this will set the file's access and
modification times to the current time (i.e. equivalent to the example
above.)
utime undef, undef, @ARGV;
Under NFS this will use the time of the NFS server, not the time of
the local machine. If there is a time synchronization problem, the
NFS server and local machine will have different times. The Unix
touch(1) command will in fact normally use this form instead of the
one shown in the first example.
Note that only passing one of the first two elements as undef will
be equivalent of passing it as 0 and will not have the same effect as
described when they are both undef. This case will also trigger an
uninitialized warning.
The values are returned in an apparently random order. The actual
random order is subject to change in future versions of perl, but it
is guaranteed to be the same order as either the keys or each
function would produce on the same (unmodified) hash. Since Perl
5.8.1 the ordering is different even between different runs of Perl
for security reasons (see Algorithmic Complexity Attacks in the perlsec manpage).
As a side effect, calling values() resets the HASH's internal iterator,
see each. (In particular, calling values() in void context resets
the iterator with no other overhead.)
Note that the values are not copied, which means modifying them will modify the contents of the hash:
for (values %hash) { s/foo/bar/g } # modifies %hash values
for (@hash{keys %hash}) { s/foo/bar/g } # same
If BITS is 8, ``elements'' coincide with bytes of the input string.
If BITS is 16 or more, bytes of the input string are grouped into chunks
of size BITS/8, and each group is converted to a number as with
pack()/unpack() with big-endian formats n/N (and analogously
for BITS==64). See pack for details.
If bits is 4 or less, the string is broken into bytes, then the bits
of each byte are broken into 8/BITS groups. Bits of a byte are
numbered in a little-endian-ish way, as in 0x01, 0x02,
0x04, 0x08, 0x10, 0x20, 0x40, 0x80. For example,
breaking the single input byte chr(0x36) into two groups gives a list
(0x6, 0x3); breaking it into 4 groups gives (0x2, 0x1, 0x3, 0x0).
vec may also be assigned to, in which case parentheses are needed
to give the expression the correct precedence as in
vec($image, $max_x * $x + $y, 8) = 3;
If the selected element is outside the string, the value 0 is returned. If an element off the end of the string is written to, Perl will first extend the string with sufficiently many zero bytes. It is an error to try to write off the beginning of the string (i.e. negative OFFSET).
The string should not contain any character with the value > 255 (which
can only happen if you're using UTF-8 encoding). If it does, it will be
treated as something which is not UTF-8 encoded. When the vec was
assigned to, other parts of your program will also no longer consider the
string to be UTF-8 encoded. In other words, if you do have such characters
in your string, vec() will operate on the actual byte string, and not the
conceptual character string.
Strings created with vec can also be manipulated with the logical
operators |, &, ^, and ~. These operators will assume a bit
vector operation is desired when both operands are strings.
See Bitwise String Operators in the perlop manpage.
The following code will build up an ASCII string saying 'PerlPerlPerl'.
The comments show the string after each step. Note that this code works
in the same way on big-endian or little-endian machines.
my $foo = '';
vec($foo, 0, 32) = 0x5065726C; # 'Perl'
# $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
vec($foo, 2, 16) = 0x5065; # 'PerlPe'
vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
vec($foo, 21, 4) = 7; # 'PerlPerlPer'
# 'r' is "\x72"
vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
# 'l' is "\x6c"
To transform a bit vector into a string or list of 0's and 1's, use these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be used in place of the *.
Here is an example to illustrate how the bits actually fall in place:
#!/usr/bin/perl -wl
print <<'EOT';
0 1 2 3
unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
EOT
for $w (0..3) {
$width = 2**$w;
for ($shift=0; $shift < $width; ++$shift) {
for ($off=0; $off < 32/$width; ++$off) {
$str = pack("B*", "0"x32);
$bits = (1<<$shift);
vec($str, $off, $width) = $bits;
$res = unpack("b*",$str);
$val = unpack("V", $str);
write;
}
}
}
format STDOUT =
vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
$off, $width, $bits, $val, $res
.
__END__
Regardless of the machine architecture on which it is run, the above example should print the following table:
0 1 2 3
unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
wait(2) system call on your system: it waits for a child
process to terminate and returns the pid of the deceased process, or
-1 if there are no child processes. The status is returned in $?.
Note that a return value of -1 could mean that child processes are
being automatically reaped, as described in the perlipc manpage.
-1 if there is no such child process. On some
systems, a value of 0 indicates that there are processes still running.
The status is returned in $?. If you say
use POSIX ":sys_wait_h";
#...
do {
$kid = waitpid(-1, WNOHANG);
} until $kid > 0;
then you can do a non-blocking wait for all pending zombie processes.
Non-blocking wait is available on machines supporting either the
waitpid(2) or wait4(2) system calls. However, waiting for a particular
pid with FLAGS of 0 is implemented everywhere. (Perl emulates the
system call by remembering the status values of processes that have
exited but have not been harvested by the Perl script yet.)
Note that on some systems, a return value of -1 could mean that child
processes are being automatically reaped. See the perlipc manpage for details,
and for other examples.
eval() block is looking for a list value. Returns false if the context is
looking for a scalar. Returns the undefined value if the context is
looking for no value (void context).
return unless defined wantarray; # don't bother doing more
my @a = complex_calculation();
return wantarray ? @a : "@a";
This function should have been named wantlist() instead.
die, but doesn't exit or throw
an exception.
If LIST is empty and $@ already contains a value (typically from a
previous eval) that value is used after appending "\t...caught"
to $@. This is useful for staying almost, but not entirely similar to
die.
If $@ is empty then the string "Warning: Something's wrong" is used.
No message is printed if there is a $SIG{__WARN__} handler
installed. It is the handler's responsibility to deal with the message
as it sees fit (like, for instance, converting it into a die). Most
handlers must therefore make arrangements to actually display the
warnings that they are not prepared to deal with, by calling warn
again in the handler. Note that this is quite safe and will not
produce an endless loop, since __WARN__ hooks are not called from
inside one.
You will find this behavior is slightly different from that of
$SIG{__DIE__} handlers (which don't suppress the error text, but can
instead call die again to change it).
Using a __WARN__ handler provides a powerful way to silence all
warnings (even the so-called mandatory ones). An example:
# wipe out *all* compile-time warnings
BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
my $foo = 10;
my $foo = 20; # no warning about duplicate my $foo,
# but hey, you asked for it!
# no compile-time or run-time warnings before here
$DOWARN = 1;
# run-time warnings enabled after here
warn "\$foo is alive and $foo!"; # does show up
See the perlvar manpage for details on setting %SIG entries, and for more
examples. See the Carp module for other kinds of warnings using its
carp() and cluck() functions.
select function) may be set
explicitly by assigning the name of the format to the $~ variable.
Top of form processing is handled automatically: if there is
insufficient room on the current page for the formatted record, the
page is advanced by writing a form feed, a special top-of-page format
is used to format the new page header, and then the record is written.
By default the top-of-page format is the name of the filehandle with
``_TOP'' appended, but it may be dynamically set to the format of your
choice by assigning the name to the $^ variable while the filehandle is
selected. The number of lines remaining on the current page is in
variable $-, which can be set to 0 to force a new page.
If FILEHANDLE is unspecified, output goes to the current default output
channel, which starts out as STDOUT but may be changed by the
select operator. If the FILEHANDLE is an EXPR, then the expression
is evaluated and the resulting string is used to look up the name of
the FILEHANDLE at run time. For more on formats, see the perlform manpage.
Note that write is not the opposite of read. Unfortunately.
tr///. See the perlop manpage.
| perlfunc - Perl builtin functions |