=head1 NAME perldebug - Perl debugging =head1 DESCRIPTION First of all, have you tried using the B<-w> switch? =head1 The Perl Debugger If you invoke Perl with the B<-d> switch, your script runs under the Perl source debugger. This works like an interactive Perl environment, prompting for debugger commands that let you examine source code, set breakpoints, get stack backtraces, change the values of variables, etc. This is so convenient that you often fire up the debugger all by itself just to test out Perl constructs interactively to see what they do. For example: $ perl -d -e 42 In Perl, the debugger is not a separate program the way it usually is in the typical compiled environment. Instead, the B<-d> flag tells the compiler to insert source information into the parse trees it's about to hand off to the interpreter. That means your code must first compile correctly for the debugger to work on it. Then when the interpreter starts up, it preloads a special Perl library file containing the debugger. The program will halt I the first run-time executable statement (but see below regarding compile-time statements) and ask you to enter a debugger command. Contrary to popular expectations, whenever the debugger halts and shows you a line of code, it always displays the line it's I to execute, rather than the one it has just executed. Any command not recognized by the debugger is directly executed (C'd) as Perl code in the current package. (The debugger uses the DB package for keeping its own state information.) For any text entered at the debugger prompt, leading and trailing whitespace is first stripped before further processing. If a debugger command coincides with some function in your own program, merely precede the function with something that doesn't look like a debugger command, such as a leading C<;> or perhaps a C<+>, or by wrapping it with parentheses or braces. =head2 Debugger Commands The debugger understands the following commands: =over 12 =item h [command] Prints out a help message. If you supply another debugger command as an argument to the C command, it prints out the description for just that command. The special argument of C produces a more compact help listing, designed to fit together on one screen. If the output of the C command (or any command, for that matter) scrolls past your screen, precede the command with a leading pipe symbol so that it's run through your pager, as in DB> |h You may change the pager which is used via C command. =item p expr Same as C in the current package. In particular, because this is just Perl's own C function, this means that nested data structures and objects are not dumped, unlike with the C command. The C filehandle is opened to F, regardless of where STDOUT may be redirected to. =item x expr Evaluates its expression in list context and dumps out the result in a pretty-printed fashion. Nested data structures are printed out recursively, unlike the real C function in Perl. See L if you'd like to do this yourself. The output format is governed by multiple options described under L<"Options">. =item V [pkg [vars]] Display all (or some) variables in package (defaulting to C
) using a data pretty-printer (hashes show their keys and values so you see what's what, control characters are made printable, etc.). Make sure you don't put the type specifier (like C<$>) there, just the symbol names, like this: V DB filename line Use C<~pattern> and C for positive and negative regexes. This is similar to calling the C command on each applicable var. =item X [vars] Same as C. =item T Produce a stack backtrace. See below for details on its output. =item s [expr] Single step. Executes until the beginning of another statement, descending into subroutine calls. If an expression is supplied that includes function calls, it too will be single-stepped. =item n [expr] Next. Executes over subroutine calls, until the beginning of the next statement. If an expression is supplied that includes function calls, those functions will be executed with stops before each statement. =item r Continue until the return from the current subroutine. Dump the return value if the C option is set (default). =item Repeat last C or C command. =item c [line|sub] Continue, optionally inserting a one-time-only breakpoint at the specified line or subroutine. =item l List next window of lines. =item l min+incr List C lines starting at C. =item l min-max List lines C through C. C is synonymous to C<->. =item l line List a single line. =item l subname List first window of lines from subroutine. I may be a variable that contains a code reference. =item - List previous window of lines. =item w [line] List window (a few lines) around the current line. =item . Return the internal debugger pointer to the line last executed, and print out that line. =item f filename Switch to viewing a different file or C statement. If I is not a full pathname found in the values of %INC, it is considered a regex. Ced strings (when accessible) are considered to be filenames: C and C access the body of the 7th Ced string (in the order of execution). The bodies of the currently executed C and of Ced strings that define subroutines are saved and thus accessible. =item /pattern/ Search forwards for pattern (a Perl regex); final / is optional. =item ?pattern? Search backwards for pattern; final ? is optional. =item L List all breakpoints and actions. =item S [[!]regex] List subroutine names [not] matching the regex. =item t Toggle trace mode (see also the C option). =item t expr Trace through execution of C. See L for examples. =item b [line] [condition] Set a breakpoint before the given line. If I is omitted, set a breakpoint on the line about to be executed. If a condition is specified, it's evaluated each time the statement is reached: a breakpoint is taken only if the condition is true. Breakpoints may only be set on lines that begin an executable statement. Conditions don't use C: b 237 $x > 30 b 237 ++$count237 < 11 b 33 /pattern/i =item b subname [condition] Set a breakpoint before the first line of the named subroutine. I may be a variable containing a code reference (in this case I is not supported). =item b postpone subname [condition] Set a breakpoint at first line of subroutine after it is compiled. =item b load filename Set a breakpoint before the first executed line of the I, which should be a full pathname found amongst the %INC values. =item b compile subname Sets a breakpoint before the first statement executed after the specified subroutine is compiled. =item d [line] Delete a breakpoint from the specified I. If I is omitted, deletes the breakpoint from the line about to be executed. =item D Delete all installed breakpoints. =item a [line] command Set an action to be done before the line is executed. If I is omitted, set an action on the line about to be executed. The sequence of steps taken by the debugger is 1. check for a breakpoint at this line 2. print the line if necessary (tracing) 3. do any actions associated with that line 4. prompt user if at a breakpoint or in single-step 5. evaluate line For example, this will print out $foo every time line 53 is passed: a 53 print "DB FOUND $foo\n" =item a [line] Delete an action from the specified line. If I is omitted, delete the action on the line that is about to be executed. =item A Delete all installed actions. =item W expr Add a global watch-expression. We hope you know what one of these is, because they're supposed to be obvious. B: It is far too easy to destroy your watch expressions by accidentally omitting the I. =item W Delete all watch-expressions. =item O booloption ... Set each listed Boolean option to the value C<1>. =item O anyoption? ... Print out the value of one or more options. =item O option=value ... Set the value of one or more options. If the value has internal whitespace, it should be quoted. For example, you could set C to call B with those specific options. You may use either single or double quotes, but if you do, you must escape any embedded instances of same sort of quote you began with, as well as any escaping any escapes that immediately precede that quote but which are not meant to escape the quote itself. In other words, you follow single-quoting rules irrespective of the quote; eg: C or C. For historical reasons, the C<=value> is optional, but defaults to 1 only where it is safe to do so--that is, mostly for Boolean options. It is always better to assign a specific value using C<=>. The C