Inline::Files::Virtual - Multiple virtual files in a single file


Inline::Files::Virtual - Multiple virtual files in a single file


This document describes version 0.53 of Inline::Files::Virtual, released May 25, 2001.


    use Inline::Files::Virtual;
    # Load actual file, extracting virtual files that start with "^<VF>\n"
    @virtual_filenames = vf_load($actual_file, "^<VF>\n");

    # Open one of the virtual files for reading
    open(FILE, $virtual_filenames[0]) or die;

    print while <FILE>;

    # Open one of the virtual files for appending
    open(FILE, ">> $virtual_filenames[1]") or die;

    print FILE "extra text";
    printf FILE "%6.2", $number;


    # Actual file will be updated at this point

=head1 WARNING

This module is still experimental. Careless use of it will almost certainly cause the source code in files that use it to be overwritten. You are strongly advised to use the Inline::Files module instead.

If you chose to use this module anyway, you thereby agree that the authors will b<under no circumstances> be responsible for any loss of data, code, time, money, or limbs, or for any other disadvantage incurred as a result of using Inline::Files.


This module allows you to treat a single disk file as a collection of virtual files, which may then be individually opened for reading or writing. Virtual files which have been modified are written back to their actual disk at the end of the program's execution (or earlier if the vf_save subroutine is explicitly called).

Each such virtual file is introduced by a start-of-virtual-file marker (SOVFM). This may be any sequence (or pattern) of characters that marks the beginning of the content of a virtual file. For example, the string "--" might be used:

        Contents of virtual
        file number 1
        Contents of virtual
        file number 2
        Contents of virtual
        file number 3

or the pattern /##### \w+ #####/:

        ##### VF1 #####
        Contents of virtual
        file number 1
        ##### VF2 #####
        Contents of virtual
        file number 2
        ##### VF3 #####
        Contents of virtual
        file number 3

Note that the SOVFM is not considered to be part of the file contents.


The module exports the following methods:

vf_load $file, $SOVFM_pattern
This subroutine is called to load an actual disk file containing one or more virtual files. The first argument specifies the name of the file to be loaded as a string. The second argument specifies a pattern (as either a string or qr regex) that matches each start-of-virtual-file marker within the file. For example, if the file /usr/local/details.dat contains:
        =info names


        =info numbers


        =info comment

        Dangerous to know

then you could load it as three virtual files with:
        @virtual_filenames =
                vf_load("/usr/local/details.dat", qr/^=info\s+\S+\s*?\n/);

Note that, because the actual file is decomposed into virtual files using a split, it is vital that the pattern does not contain any capturing parentheses.

On success, vf_load returns a list of virtual filenames for the virtual files. Each virtual filename consists of the actual name of the file containing the virtual file, concatenated with the offset of the virtual file's SOVFM within the actual file. For example, the above call to vf_load would return three virtual filenames:


When any of these virtual filenames is subsequently used in an C<open>, the 
corresponding virtual file is opened.

vf_save @actual_filenames
This subroutine causes the virtual files belonging to the nominated actual file (or files) to be written back to disk. If vf_save is called without arguments, then all currently loaded virtual files are saved to their respective actual files at that point.

vf_save is automatically called in an END block at the termination of any program using the module.

vf_marker $virtual_filename
This subroutine returns the SOVFM that preceded the nominated virtual file.

The module also modifies the open, close, print, printf, read, getline, getc, seek, tell, and truncate built-in functions so that they operate correctly on virtual files.

As a special case, it is also possible to use the raw SOVFM as a virtual file name:

    use Inline::Files::Virtual;
    vf_load $filename, qr/__[A-Z]+__/;
    open FILE, "__MARKER__";
    # and in the file that was vf_load-ed
    file contents here

However, this always opens the very first virtual file with that SOVFM, no matter how often it is called, or how many such markers appear in the file.

Handling ``implicit'' virtual start-of-virtual-file markers

Sometimes an SOVFM is ``implicit''. That is, rather thanb being a separate marker for the start of a virtual file, it is the first part of the actual data of the virtual file. For example, consider the following XML file:

                <DESC>This is data set 1</DESC>
                <DATUM/>datum 1
                <DATUM/>datum 2
                <DATUM/>datum 3
                <DESC>This is data set 2</DESC>
                <DATUM/>datum 4
                <DATUM/>datum 5
                <DATUM/>datum 6

Each of the C<E<lt>DATAE<gt>...E<lt>/DATAE<gt>> blocks could be treated
as a separate virtual file by specifying:
        @datasets = vf_load("data.xml", '<DATA>');

But this would cause the individual virtual files to contain invalid
XML, such as:
                <DESC>This is data set 1</DESC>
                <DATUM/>datum 1
                <DATUM/>datum 2
                <DATUM/>datum 3

One can indicate that the nominated SOVFMs are also part of the virtual files' contents, by specifying the markers as a look-ahead pattern:

        @datasets = vf_load("data.xml", '(?=<DATA>)');

This causes C<vf_load> to identify the sequence C<E<lt>DATAE<gt>> as a
start-of-virtual-file marker but not consume it, thereby leaving it as
the initial sequence of the virtual file's content.


Could not vf_load '%s'
The module could not open the specified disk file and read it in as a set of virtual files.

Unable to complete vf_save
The module could not open the specified disk file and write it out as a set of virtual files. A preceding warning may indicate which virtual file caused the problem.

Virtual file not open for input
An attempt was made to getline, getc, or read a virtual file that was opened for output only. (Warning only)

Virtual file not open for output
An attempt was made to print or printf a virtual file that was opened for input only. (Warning only)


Damian Conway (


Brian Ingerson (


Copyright (c) 2001. Damian Conway. All rights reserved.

This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.


 Inline::Files::Virtual - Multiple virtual files in a single file