IO::InSitu(3)

NAME
IO::InSitu - Avoid clobbering files opened for both input and output

VERSION
This document describes IO::InSitu version 0.0.1

SYNOPSIS
    use IO::InSitu;

    my ($in, $out) = open_rw($infile_name, $outfile_name);

    for my $line (<$in>) {
        $line =~ s/foo/bar/g;
        print {$out} $line;
    }

DESCRIPTION
When users want to do in-situ processing on a file, they often specify it as both the input and output file:

    > myapp -i sample_data -o sample_data -op=normalize

But, if the "-i" and "-o" flags are processed independently, the program will usually open the file for input, open it again for output (at which point the file will be truncated to zero length), and then attempt to read in the first line of the now-empty file:

    # Open both filehandles...
    use Fatal qw( open );
    open my $src,  '<', $source_file;
    open my $dest, '>', $destination_file;

    # Read, process, and output data, line-by-line...
    while (my $line = <$src>) {
        print {$dest} transform($line);
    }

Not only does this not perform the requested transformation on the file, it also destroys the original data. Fortunately, this problem is extremely easy to avoid: just make sure that you unlink the output file before you open it:

    # Open both filehandles...
    use Fatal qw( open );
    open my $src,  '<', $source_file;
    unlink $destination_file;
    open my $dest, '>', $destination_file;

    # Read, process, and output data, line-by-line...
    while (my $line = <$src>) {
        print {$dest} transform($line);
    }

If the input and output files are different, unlinking the output file merely removes a file that was about to be rewritten anyway. Then the second open simply recreates the output file, ready for writing.

If the two filenames actually refer to a single in-situ file, unlinking the output filename removes that filename from its directory, but doesn't remove the file itself from the filesystem. The file is already open through the filehandle in $input, so the filesystem will preserve the unlinked file until that input filehandle is closed. The second open then creates a new version of the in-situ file, ready for writing.

The only limitation of this technique is that it changes the inode of any in-situ file . That can be a problem if the file has any hard-linked aliases, or if other applications are identifying the file by its inode number. If either of those situations is possible, you can preserve the in-situ file's inode by using the open_rw() subroutine that is exported from this module:

    # Open both filehandles...
    use IO::InSitu;
    my ($src, $dest) = open_rw($source_file, $destination_file);

    # Read, process, and output data, line-by-line...
    while (my $line = <$src>) {
        print {$dest} transform($line);
    }

INTERFACE

"($in_fh, $out_fh) = open_rw($infile_name, $outfile_name, \%options)"
"($in_fh, $out_fh) = open_rw($in_out_file_name, \%options)"
    The open_rw() subroutine takes the names of two files: one to be opened for reading, the other for writing. If you only give it a single filename, it opens that file for both reading and writing.

    It returns a list of two filehandles, opened to those two files. However, if the filename(s) refer to the same file, open_rw() first makes a temporary copy of the file, which it opens for input. It then opens the original file for output. In such cases, when the input filehandle is eventually closed, IO::InSitu arranges for the temporary file to be automatically deleted.

    This approach preserves the original file's inode, but at the cost of making a temporary copy of the file. The name of the temporary is usually formed by appending '.bak' to the original filename, but this can be altered, by passing the 'tmp' option to open_rw():

        my ($src, $dest) = open_rw($source, $destination, {tmp=>\&temp_namer});

    The 'tmp' option expects to be passed a reference to a subroutine. That subroutine will be called and passed the name of the input file. It is expected to return the name of the back-up file. For example:

        sub tmp_namer {
            my ($input_file) = @_;

            return "$input_file.orig";
        }

DIAGNOSTICS

"Usage: open_rw( $in_filename, $out_filename, \%options )"
    You called open_rw() with less than two arguments, or more than three. Pass the function two filenames, and (optionally) a reference to any options.

"Can't open non-existent input file '%s'"
    You specified an input file that does not exist on the filesystem. Usually caused by a misspelling, or an incorrect file path.

"Can't open input file '%s'"
    You specified an input file that exists on the filesystem but could not be opened for reading. Usually caused by insufficient file permissions.

"Can't open copy of input file '%s'"
    The module was unable to create (or access) its back-up copy of the input file. This is usually caused by insufficient file permissions on the directory where the back-up is supposed to be written.

"Can't open output file '%s'"
    You specified an output file that could not be opened for writing. This usually means the directory into which the output file was to be placed is non-existent or you don't have write permission to the directory or the output file.

CONFIGURATION AND ENVIRONMENT
IO::InSitu requires no configuration files or environment variables.

DEPENDENCIES
None.

INCOMPATIBILITIES
None reported.

BUGS AND LIMITATIONS
No bugs have been reported.

Please report any bugs or feature requests to "bug-io-insitu@rt.cpan.org", or through the web interface at <http://rt.cpan.org>.

AUTHOR
Damian Conway "<DCONWAY@cpan.org>"

LICENCE AND COPYRIGHT
Copyright (c) 2005, Damian Conway "<DCONWAY@cpan.org>". All rights reserved.

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

DISCLAIMER OF WARRANTY
BECAUSE THIS SOFTWARE IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE SOFTWARE, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE SOFTWARE "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE SOFTWARE IS WITH YOU. SHOULD THE SOFTWARE PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR, OR CORRECTION.

IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE SOFTWARE AS PERMITTED BY THE ABOVE LICENCE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE SOFTWARE (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE SOFTWARE TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.