1IO::InSitu(3)         User Contributed Perl Documentation        IO::InSitu(3)
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NAME

6       IO::InSitu - Avoid clobbering files opened for both input and output
7

VERSION

9       This document describes IO::InSitu version 0.0.1
10

SYNOPSIS

12           use IO::InSitu;
13
14           my ($in, $out) = open_rw($infile_name, $outfile_name);
15
16           for my $line (<$in>) {
17               $line =~ s/foo/bar/g;
18               print {$out} $line;
19           }
20

DESCRIPTION

22       When users want to do in-situ processing on a file, they often specify
23       it as both the input and output file:
24
25           > myapp -i sample_data -o sample_data -op=normalize
26
27       But, if the "-i" and "-o" flags are processed independently, the
28       program will usually open the file for input, open it again for output
29       (at which point the file will be truncated to zero length), and then
30       attempt to read in the first line of the now-empty file:
31
32           # Open both filehandles...
33           use Fatal qw( open );
34           open my $src,  '<', $source_file;
35           open my $dest, '>', $destination_file;
36
37           # Read, process, and output data, line-by-line...
38           while (my $line = <$src>) {
39               print {$dest} transform($line);
40           }
41
42       Not only does this not perform the requested transformation on the
43       file, it also destroys the original data. Fortunately, this problem is
44       extremely easy to avoid: just make sure that you unlink the output file
45       before you open it:
46
47           # Open both filehandles...
48           use Fatal qw( open );
49           open my $src,  '<', $source_file;
50           unlink $destination_file;
51           open my $dest, '>', $destination_file;
52
53           # Read, process, and output data, line-by-line...
54           while (my $line = <$src>) {
55               print {$dest} transform($line);
56           }
57
58       If the input and output files are different, unlinking the output file
59       merely removes a file that was about to be rewritten anyway. Then the
60       second open simply recreates the output file, ready for writing.
61
62       If the two filenames actually refer to a single in-situ file, unlinking
63       the output filename removes that filename from its directory, but
64       doesn't remove the file itself from the filesystem. The file is already
65       open through the filehandle in $input, so the filesystem will preserve
66       the unlinked file until that input filehandle is closed. The second
67       open then creates a new version of the in-situ file, ready for writing.
68
69       The only limitation of this technique is that it changes the inode of
70       any in-situ file . That can be a problem if the file has any hard-
71       linked aliases, or if other applications are identifying the file by
72       its inode number. If either of those situations is possible, you can
73       preserve the in-situ file's inode by using the "open_rw()" subroutine
74       that is exported from this module:
75
76           # Open both filehandles...
77           use IO::InSitu;
78           my ($src, $dest) = open_rw($source_file, $destination_file);
79
80           # Read, process, and output data, line-by-line...
81           while (my $line = <$src>) {
82               print {$dest} transform($line);
83           }
84

INTERFACE

86       "($in_fh, $out_fh) = open_rw($infile_name, $outfile_name, \%options)"
87       "($in_fh, $out_fh) = open_rw($in_out_file_name, \%options)"
88           The "open_rw()" subroutine takes the names of two files: one to be
89           opened for reading, the other for writing. If you only give it a
90           single filename, it opens that file for both reading and writing.
91
92           It returns a list of two filehandles, opened to those two files.
93           However, if the filename(s) refer to the same file, "open_rw()"
94           first makes a temporary copy of the file, which it opens for input.
95           It then opens the original file for output. In such cases, when the
96           input filehandle is eventually closed, IO::InSitu arranges for the
97           temporary file to be automatically deleted.
98
99           This approach preserves the original file's inode, but at the cost
100           of making a temporary copy of the file. The name of the temporary
101           is usually formed by appending '.bak' to the original filename, but
102           this can be altered, by passing the 'tmp' option to "open_rw()":
103
104               my ($src, $dest) = open_rw($source, $destination, {tmp=>\&temp_namer});
105
106           The 'tmp' option expects to be passed a reference to a subroutine.
107           That subroutine will be called and passed the name of the input
108           file. It is expected to return the name of the back-up file. For
109           example:
110
111               sub tmp_namer {
112                   my ($input_file) = @_;
113
114                   return "$input_file.orig";
115               }
116

DIAGNOSTICS

118       "Usage: open_rw( $in_filename, $out_filename, \%options )"
119           You called "open_rw()" with less than two arguments, or more than
120           three.  Pass the function two filenames, and (optionally) a
121           reference to any options.
122
123       "Can't open non-existent input file '%s'"
124           You specified an input file that does not exist on the filesystem.
125           Usually caused by a misspelling, or an incorrect file path.
126
127       "Can't open input file '%s'"
128           You specified an input file that exists on the filesystem but could
129           not be opened for reading.  Usually caused by insufficient file
130           permissions.
131
132       "Can't open copy of input file '%s'"
133           The module was unable to create (or access) its back-up copy of the
134           input file. This is usually caused by insufficient file permissions
135           on the directory where the back-up is supposed to be written.
136
137       "Can't open output file '%s'"
138           You specified an output file that could not be opened for writing.
139           This usually means the directory into which the output file was to
140           be placed is non-existent or you don't have write permission to the
141           directory or the output file.
142

CONFIGURATION AND ENVIRONMENT

144       IO::InSitu requires no configuration files or environment variables.
145

DEPENDENCIES

147       None.
148

INCOMPATIBILITIES

150       None reported.
151

BUGS AND LIMITATIONS

153       No bugs have been reported.
154
155       Please report any bugs or feature requests to
156       "bug-io-insitu@rt.cpan.org", or through the web interface at
157       <http://rt.cpan.org>.
158

AUTHOR

160       Damian Conway  "<DCONWAY@cpan.org>"
161
163       Copyright (c) 2005, Damian Conway "<DCONWAY@cpan.org>". All rights
164       reserved.
165
166       This module is free software; you can redistribute it and/or modify it
167       under the same terms as Perl itself.
168

DISCLAIMER OF WARRANTY

170       BECAUSE THIS SOFTWARE IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
171       FOR THE SOFTWARE, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT
172       WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER
173       PARTIES PROVIDE THE SOFTWARE "AS IS" WITHOUT WARRANTY OF ANY KIND,
174       EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
175       WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
176       ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE SOFTWARE IS WITH
177       YOU. SHOULD THE SOFTWARE PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL
178       NECESSARY SERVICING, REPAIR, OR CORRECTION.
179
180       IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
181       WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
182       REDISTRIBUTE THE SOFTWARE AS PERMITTED BY THE ABOVE LICENCE, BE LIABLE
183       TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL, OR
184       CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE
185       SOFTWARE (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING
186       RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A
187       FAILURE OF THE SOFTWARE TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF
188       SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
189       DAMAGES.
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193perl v5.34.0                      2021-07-22                     IO::InSitu(3)
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