file lock and update

[CColin]

Hi, I have a program which modifies a file in place using a temporary file

open(OLD, "< $old")         or die "can't open $old: $!";
open(NEW, "> $new")         or die "can't open $new: $!";
my @array = <OLD>;
my $required_element_details = shift(@array);

foreach my $remaining_details (@array) {
print NEW "$remaining_details";
}

close(OLD)   
close(NEW)                 

rename($new, $old)

Problem is this file can be accessed at the same time by the same program so the reading and writing process needs to be locked. Have tried:

open(OLD, "< $old")         or die "can't open $old: $!";
open(NEW, "> $new")         or die "can't open $new: $!";
flock (OLD, 2);
flock (NEW, 2);

[continue as before including renaming the file before the filehandles
+ are closed]

But this doesn't work when both programs are trying at the same time, instead both programs read the same initial element (and write the same results). Any suggestions? Colin

[McDarren]

Hi Colin,

You probably want to use sysopen rather than open. The canonical way to do this (on a single output file) is something along the following lines:

use Fcntl qw(:DEFAULT :flock);

sysopen(OUT, $outfile, O_WRONLY | O_CREAT)
   or die "Cannot open $outfile for writing:$!\n";
flock(OUT, LOCK_EX)
   or die "Cannot get a lock on $outfile:$!\n";
truncate(OUT, 0)
   or die "Cannot truncate $outfile:$!\n";

There are a couple of very good discussions regarding file locking in the Camel (3rd Ed.). If you happen to have a copy, look in Chapters 16.2.1 and 23.2.2.

Otherwise, you can refer to the docs for sysopen and flock.
perldoc -q lock also gives some useful examples.

Hope this helps,
Darren :)

[mickeyn]

Hey,

check NL::File::Lock

HTH,
Mickey

[roboticus]

CColin:

A minor nit in terminology: Your program doesn't modify the file in place. "Modify in place" means to open the file in read/write mode and alter the file without creating a new copy. (Not terribly safe for many applications, though!) Here's an example of modifying a file in place:

$ cat 659032.pl
#!/usr/bin/perl -w
use warnings;
use strict;

# Open in Read/Write mode and modify it in place
open FH, "+<", "a_file.txt" or die "Can't open file!";
print FH "Foo bar baz!";
close FH;

$ cat a_file.txt
Now is the time for all good men to
come to the aid of their party.

$ perl 659032.pl

$ cat a_file.txt
Foo bar baz!ime for all good men to
come to the aid of their party.

$

...roboticus