File reade and recreate

adriang has asked for the wisdom of the Perl Monks concerning the following question:

Hi all,

I want to create a counter using a file but this don't work

#!/usr/bin/perl
use warnings;
use strict;
use Carp;

open(my $count_file,"+<","count.txt") or croak "cant open";
my $count=<$count_file>;
$count++;
print $count_file $count;
print $count;

__END__
[download]

only this work:

#!/usr/bin/perl
use warnings;
use strict;
use Carp;

open(my $count_file,"<","count.txt") or croak "cant open";
my $count=<$count_file>;
$count++;
open($count_file,">","count.txt") or croak "cant open";
print $count_file $count;
print $count;

__END__
[download]

I want to use OPEN only once.

Thanks in advace, Adrian.

Comment on File reade and recreate Select or Download Code

Replies are listed 'Best First'.
Re: File reade and recreate by GrandFather (Saint) on Aug 20, 2014 at 07:58 UTC
Add: `seek $count_file, 0, 0;` [download] before the `print $count_file $count;` in your first version of the code. Perl is the programming world's equivalent of English	[reply] [d/l] [select]
Re^2: File reade and recreate by adriang (Sexton) on Aug 20, 2014 at 10:18 UTC
Super, Thanks.	[reply]
Re: File reade and recreate by MidLifeXis (Monsignor) on Aug 20, 2014 at 13:42 UTC
You may also want to research how to lock the file from read through write. If multiple processes have the opportunity of simultaneous execution, you have a race condition where you can get anything from the correct behavior to missing one of the updates, to corrupting the data in the file. In this case I don't think you have the possibility of corruption (that scenario can happen more easily on files with larger datasets), but you may miss one of the updates. ProcA starts and reads the counter. ProcB gets control, reads the counter (same value), increments, and writes the counter. ProcA now resumes, increments the (now outdated) counter, and writes it. Your counter is now one less than it should be. --MidLifeXis	[reply]
Re^2: File reade and recreate by adriang (Sexton) on Aug 21, 2014 at 07:53 UTC
Thanks for the suggestion `open($file_read,"+<","$file") or croak "cant open $file for read"; flock($file_read,2); my $count=<$file_read>; $count++; seek($file_read, 0, "0"); print $file_read $count; flock($file_read,8); close($file_read);` [download]	[reply] [d/l]