PERL CGI program

Question

I was trying out an elementary Perl/CGI script to keep track of visitors coming to a web page. The Perl code looks like this:

#!/usr/bin/perl
#KEEPING COUNT OF VISITORS IN A FILE
use CGI':standard';
print "content-type:text/html\n\n";
#opening file in read mode
open (FILE,"<count.dat");
$cnt= <FILE>;
close(FILE);
$cnt=$cnt+1;
#opening file to write
open(FILE,">count.dat");
print FILE $cnt;
close(FILE);
print "Visitor count: $cnt";

The problem is that the web page does not increment the count of visitors on each refresh. The count remains at the initital value of $cnt , ie 1. Any ideas where the problem lies?

Answer 1

You never test if the attempt to open the file handle works. Given a file which I had permission to read from and write to that contained a single number and nothing else, the code behaved as intended. If the file did not exist then the count would always be 1 , if it was read-only then it would remain at whatever the file started at.

More general advice:

• use strict; and use warnings; (and correct code based on their complaints)
• Use the three argument call to open as per the first example in the documentation
• When you open a file always || handle_the_error_in($!); || handle_the_error_in($!);
• Don't use a file to store data like this, you have potential race conditions.
• Get the name of the language correct

Answer 2

Here's an alternate solution that uses only one open() and creates the file if it doesn't already exist. Locking eliminates a potential race condition among multiple up-daters.

#!/usr/bin/env perl
use strict;
use warnings;
use Fcntl qw(:DEFAULT :flock);
my $file = 'mycount';
sysopen(my $fh, $file, O_RDWR|O_CREAT) or die "Can't open '$file' $!\n";
flock($fh, LOCK_EX) or die "Can't lock $file: $!\n";
my $cnt = <$fh>;
$cnt=0 unless $cnt;
$cnt++;
seek $fh, 0, 0;
print ${fh} $cnt;
close $fh or die "Can't close $file: $\n";
print "Visitor count: $cnt\n";

Answer 3

The most obvious thing that you would have forgotten is to change permissions of the file count.dat Do this :

sudo chmod 777 count.dat

That should do the trick

Answer 4

A few potential reasons:

• 'count.dat' is not being open ed for reading. Always test with or die $!; at minimum to check if the file opened or not

• The code is not being executed and you think it is

Answer 5

You will need to close the webpage and reopen it again. Just refreshing the page won't increment the count.