Perl CGI. no output until backend script is done

Shell Programming and Scripting
1

It is a basic Perl CGI question, I want to print out "Processing ... " while backend script /script/wait.pl is still running.

But acctually, nothing appeared in browser untill /script/wait.pl finished.

 
print "Content-type:text/html\r\n\r\n";
print '<html>';
print '<head>';
print '</head>';
print '<body>';
print '<h2>Processing ... </h2>';
`/script/wait.pl`;
print '</body>';
print '</html>';
2

Perl buffers output on stdout by default untill the buffer is full, or a newline is found. You can force output either by adding ',"\n"' to the end of your print statements, or by disabling output buffering by adding this line near the top:

$| = 1;

But even then it's not guaranteed that the browser will start rendering the page before it received all the content.

3

Hi,
It should print..What is the contents of wait.pl? Do you see anything in the error_log of your webserver?

-Raja

4

$| doesn't work, To make it simple, I tested following script. I expect "OK, starting time consuming process " appeared first, then "All done" aftter 5 seconds. But they all appeared all together after 5 secs.

BTW: I use apache2 with mod_perl2

#!/usr/bin/perl -wT

use strict;
$| = 1;

print "Content-type: text/plain\n\n";

print "OK, starting time consuming process ... \n";


for ( my $loop = 1; $loop <= 5; $loop++ ) {
    print "Iteration: $loop\n";
    ## Perform some time consuming task here ##
    sleep 1;
}
print "All Done!\n";
5

I found two solutions for the issue.

Solution #1

#!/usr/bin/perl -w
use CGI qw(:standard);
$|=1;
my $r = shift;
print header('text/html');
print start_html;
$r->print ("Processing ... <br>");
foreach $i (1..2) {
    sleep 2;
   $r->print ("Item .. $i <br>");
}
$r->print ("DONE <br>");
print end_html;

Solution #2

 
#!/usr/bin/perl -w
use CGI qw(:standard);
my $r = shift;
print header('text/html');
print start_html;
print br ("Processing ... ");
 $r->rflush;
foreach $i (1..2) {
    sleep 2;
    print br  ("Item .. $i ");
    $r->rflush;
}
print br ("DONE");
print end_html;

For more information, please check my post
http://honglus.blogspot.com/2010/08/resolving-perl-cgi-buffering-issue.html