Re: How a web server sending data to a CGI perl script ?

[Anonymous Monk]

So, I don't know much about CGI. Your questions look pretty general to me, though. It seems you're asking about what incantations to use to put stuff in environment and redirect standard streams. Here is an example. I assume it's close enough to how CGI works. Note that there is no error handling whatsoever (I leave it as an exercise for the reader). Also note that it assumes Unix-like operating system.

# server.pl
use strict;
use warnings;

use POSIX ();

my $cgis = 0;

$SIG{CHLD} = sub {
    while ( waitpid( -1, POSIX::WNOHANG ) > 0 ) {
        $cgis -= 1;
    }
};

while (1) {
    print '(Ctrl-D to quit)> ';
    my $request = <STDIN>;
    last if not defined $request;
    chomp $request;

    print "Server: about to fork a CGI client\n";

    $ENV{foo} = 'bar';     # use some real
    $ENV{baz} = 'quux';    # CGI variables

    pipe my ( $child_stdin,       $input_to_child, );
    pipe my ( $output_from_child, $child_stdout, );

    my $pid = fork;

    if ( $pid == 0 ) {
        close $input_to_child;
        close $output_from_child;

        open STDIN,  '<&', $child_stdin;
        open STDOUT, '>&', $child_stdout;
        exec {'perl'} 'perl', 'client.pl';
    }
    else {
        close $child_stdin;
        close $child_stdout;

        print "Server: forked process $pid\n";

        $cgis += 1;

        print $input_to_child $request;
        close $input_to_child;
        print
          "Server: reading child response\n",
          <$output_from_child>;
    }
}

print "Server exiting, remaining children: $cgis\n";

and client

# client.pl

use strict;
use warnings;

my $input = <STDIN>;

print <<END

This is a pseudo-CGI client reporting.
Process number $$
Got input "$input"
\$ENV{foo} is "$ENV{foo}"
\$ENV{baz} is "$ENV{baz}"
End of report

END

[Anonymous Monk]

come to think of it, Perl's open is not entirely appropriate for redirecting. It's better to use low-level dup2, otherwise there are some bad interactions when standard streams are already closed (which must be the case for a server).

    if ( $pid == 0 ) {
        close $input_to_child;
        close $output_from_child;

        if ( fileno $child_stdout != POSIX::STDOUT_FILENO ) {
            POSIX::dup2( fileno $child_stdout, POSIX::STDOUT_FILENO );
            close $child_stdout;
        }
        if ( fileno $child_stdin != POSIX::STDIN_FILENO ) {
            POSIX::dup2( fileno $child_stdin, POSIX::STDIN_FILENO );
            close $child_stdin;
        }
        exec {'perl'} 'perl', 'client.pl';
    }

[Anonymous Monk]

which must be the case for a server

come to think of it more, that shouldn't be the case... I must be tired today :) Anyway, I changed the code mainly because I always thought Perl's open used dup2, but then I checked the doc and it actually says: dup(2). So it's probably better to use POSIX::dup2 directly.

[exilepanda]

It seems you're asking about what incantations to use to put stuff in environment and redirect standard streams

Yes exactly it is!

Though your script won't work for me (my fault didn't mention I am on a Win7 box).  <$output_from_child>; will never return. But at least give me a hint to start from a pipe() fork() pairs... But that's okay to leave this homework for me. ( wondering if anything to do about the $SIG{CHLD}, anyway.. )

Again, thank you very much!=)