Redirecting C Program output to perl?

BrandonBlock

Hi guys,

Bit of a problem here - I have a Unix C Program (which I didn't write) and it constantly outputs info every few seconds while it is running (using printf , then fflush (stdout)).

Now I am writing a perl program that needs to capture this output in real time. But usings system(); and exec(); to call the C program, they both need to wait until the process is completed before returning to perl.

Is there any way around this? Is it possible to capture data from a running process in perl or do you need to wait until it is complete?

Thanks in advance for any help, would be very much appreciated.

Webmonkey

if its on linux can you not pipe/redirect the output into the perl program?

You could also write to file and read in perl

BrandonBlock

Well , the "C Program" is actually called from within perl using a button, and may not have to be executed at all so I can't redirect the output into perl at runtime anyway. I'll give the file thing a try though but I'm not sure if bash redirects to files in "realtime" so to speak. The C Program is running like this example: (in minutes and seconds)

0:00 - Perl prog begins executing
0:35 - Perl prog calls C prog (i.e. user presses button)
0:36 - C prog outputs something
(0:36) - (Perl prog needs that output instantly)
0:40 - C prog outputs something else
(0:40) - (Again, perl needs this instantly, and so on)
0:41 - C prog outputs something else
0:53 - C prog outputs something else
0:54 - Perl prog kills C prog

Webmonkey

Ah i see - am I guess you could write to file but I guess you could use threads to overcome the problem of having to wait for the C program to finish?

I amn't too familiar with perl though

IRLConor

Here's an example, using ping in place of your C program.

#!/usr/bin/perl

use strict;
use warnings;

if (my $ping_pid = open(PING, "ping 127.0.0.1 |")) {
    while (my $ping_output = <PING>) {
        # Do something with the output, let's say print
        print $ping_output;

        # Kill the C program based on some arbitrary condition (in this case
        # the output of the program itself).
        if ($ping_output =~ /icmp_seq=69 /o) {
            kill 9, $ping_pid;
        }
    }
}

carveone

BrandonBlock wrote: »

Well , the "C Program" is actually called from within perl using a button, and may not have to be executed at all so I

If you get to execute the C program yourself from your own perl program, then IRLConor's example is the one to use. The program run with 'open' runs immediately in parallel with the perl program. You get to read its output whenever you feel like it.

If you don't have control (it's not your perl program), then you'll have to play games with either named pipes or doing a fork, exec:

Eg:
open FOO, "cat -n file|";
or
open FOO, "-|" or exec 'cat', '-n', 'file';

In the second instance, reading from FOO gets the STDOUT of the execed cat.

Fun eh!

Conor.

BrandonBlock

Cheers lads, IRLConor's method is working to an extent, but the guy who wrote the C added a fflush(stdout) after every printf so that only the last line is ever displayed on the stdout (bash shell in this case) which is causing problems with the perl program. I might have to try recompiling his code and remove that line. Thanks all the same

carveone

BrandonBlock wrote: »

Cheers lads, IRLConor's method is working to an extent, but the guy who wrote the C added a fflush(stdout) after every printf so that only the last line is ever displayed on the stdout (bash shell in this case) which is causing problems with the perl program. I might have to try recompiling his code and remove that line. Thanks all the same

Can't imagine that would make any difference! As a test you might try redirecting the C program into a file and having a look at the file to see what's coming out. I've seen programs decide to print with \r instead of \n, leading to each new line erasing the previous line on the console.

C.

BrandonBlock

carveone wrote: »

Can't imagine that would make any difference! As a test you might try redirecting the C program into a file and having a look at the file to see what's coming out. I've seen programs decide to print with \r instead of \n, leading to each new line erasing the previous line on the console.

C.

Ahh brilliant!! That was it , thanks so much! I recompiled yer mans program with \n in place of \r and it's working now. Thanks to everyone else who helped aswell.