Hi jeanluca, I think you just got it the wrong way round :)   You have to dup the output side of the pipe to file descriptor 63 (my_prog.pl needs to write to the output side...), and then simply read from the input side within your C program. Also, it's more common to swap the roles of parent and child, i.e. run the command in the child (via one of the exec functions), and read its output in the parent process. Here's a modified version of your program that should work (with my_prog.pl as you had it):

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/wait.h>

int main()
{
    const char *cmd = "./my_prog.pl";
    const int RESPONSE_FD = 63;
    int pfd[2], status;
    char buffer[100];
    int rv;

    status = pipe(pfd);
    if(status == -1)
    {
        printf("Could not open pipe\n");
        exit(1);
    }
    else
    {
        printf("The filedescriptors are: %d %d\n", pfd[0], pfd[1]);
    }

    status = fork();
    if(status == 0)
    {
        printf("This is the child\n");
        /* dup output/writing side of pipe */
        status = dup2(pfd[1], RESPONSE_FD);
        /* close unused input side */
        close(pfd[0]);
        if(status == -1)
        {
            printf("dup2 failed\n");
            exit(1);
        }
        execl(cmd, cmd, (char *)0);
    }
    else
    {
        printf("This is the parent\n");
        /* close unused output side of pipe */
        close(pfd[1]);
        /* read from input side */
        while(read(pfd[0], buffer, sizeof(buffer))>0)
        {
            printf("%s", buffer);
        }
        wait(&rv);
        fprintf(stderr,"%s exited with value %d\n", cmd, rv);
    }
    return 0;
}
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Output:

The filedescriptors are: 4 6
This is the child
This is the parent
this is a message
./my_prog.pl exited with value 0
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Update: minor fixes, so that it also does compile with -Wall :)

Yes!!!
Thanks a lot, I really really appreciate it!!
And it wasn't even a perl problem!!

LuCa

The context in which the utility is ultimately executed may differ from that in which system() was called. For example, file descriptors that have the FD_CLOEXEC flag set are closed, and the process ID and parent process ID are different. Also, if the executed utility changes its environment variables or its current working directory, that change is not reflected in the caller’s context.

Try these. They are super simple and don't take errors and broken-pipe conditions into account. Start the c program, then start the perl program. The perl program could be launched from c if you wanted.

#include <stdio.h>

int main(void)
{
FILE *fp;
/* char *line;  updated correction */
char line[256];   

if (mkfifo ("age-pipe", 0755) != 0)
{printf ("Could not make this fifo\n");}
else {printf ("FIFO was successfully made!\n");}

fp = fopen ("age-pipe", "r");

for(;;){
fgets (line,256, fp);
printf ("message: %s\n", line);
}
close (fp);
}
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#!/usr/bin/perl
use strict;
use warnings;

open(FIFO, "> age-pipe") or die $!;

select FIFO; $|=1;
select STDOUT;$|=1;

my $input = 'a';
while(1){
print "Enter your age:\n";
#my $input = <STDIN>;
print FIFO "$input\n";
sleep(1);
$input++
}

close(FIFO);
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---

I'm not really a human, but I play one on earth. Cogito ergo sum a bum

When I run your example I get

~/tmp/cperl> ./a.out
FIFO was successfully made!
Segmentation fault
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I get the segmentation fault (from the c program) when I start the perl script :(

Anyway, what is so different about this approach and simply using the number 63 ?

LuCa