/* gcc callfilter.c -o callfilter */
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>

int main (int argc, char *argv[]) {
   int writer[2]; /* From the parent's point of view */
   int reader[2]; /* From the parent's point of view */
   int pid;
   char *msg = "hullo";
   char buf[10];
   int nread;

   /* Get pipes on */
   if (pipe(reader)) {
      perror("reader pipe(): ");
      return 1;
   }
   if (pipe(writer)) {
      perror("writer pipe(): ");
      return 1;
   }

   pid = fork();
   if (pid < 0) {
      perror("fork(): ");
      return 1;
   }
   else if (pid == 0) { /* In the child */
      /* close unneeded endpoints */
      close(reader[0]);
      close(writer[1]);
      /* Duplicate handles on STDIN and STDOUT for child */
      dup2(writer[0], 0);
      dup2(reader[1], 1);
      /* call filter */
      execl("./filter.pl", "./filter.pl", NULL); /* patched, thanks to djp */
      perror("execl(): ");
      return 1;
   }
   else { /* In the parent */
      /* close unneeded endpoints */
      close(reader[1]);
      close(writer[0]);
      /* Send something to the filter */
      write(writer[1], msg, strlen(msg));
      close(writer[1]);
      /* Read response from the filter */
      printf("filter says:\n");
      while ((nread = read(reader[0], buf, 9)) > 0) {
         buf[nread] = '\x0';
         printf("%s", buf);
      }
      if (nread < 0) {
         perror("read(): ");
         return 1;
      }
      printf("\n");
      close(reader[0]);
   }

   return 0;
}

##</code><code>##

#!/usr/bin/perl
use strict;
use warnings;
local $/;
my $in = <STDIN>;
print {*STDOUT} scalar reverse $in;

##</code><code>##

poletti@PolettiX:~/sviluppo/perl/dup$ ./callfilter
filter says:
olluh