Programming in C using pipes to send info backwards - c

Question

I'm trying to figure out how to use pipes. I've managed to set up pipes to send information from C1 (child 1) to C2 (child 2) however, when I try to do the opposite e.g. sending an integer "10" from C2 to C1 it gets stuck after my first system("ps -o pid,ppid,pgid,sess,comm") call. In general I don't seem to have problem sending stuff forward however problem seems to arise when I try to do stuff backwards. Am I missing some logical steps here?
./in reads an integer and prints it and ./out takes an integer and writes it. They seems to be working perfectly okay.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <assert.h>
#define READ 0
#define WRITE 1
int main()
{
int fds[2];
pid_t pid;
int test;
if(pipe(fds)<0)
{
exit(0);
}
pid = fork();
if(pid==0)
{//C1
test = close(READ); assert(test==0);
test = dup(fds[READ]); assert(test==READ);
test = close(fds[READ]); assert(test==0);
test = close(fds[WRITE]); assert(test==0);
sleep(1);
execlp("./in", "./in", NULL);
exit(1);
}
system("ps -o pid,ppid,pgid,sess,comm");
wait(0);
system("ps -o pid,ppid,pgid,sess,comm");
sleep(2);
pid = fork();
if(pid==0)
{//C2
test = close(WRITE); assert(test==0);
test = dup(fds[WRITE]); assert(test==WRITE);
test = close(READ); assert(test==0);
test = close(fds[READ]); assert(test==0);
test = close(fds[WRITE]); assert(test==0);
sleep(2);
execlp("./out", "./out", "10", NULL);
exit(0);
}
sleep(1);
test = close(fds[READ]); assert(test==0);
test = close(fds[WRITE]); assert(test==0);
system("ps -o pid,ppid,pgid,sess,comm");
wait(0);
system("ps -o pid,ppid,pgid,sess,comm");
return 0;
}

Answer 1

The reason your process is getting stuck is the wait() system call after calling system(). system() actually calls wait() itself so your wait() is waiting for child 1 to finish but child 1 is itself probably blocked on IO.
For bidirectional communication, you really need two pipes, one for communication in each direction. With the second pipe you do pretty much exactly what you do at the moment except for child 1 you replace stdout and for child 2 you replace stdin.

Answer 2

the dup() function returns a file descriptor.
The child code is overlaying that file descriptor with the returned value from a call to close() Usually the returned value from close() is 0, which is NOT a file descriptor.
There is no communication possible via a pipe in the posted code as both end of the pipe are closed before calling execlp() for both child 1 and child 2.