Parallel pipes not closing in C? - c

Question

I'm trying to pipe in C in parallel but for some reason it's not closing... it's just waiting.... Not sure if I'm describing this well cuz i'm new at this, but here's the code
... some code up here
if(child_pid == 0) {
if(p0 >=0 && p1 == -1) {
dup2(p0, STDIN_FILENO);
close(p0);
close(p1);
}
else if (p0 == -1 && p1 >= 0) {
dup2(p1, STDOUT_FILENO);
close(p0);
close(p1);
}
else if (p0 >= 0 && p1 >=0) {
dup2(p0, STDIN_FILENO);
dup2(p1, STDOUT_FILENO);
close(p0);
close(p1);
}
if (input)
iofunc(input, 0);
if (output)
iofunc(output, 1);
if (sub){
command_exec (SUBSHELL_COMMAND, sub, -1, -1, -1);
exit(0);
}
else {
execvp(w[0], w);
exit(0);
}
}
else {
if (waitpid (-1, &child_status, 0) != child_pid)
child_status =-1;
close(p0);
close(p1);
return WEXITSTATUS(child_status);
}
... some code down here
when p0 == -1 && p1 >= 0, the process exits/returns, but for some reason, when p0 >= 0 && p1 == -1, the process just hangs and the pipe doesn't close. I ran a command ls | cat using two parallel child processes with ls writing to p1 and the cat reading from p0. The output was correct, but the pipe didn't close and cat never exited. What is going on?
#William I tried the following in my parent else block, but it didn't work
close(p0);
close(p1);
if (waitpid (-1, &child_status, 0) != child_pid)
child_status =-1;
#William here is the code that calls a function that calls the above code
int i = 0;
int num = 2;
int status;
pid_t pid;
pid_t pids[num];
int fd[2];
int p = pipe(fd);
int a;
int b;
//printf("pipe0:%d\n", fd[0]);
//printf("pipe1:%d\n", fd[1]);
for (i = 0; i < num; i++) {
if ((pids[i] = fork()) < 0) {
error(1,0, "child not forked");
} else if (pids[i] == 0) {
if (i == 0){
b = command_exec (c->type,
c->u.command[1],
0, f[0], p1);
}
else if (i == 1){
a = command_exec (c->type,
c->u.command[0],
1, p0, fd[1]);
}
exit(a && b);
}
}
while (num > 0) {
pid = wait(&status);
printf("Child with PID %ld exited with status 0x%x.\n", (long)pid, status);
num--;
}
The printf only prints the child process that finishes ls but not cat

Answer 1

Most likely, the parent is holding open the other side of the pipe. The ls has an open file descriptor on the write side of the pipe into cat, but so does the parent. cat will not exit until all the write side file descriptors are closed. Probably, you just need to reverse your code and close the pipes before you call waitpid instead of after.