C - Executing bash command with 2 pipes - c

Question

I have an assignment for which I have to create C programm which has to do the following: The mother proccess has to send via pipe to the child proccess the result of the command ps hax -o user | sort | uniq -c. The child proccess has to take this result and write it to a file users.log.
This is the code I have written:
int main(int argc, char **argv)
{
int file_number;
pid_t pid, pid_2, pid_3, pid_4;
int n, pd[2],fd[2], zd[2];
char line[MAXLINE];
if ( pipe(pd) < 0 || pipe(fd) < 0 || pipe(zd) < 0)
{
perror("cannot create pipe."); // failure on create pipe
exit(1);
}
if ( (pid = fork()) < 0)
{
perror("cannot fork"); // failure on creating child proccess
exit(1);
}
else if ( pid > 0) // mother proccess
{
dup2(zd[1], STDOUT_FILENO);
close(zd[0]);
close(zd[1]);
if ( (pid_4 = fork()) < 0)
{
perror("cannot fork.");
exit(1);
}
else if (pid_4 > 0)
{
dup2(pd[1], STDOUT_FILENO);
close(pd[0]);
close(pd[1]);
close(fd[0]);
close(fd[1]);
execl("/bin/ps", "ps", "hax", "-o", "user", (char *)0);
}
if ( (pid_2 = fork()) < 0)
{
perror("cannot fork.");
exit(1);
}
else if (pid_2 == 0)
{
dup2(pd[0], 0);
dup2(fd[1], 1);
close(pd[0]);
close(pd[1]);
close(fd[0]);
close(fd[1]);
execl("/bin/sort", "sort", "", (char *)0);
}
if ( (pid_3 = fork()) < 0)
{
perror("cannot fork.");
exit(1);
}
else if (pid_3 == 0)
{
dup2(fd[0], 0);
close(pd[0]);
close(pd[1]);
close(fd[0]);
close(fd[1]);
execl("/bin/uniq", "uniq", "-c", (char *)0);
}
close(pd[0]);
close(pd[1]);
close(fd[0]);
close(fd[1]);
pid_t terminated;
int stat;
while ((terminated = wait(&stat)) > 0)
{
printf("Child %d terminated.\n", terminated);
}
}
else // child proccess
{
FILE *fptr = fopen("users.log", "a+");
if (fptr == NULL)
fprintf(stderr, "could not open file.\n");
file_number = fileno(fptr); // getting the file descriptor of fptr
dup2(zd[0], STDIN_FILENO);
close(zd[1]);
close(zd[0]);
n = read(zd[0], line, MAXLINE);
printf("Output: (%.*s)\n", n, line);
write(file_number, line, n);
}
return 0;
}
Running the code seems not to produce the desired result. Nothing is written to the file and I get this error:
Signal 17 (CHLD) caught by hax (procps-ng version 3.3.10).
hax:display.c:66: please report this bug
What am I missing here?