linux - exec'd program not terminating - c

Question

I am writing program in C on Linux which has to fork 2 children.
First child will send two random numbers over pipe to the second child. It will listen for SIGUSR1 signal and will then terminate.
The second child will duplicate(dup2) pipe input as STDIN and file fp as STDOUT. It will then execl program which will print out some data according to its input and end.
My problem is, that the execl'd program will never terminate and I don't know why. Any help or tips will be appreciated.
main.c (parent):
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/wait.h>
#include <sys/types.h>
const int BUFFER_SIZE = 30;
int pipefd[2] = {0,0};
int parent_pid = 0;
int first_pid = 0;
int second_pid = 0;
int sleep_time = 5;
int debug = 0;
FILE *fp;
void parent_func() {
int wstatus = 0;
sleep(sleep_time);
kill(first_pid, SIGUSR1);
wait(&wstatus);
waitpid(second_pid, &wstatus, 0);
}
static void sigusr1_handler(int sig) {
if (sig == SIGUSR1) {
fputs("TERMINATED", stderr);
close(pipefd[1]);
exit(0);
}
}
void first_func() {
struct sigaction act;
char buffer[BUFFER_SIZE];
close(pipefd[0]);
memset(&act, '\0', sizeof(act)); // clear the sigaction struct
act.sa_handler = &sigusr1_handler; // sets function to run on signal
if (sigaction(SIGUSR1, &act, NULL) < 0) { // assign sigaction
fputs("cannot assign sigaction - exiting...", stderr);
exit(1);
}
while (1) {
sprintf(buffer, "%d %d\n", rand(), rand());
write(pipefd[1], buffer, strlen(buffer));
puts(buffer);
sleep(1);
}
}
void second_func() {
close(pipefd[1]);
fp = fopen("out.txt", "w");
char buf[30];
dup2(pipefd[0], STDIN_FILENO);
close(pipefd[0]);
//dup2(fileno(fp), STDOUT_FILENO);
execl("./test", "", NULL);
perror("Error");
}
int main(int argc, char *argv[]) {
int fork_val = 0;
parent_pid = getpid();
if (pipe(pipefd)) {
fputs("cannot create pipe - exiting...", stderr);
return 1;
}
if (debug) {
sleep_time *= 10;
}
if ((fork_val = fork()) == -1) {
fputs("cannot fork process - exiting...", stderr);
return 1;
} else if (fork_val == 0) {
first_func();
} else {
first_pid = fork_val;
if ((fork_val = fork()) == -1) {
fputs("cannot fork process - exiting...", stderr);
return 1;
} else if (fork_val == 0) {
second_func();
} else {
second_pid = fork_val;
parent_func();
}
}
fclose(fp);
exit(0);
}
test.c (the execl'd file):
#include "nd.h"
#include "nsd.h"
#include <stdio.h>
#include <stdlib.h>
int main() {
int num1 = 0;
int num2 = 0;
char buffer[100];
while (fgets(buffer, 100, stdin) != NULL) {
if (sscanf(buffer, "%d %d", &num1, &num2) == 2) {
(num1 < 0) ? num1 = (num1 * -1) : num1;
(num2 < 0) ? num2 = (num2 * -1) : num2;
if (num1 == 1 || num2 == 1) {
puts("1");
} else if (num1 == num2) {
if (nd(num1) == 1) {
puts("prime");
} else {
printf("%d\n", num1);
}
} else if (nd(num1) == 1 && nd(num2) == 1) {
puts("prime");
} else {
printf("%d\n", nsd(num1, num2));
}
} else {
fputs("error\n", stderr);
}
}
fputs("DONE", stderr);
exit(0);
}

Answer 1

To be able to detect an end of file from a pipe you need to read from a empty pipe with no writer (no process with an open for writing descriptor).
As your writer (first_func()) never closes its descriptor and always writes something in a never ending loop the reader will either wait for some data or read some data.
Be also careful about closing non useful descriptors, if not you may encounter some problems with pipes, such has a single process that is a reader and a writer, so being unable to detect the end of file...