I am trying to write a semaphore using signals. code in main the checks that it works, it suppose to print (something like):

Thread 2 trying to enter critical section…
Thread 2 entered critical section.
Thread 1 trying to enter critical section…
Thread 2 leaving critical section.
Thread 1 entered critical section.
Thread 1 leaving critical section.

where every thread enters and leaves.

But it prints:

Thread 2 trying to enter critical section…
Thread 2 entered critical section.
Thread 1 trying to enter critical section…
Thread 2 leaving critical section.

and doesn’t print that the other thread is also entering and leaving the critical section, also it doesn’t terminate. My guess is that I don’t understand how the sigsusppend() is working. Thank you in advance.

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>
#include <pthread.h>

// Global semaphore
volatile sig_atomic_t semaphore = 1;

// Signal handling function
void signal_handler(int sig) {
    semaphore = 1;  // Release the semaphore
}

// Initialize the semaphore
void sem_init() {
    struct sigaction sa;
    sa.sa_handler = signal_handler;
    sa.sa_flags = 0; // No flags
    sigemptyset(&sa.sa_mask);
    sigaction(SIGUSR1, &sa, NULL);

    // Block SIGUSR1 to prevent premature handling
    sigset_t set;
    sigemptyset(&set);
    sigaddset(&set, SIGUSR1);
    pthread_sigmask(SIG_BLOCK, &set, NULL);
}

// Wait (down operation) on the semaphore
void sem_down() {
    sigset_t set;
    sigemptyset(&set);
    sigaddset(&set, SIGUSR1);
    while (!semaphore) {
        sigsuspend(&set); // Replace pause with sigsuspend for targeted waiting
    }
    semaphore = 0; // Take the semaphore
}

// Signal (up operation) on the semaphore
void sem_up() {
    semaphore = 1;
    kill(getpid(), SIGUSR1); // Send signal to release semaphore
}

// A simple thread function that uses the semaphore
void* thread_func(void* arg) {
    printf("Thread %ld trying to enter critical section...n", (long)arg);
    sem_down();
    printf("Thread %ld entered critical section.n", (long)arg);
    sleep(2); // Simulate critical section work
    sem_up();
    printf("Thread %ld leaving critical section.n", (long)arg);
    return NULL;
}

// Main function to demonstrate semaphore usage
int main() {
    sem_init();

    pthread_t t1, t2;
    pthread_create(&t1, NULL, thread_func, (void*)1);
    pthread_create(&t2, NULL, thread_func, (void*)2);

    pthread_join(t1, NULL);
    pthread_join(t2, NULL);

    return 0;
}