sem_post does not prioritise sem_wait call in other thread - c

Question

I am working on my project where it is crucial that everything is cleaned up properly and all buffered log messages are saved to a file etc.. I am calling exit from another thread and I was thinking to use a semaphore to wait for the cleanup in main thread to occur before the program fully exits. The problem is that when I call sem_post from my exit handler registred with atexit, sem_wait will not immediately decrement the semaphore thus sem_wait in my exit handler will exit immediately because the semaphore is greater than zero.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <semaphore.h>
#include <pthread.h>
static sem_t sem;
void *do_work(void *arg) {
// if something fails
printf("Something failed!\n");
exit(1);
}
void exit_handler() {
sem_post(&sem); // wake up main thread
sem_wait(&sem); // wait for cleanup in main
sem_destroy(&sem);
}
int main() {
pthread_t worker_thread;
sem_init(&sem, 0, 0);
atexit(exit_handler);
pthread_create(&worker_thread, NULL, do_work, NULL);
sem_wait(&sem); // block this thread until work is done
// simulate some cleanup
usleep(1000000);
printf("This never gets called!\n");
sem_post(&sem); // destroy semaphore
}
This example demonstrates my problem. Is there any solution to this problem? I mustn't put the cleanup in my exit handler because I have a lot of local resources in the main function that needs to be cleaned up in my actual program.

Answer 1

You can't control which thread will return from sem_wait() after a call to sem_post(). You need to use two semaphores:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <semaphore.h>
#include <pthread.h>
static sem_t wakeupMain;
static sem_t cleanupDone;
void *do_work(void *arg) {
// if something fails
printf("Something failed!\n");
exit(1);
}
void exit_handler() {
sem_post(&wakeupMain); // wake up main thread
sem_wait(&cleanupDone); // wait for cleanup in main
sem_destroy(&wakeupMain);
sem_destroy(&cleanupDone);
}
int main() {
pthread_t worker_thread;
sem_init(&wakeupMain, 0, 0);
sem_init(&cleanupDone, 0, 0);
atexit(exit_handler);
pthread_create(&worker_thread, NULL, do_work, NULL);
sem_wait(&wakeupMain); // block this thread until work is done
// simulate some cleanup
usleep(1000000);
printf("This never gets called!\n");
sem_post(&cleanupDone); // destroy semaphore
}