Initialise semaphores using sem_open - c

Question

In my producer and consumer problem, I use sem_open() to initialize the semaphores. How could I test if sem_open() works in a correct way?
The program can be compiled, but when I run the program it does not print anything. I test the program and found the problem maybe about sem_open(). I find if I comment the sem_open() in the program, the program will run correctly.
#include <stdio.h>
#include <pthread.h>
#include <semaphore.h>
#include <stdlib.h>
#include <unistd.h>
#define BUFFER_SIZE 10
int buffer[BUFFER_SIZE];
pthread_t pro_thread, con_thread;
pthread_mutex_t mutex;
int counter = 0;
sem_t *empty, *full;
void print_buffer(int counter) {
for (int i = 0; i < counter; i ++)
{
printf("*");
}
printf("\n");
}
void* producer(void* var) {
int item;
while(1) {
item = rand() % 100 + 1;
sem_wait(empty);
pthread_mutex_lock(&mutex);
while (counter == BUFFER_SIZE)
; // waiting
if(counter < BUFFER_SIZE) {
buffer[counter] = item;
counter ++;
printf("Producer: ");
print_buffer(counter);
}
sleep(1);
pthread_mutex_unlock(&mutex);
sem_post(full);
}
}
void* consumer(void* var) {
int item;
while(1) {
sem_wait(full);
pthread_mutex_lock(&mutex);
while (counter == 0)
; // waiting
if(counter > 0) {
counter --;
print_buffer(counter);
}
sleep(1);
pthread_mutex_unlock(&mutex);
sem_post(empty);
}
}
int main(int argc, char *argv[]) {
pthread_mutex_init(&mutex, NULL);
empty = sem_open("/mysem", O_CREAT, 0644, BUFFER_SIZE);
full = sem_open("/mysem", O_CREAT, 0644, 0);
pthread_create(&pro_thread, NULL, producer, NULL);
pthread_create(&con_thread, NULL, consumer, NULL);
pthread_exit(NULL);
return 0;
}

Answer 1

As stated in the sem_open man page:
The semaphore is identified by name.
Since your code provides the same name value (/mysem) for both sem_open calls it results in referencing the same semaphore for both full and empty. That's clearly not what the logic of the program should be. Instead, open different semaphores for each. It's also best practice to check the return values of all function calls.
empty = sem_open("/empty_sem", O_CREAT, 0644, BUFFER_SIZE);
if (empty == SEM_FAILED) {
perror("Failed to open semphore for empty");
exit(-1);
}
full = sem_open("/full_sem", O_CREAT, 0644, 0);
if (full == SEM_FAILED) {
sem_close(empty);
perror("Failed to open semphore for full");
exit(-1);
}