How can I get multiple calls to sem_open working in C?

Question

I'm experiencing a lot of difficulty getting Semaphores to work on a Linux based system in C.

The process of my application is such:

1. Application starts
2. Application forks into child/parent
3. Each process uses sem_open with a common name to open the semaphore.

If I create the semaphore before forking, it works fine. However, requirements prevent me from doing so. When I try to call sem_open for the second time, I get a "Permission Denied" error (via errno).

Is it possible to do this in any way? Or is there any way to open the semaphore in one process and used a shared memory mechanism to share it with the child process?

Answer 1

Are you using the 4 parameter or 2 parameter version of sem_open?

Make sure to use the 4 parameter version and use a mode that will allow other processes to open the semaphore. Assuming all the processes are owned by the same user, a mode of 0600 (S_IRUSR | S_IWUSR) will be sufficient.

You may also want to verify that you umask is not masking out any of the necessary permissions.

Answer 2

Don't forget to specify the mode and value parameter when using O_CREAT in the flags. Here is a working example :

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <semaphore.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/wait.h>

static void parent(void)
{
    sem_t * sem_id;
    sem_id=sem_open("mysem", O_CREAT, 0600, 0);
    if(sem_id == SEM_FAILED) {
        perror("parent sem_open");
        return;
    }
    printf("waiting for child\n");
    if(sem_wait(sem_id) < 0) {
        perror("sem_wait");
    }
}

static void child(void)
{
    sem_t * sem_id;
    sem_id=sem_open("mysem", O_CREAT, 0600, 0);
    if(sem_id == SEM_FAILED) {
        perror("child sem_open");
        return;
    }
    printf("Posting for parent\n");
    if(sem_post(sem_id) < 0) {
        perror("sem_post");
    }
}

int main(int argc, char *argv[])
{
    pid_t pid;
    pid=fork();
    if(pid < 0) {
        perror("fork");
        exit(EXIT_FAILURE);
    }

    if(!pid) {
        child();    
    } else {
        int status;
        parent();
        wait(&status);
    }
    return 0;
}