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.