无法理解命名信号量初始化

Question

I need to share a ressource between threads using named semaphores. I have a main thread, and X subthreads.

Do I have to use sem_open with the O_CREAT flag in my main thread, and then call sem_open again in every subthreads, with the same name but without flags?

Or do I only have to call sem_open once in my main thread and then use it's return value in all subthreads?

I tried both but I didn't get anything close to expected behaviour...

Answer 1

You must use wait and signal functions in your threads after open your semaphore:

The sem_open(3) function creates a new named semaphore or opens an existing named semaphore. After the semaphore has been opened, it can be operated on using sem_post(3) and sem_wait(3). When a process has finished using the semaphore, it can use sem_close(3) to close the semaphore. When all processes have finished using the semaphore, it can be removed from the system using sem_unlink(3).

Reference : sem_overview(7) — Linux manual page