PHP's sem_acquire() - at what order do the 'waiting' processes get executed

Question

So semaphores in PHP are very convenient to synchronize jobs/tasks (process, to be more precise).

Say 1 process gains access to a semaphore and starts doing its thing (=work). 3 other processes are firing sem_acquire() to get access as well, at some order. They all get blocked, waiting for the semaphore to be free and assigned to each.

Question is: at what order will the 3 individual processes get executed? Common logic says that they'll get executed in first come (and gets blocked), first served (release to work), but I didn't see this statement in the official documentation.

Can anyone confirm or correct me? Thanks!

Answer 1

The PHP sem_acquire() function calls through to the semop() system call, which does not appear to make any guarantees regarding what order waiting processes will acquire a semaphore in.

Answer 2

I wrote a script demonstrating, that YES, they are served on first-come first-serve basis. It was crucial for my app.

You can demonstrate it as well - create following thread.php and then call it using this shell script.

UPDATE: wikipedia for semaphores explicitly mentions an associated FIFO buffer to avoid "starvation".