What Is Wrong With PHP’s Semaphore Extension
Lack of a true Semaphore
sem_release() releases the semaphore if it is currently acquired by the calling process, otherwise a warning is generated.
By far the worst, this limitation makes a whole class of problems much more difficult to solve, and in fact means that PHP’s Semaphore extension, despite the name, is really a mutex (a semaphore with ownership, i.e., only the acquirer may release). Furthermore, it is inconsistent with the behavior of equivalent functions in other languages such as C and Python.
Please see phm\Lock\Semaphore for a kludgy workaround using mutexes, shared memory, and a message queue.
Undefined error handling
What happens if you sem_get() using a key that is already in use by a different type of resource, say, a System V shared memory segment? This is an issue because ftok() (the normal way of generating IPC keys for this stuff) is collision-prone, given a large enough filesystem.
Undefined behavior of sem_get()
[T]he documentation does not stipulate what will happen if the max_acquire paramter is varied upon successive invocations of the sem_get method. [S]o setting it to 100, then to 1 on 2 successive calls will have an undefined behavior.
This is avoidable, but it would be nice if this were fixed.
Cannot disable semaphore auto-releasing
If I write a program with a sem_get and auto_release=0, in case of shutdown, an other program should not be available to take the semaphore.
A semaphore may be deleted when other processes are waiting to acquire it
sem_remove() should take the SYSVSEM_USAGE (see sysvsem.c) count into consideration when it is called. Only if this count is == 1 should the semaphore be removed. This will allow the last process that is using the semaphore to remove it from the system.
The mantra among the PHP community seems to be “less bitchin’, more fixin’”, which is admirable. I do not wish to criticize or complain, but rather to document the need and hopefully encourage someone who has the background in C development to fix this (alas, I do not have this).