If you want to implement this sort of semaphore access (which is usually what is needed if doing ipc) one thing you can do is create shared memory which is simply an int. Use this int between processes as the value of the semaphore. You can write wrapper functions which use this to allow you to do the desired readers-writers stuff or just use the shared memory plain. I wouldn't be too worried about race conditions in this case as the shared memory is only an int. If you are worried about race conditions, the wrapper functions for doing the semaphore stuff could actually use a semaphore to elimate any race conditions. Also, an added bonus of doing it this way is you can check the value of the int.
sem_release
Description
bool sem_release ( resource sem_identifier )sem_release() releases the semaphore if it is currently acquired by the calling process, otherwise a warning is generated.
如果成功则返回 TRUE，失败则返回 FALSE。
After releasing the semaphore, sem_acquire() may be called to re-acquire it.
See also sem_get() and sem_acquire().
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User Contributed Notes
danno at circumsolutions dot com
02-Apr-2001 05:44
02-Apr-2001 05:44
apua at communiplex dot com
09-Dec-1999 12:54
09-Dec-1999 12:54
The fact that sem_release only resease the semaphore if it is currently acquired by the calling process is, in fact, a misfeature since sometimes (eg in a readers-writers implementation) it is desirable to release a semaphore acquired by another process. In C you can do that.
I'm telling this because I was trying to write a chat app in php3 made of two programs: a listener and a speaker. The speaker releases a semaphore to inform that there is a new message in a shared memory var. And listeners wait in that semaphore before outputting the message.