Are there any equivalents to the futex in Linux/Unix?

Question

I\'m looking for something could be used for polling (like select, kqueue, epoll i.e. not busy polling) in C/C++. In oth

Answer 1

semaphores are not mutexes, and would work with slightly less overhead (avoiding the mutex+condvar re-lock, for example)

Note that since any solution where a thread sleeps until woken will involve a kernel syscall, it still isn't cheap. Assuming x86_64 glibc and the FreeBSD libc are both reasonable implementations, the unavoidable cost seems to be:

1. user-mode synchronisation of the count (with a CAS or similar)
2. kernel management of the wait queue and thread sleep/wait

I assume the mutex + condvar overhead you're worried about is the cond_wait->re-lock->unlock sequence, which is indeed avoided here.

Answer 2

You want semaphores and not mutexes for the signaling between the to threads..

http://man7.org/linux/man-pages/man3/sem_wait.3.html

Semaphores can be used like a counter such as if you have a queue, you increment (post) to the semaphore every time you insert a message, and your receiver decrement (wait) on the semaphore for every message it takes out. If the counter reach zero the receiver will block until something is posted.

So a typical pattern is to combine a mutex and a semaphore like;

sender:
    mutex.lock
    insert message in shared queue
    mutex.unlock
    semaphore.post

receiver:
    semaphore.wait
    mutex.lock
    dequeue message from shared structure
    mutex.unlock