sem_release
Description
bool sem_release ( int 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().
注: This function does not work on Windows systems.