The difference between the semaphore and the condition variable

Notice the difference between the semaphore and the condition variable the semaphore content is visible: http://www.cnblogs.com/charlesblc/p/6142868.html semaphore, shared memory, and Message Queuing the system V IPC Three Musketeers focus oninter-process communication, while conditional variables, mutexes, mainly focus oninter-thread communication。 The following is a reference: http://blog.chinaunix.net/uid-27164517-id-3282242.html pthread_cond_wait refers tocondition Variable, Sum aMutual exclusion lock combined with。 Inget the lock first before calling Pthread_cond_wait。 pthread_cond_wait function is executed firstAutomatic ReleaseSpecifies the lock, and then waits for the condition variable to change. The specified mutex is automatically set before the function call returnsRe-lock。

int pthread_cond_signal (pthread_cond_t * cond);

Pthread_cond_signal sends a message through the conditional variable cond, and if multiple messages are waiting, it wakes only one .

Pthread_cond_broadcast can wake up all. Call pthread_cond_signal after you want torelease the mutex immediately, because the last step of pthread_cond_wait is to re-lock the specified mutex, and if the mutex is not released after Pthread_cond_signal, Pthread_cond_wait still blocks. Regardless of the wait mode, you must mate with a mutex to prevent multiple threads from simultaneously requesting a race condition (Race Condition) for pthread_cond_wait () (or pthread_cond_timedwait (), hereinafter). Mutex mutex must be a normal lock (PTHREAD_MUTEX_TIMED_NP) or an adaptive lock (PTHREAD_MUTEX_ADAPTIVE_NP)

The type of mutex is the following: PTHREAD_MUTEX_TIMED_NP, which is the default value, which is the normal lock. When a line is Cheng, the remaining thread that requests the lock forms a wait queue, and the lock is acquired by priority after unlocking.　　This locking strategy guarantees the fairness of resource allocation. pthread_mutex_recursive_np, nested locks allow the same thread to be successfully acquired multiple times for the same lock, and unlocked by multiple unlock.　　If it is a different thread request, re-competes when the lock line threads unlocked. PTHREAD_MUTEX_ERRORCHECK_NP, checks the wrong lock, returns EDEADLK if the same thread requests the same lock, otherwise the same as the PTHREAD_MUTEX_TIMED_NP type action.　　This ensures that the simplest case of deadlocks does not occur when multiple locks are not allowed. PTHREAD_MUTEX_ADAPTIVE_NP, Adaptive Lock, the simplest type of lock, only waiting to be unlocked after re-competition.

The difference between the semaphore and the condition variable