pthread_mutex_lock, pthread_mutex_trylock, or pthread_mutex_unlock Subroutine

Purpose

Locks and unlocks a mutex.

Library

Threads Library (libpthreads.a)

Syntax

#include <pthread.h>

Description

The mutex object referenced by the mutex parameter is locked by calling pthread_mutex_lock. If the mutex is already locked, the calling thread blocks until the mutex becomes available. This operation returns with the mutex object referenced by the mutex parameter in the locked state with the calling thread as its owner.

If the mutex type is PTHREAD_MUTEX_NORMAL, deadlock detection is not provided. Attempting to relock the mutex causes deadlock. If a thread attempts to unlock a mutex that it has not locked or a mutex which is unlocked, undefined behavior results.

If the mutex type is PTHREAD_MUTEX_ERRORCHECK, then error checking is provided. If a thread attempts to relock a mutex that it has already locked, an error will be returned. If a thread attempts to unlock a mutex that it has not locked or a mutex which is unlocked, an error will be returned.

If the mutex type is PTHREAD_MUTEX_RECURSIVE, then the mutex maintains the concept of a lock count. When a thread successfully acquires a mutex for the first time, the lock count is set to one. Each time the thread relocks this mutex, the lock count is incremented by one. Each time the thread unlocks the mutex, the lock count is decremented by one. When the lock count reaches zero, the mutex becomes available for other threads to acquire. If a thread attempts to unlock a mutex that it has not locked or a mutex which is unlocked, an error will be returned.

If the mutex type is PTHREAD_MUTEX_DEFAULT, attempting to recursively lock the mutex results in undefined behavior. Attempting to unlock the mutex if it was not locked by the calling thread results in undefined behavior. Attempting to unlock the mutex if it is not locked results in undefined behavior.

If the mutex is a robust mutex and if the thread that owns the robust mutex terminates while holding the mutex lock, a call to the pthread_mutex_lock subroutine returns the EOWNERDEAD error code. In this case, the robust mutex is locked by the thread and the protected state of the robust mutex is marked as inconsistent. A call to the pthread_mutex_consistent subroutine can be used to mark the protected state of the robust mutex as consistent.

If the mutex is a robust mutex and if the protected state of the robust mutex is inconsistent, a call to the pthread_mutex_unlock subroutine marks the state of the robust mutex as permanently unusable. In this case, a call to the pthread_mutex_destroy subroutine is the only permissible operation on the robust mutex.

The function pthread_mutex_trylock is identical to pthread_mutex_lock except that if the robust mutex object referenced by the mutex parameter is currently locked (by any thread, including the current thread), the call returns immediately.

The pthread_mutex_unlock function releases the mutex object referenced by mutex. The manner in which a mutex is released is dependent upon the mutex's type attribute. If there are threads blocked on the mutex object referenced by the mutex parameter when pthread_mutex_unlock is called, resulting in the mutex becoming available, the scheduling policy is used to determine which thread will acquire the mutex. (In the case of PTHREAD_MUTEX_RECURSIVE mutexes, the mutex becomes available when the count reaches zero and the calling thread no longer has any locks on this mutex).

If a signal is delivered to a thread waiting for a mutex, upon return from the signal handler the thread resumes waiting for the mutex as if it was not interrupted.

Parameter

Return Values

If successful, the pthread_mutex_lock and pthread_mutex_unlock functions return zero. Otherwise, an error number is returned to indicate the error.

The function pthread_mutex_trylock returns zero if a lock on the mutex object referenced by the mutex parameter is acquired. Otherwise, an error number is returned to indicate the error.

Error Codes

The pthread_mutex_trylock function will fail if:

The pthread_mutex_lock, pthread_mutex_trylock and pthread_mutex_unlock functions will fail if:

The pthread_mutex_lock function will fail if:

The pthread_mutex_unlock() subroutine fails if the following error code is returned:

These subroutines will not return an EINTR error code.