Lock a Mutex

pthread_mutex_lock(3THR)

Prototype:
int	pthread_mutex_lock(pthread_mutex_t *mutex); 

#include <pthread.h>

pthread_mutex_t mutex;
int ret;

ret = pthread_ mutex_lock(&mp); /* acquire the mutex */

Use pthread_mutex_lock(3THR) to lock the mutex pointed to by mutex. When pthread_mutex_lock() returns, the mutex is locked and the calling thread is the owner. If the mutex is already locked and owned by another thread, the calling thread blocks until the mutex becomes available. (For Solaris threads, see mutex_lock(3THR).)

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 that 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 that 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. Every time a 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.

Return Values

pthread_mutex_lock() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occurs, the function fails and returns the corresponding value.

EAGAIN

The mutex could not be acquired because the maximum number of recursive locks for mutex has been exceeded.

EDEADLK

The current thread already owns the mutex.

If the symbol _POSIX_THREAD_PRIO_INHERIT is defined, the mutex is initialized with the protocol attribute value PTHREAD_PRIO_INHERIT, and the robustness argument of pthread_mutexattr_setrobust_np() is PTHREAD_MUTEX_ROBUST_NP the function fails and returns:

EOWNERDEAD

The last owner of this mutex died while holding the mutex. This mutex is now owned by the caller. The caller must attempt to make the state protected by the mutex consistent.

If the caller is able to make the state consistent, call pthread_mutex_consistent_np() for the mutex and unlock the mutex. Subsequent calls to pthread_mutex_lock() will behave normally.

If the caller is unable to make the state consistent, do not call pthread_mutex_init() for the mutex, but unlock the mutex. Subsequent calls to pthread_mutex_lock() fail to acquire the mutex and return an ENOTRECOVERABLE error code.

If the owner that acquired the lock with EOWNERDEAD dies, the next owner acquires the lock with EOWNERDEAD.

ENOTRECOVERABLE

The mutex you are trying to acquire is protecting state left irrecoverable by the mutex's previous owner that died while holding the lock. The mutex has not been acquired. This condition can occur when the lock was previously acquired with EOWNERDEAD and the owner was unable to cleanup the state and had unlocked the mutex without making the mutex state consistent.

ENOMEM

The limit on the number of simultaneously held mutexes has been exceeded.