man pages section 3: Basic Library Functions

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Updated: July 2014
 
 

pthread_mutex_trylock(3C)

Name

pthread_mutex_lock, pthread_mutex_trylock, pthread_mutex_unlock - lock or unlock a mutex

Synopsis

cc –mt [ flag... ] file... [ library... ]
#include <pthread.h>

int pthread_mutex_lock(pthread_mutex_t *mutex);
int pthread_mutex_trylock(pthread_mutex_t *mutex);
int pthread_mutex_unlock(pthread_mutex_t *mutex);

Description

The mutex object referenced by mutex 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 mutex 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 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 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 1. 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 0, 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 that 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.

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

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 mutex 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 0 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.

Return Values

If successful, the pthread_mutex_lock() and pthread_mutex_unlock() functions return 0. Otherwise, an error number is returned to indicate the error.

The pthread_mutex_trylock() function returns 0 if a lock on the mutex object referenced by mutex is acquired. Otherwise, an error number is returned to indicate the error.

Errors

The pthread_mutex_lock() and pthread_mutex_trylock() functions will fail if:

EAGAIN

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

EINVAL

The mutex was created with the protocol attribute having the value PTHREAD_PRIO_PROTECT and the calling thread's priority is higher than the mutex's current priority ceiling.

EPERM

The mutex was created with the protocol attribute having the value PTHREAD_PRIO_PROTECT and the calling thread is not in the real-time class (SCHED_RR or SCHED_FIFO scheduling class).

The pthread_mutex_trylock() function will fail if:

EBUSY

The mutex could not be acquired because it was already locked.

The pthread_mutex_lock(), pthread_mutex_trylock() and pthread_mutex_unlock() functions may fail if:

EINVAL

The value specified by mutex does not refer to an initialized mutex object.

The pthread_mutex_lock() function may fail if:

EDEADLK

The current thread already owns the mutex.

ENOMEM

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

The pthread_mutex_unlock() function will fail if:

EPERM

The mutex type is PTHREAD_MUTEX_ERRORCHECK or the mutex is a robust mutex, and the current thread does not own the mutex.

When a thread makes a call to pthread_mutex_lock() or pthread_mutex_trylock(), if the mutex is initialized with the robustness attribute having the value PTHREAD_MUTEX_ROBUST (see pthread_mutexattr_getrobust(3C)), the call will return these error values if:

EOWNERDEAD

The last owner of this mutex died while holding the mutex, or the process containing the owner of the mutex unmapped the memory containing the mutex or performed one of the exec(2) functions. This mutex is now owned by the caller. The caller must now attempt to make the state protected by the mutex consistent. If it is able to clean up the state, then it should call pthread_mutex_consistent() for the mutex and unlock the mutex. Subsequent calls to pthread_mutex_lock() and pthread_mutex_trylock() will behave normally, as before. If the caller is not able to clean up the state, pthread_mutex_consistent() should not be called for the mutex, but the mutex should be unlocked. Subsequent calls to pthread_mutex_lock() and pthread_mutex_trylock() will fail to acquire the mutex with the error value ENOTRECOVERABLE. If the owner who acquired the lock with EOWNERDEAD dies, the next owner will acquire the lock with EOWNERDEAD.

ENOTRECOVERABLE

The mutex trying to be acquired was protecting the state that has been left irrecoverable by the mutex's last owner. The mutex has not been acquired. This condition can occur when the lock was previously acquired with EOWNERDEAD, and the owner was not able to clean up the state and unlocked the mutex without calling pthread_mutex_consistent().

Attributes

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
MT-Safe
Standard

See Also

pthread_mutex_consistent(3C), pthread_mutex_init(3C), pthread_mutexattr_setprotocol(3C), pthread_mutexattr_setrobust(3C), pthread_mutexattr_settype(3C), attributes(5), standards (5)

Notes

In the current implementation of threads, pthread_mutex_lock(), pthread_mutex_unlock(), mutex_lock(), mutex_unlock(), pthread_mutex_trylock(), and mutex_trylock() do not validate the mutex type. Therefore, an uninitialized mutex or a mutex with an invalid type does not return EINVAL. Interfaces for mutexes with an invalid type have unspecified behavior.

Uninitialized mutexes that are allocated locally may contain junk data. Such mutexes need to be initialized using pthread_mutex_init() or mutex_init().