pthread_mutex_lock(3C) pthread_mutex_lock(3C)
NAME
pthread_mutex_lock, pthread_mutex_trylock, pthread_mutex_unlock - lock or unlock a mutex
SYNOPSIS
cc -mt [ flag... ] file... -lpthread [ 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);
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 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 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.
If successful, the pthread_mutex_lock() and pthread_mutex_unlock() functions return 0. Otherwise, an error number is returned to indi-
cate 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.
The pthread_mutex_lock() and pthread_mutex_trylock() functions will fail if:
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.
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.
EAGAIN The mutex could not be acquired because the maximum number of recursive locks for mutex has been exceeded.
The pthread_mutex_lock() function may fail if:
EDEADLK The current thread already owns the mutex.
The pthread_mutex_unlock() function may fail if:
EPERM The current thread does not own the mutex.
When a thread makes a call to pthread_mutex_lock() or pthread_mutex_trylock(), if the symbol _POSIX_THREAD_PRIO_INHERIT is defined and the
mutex is initialized with the protocol attribute having the value PTHREAD_PRIO_INHERIT and the robustness attribute having the value
PTHREAD_MUTEX_ROBUST_NP (see pthread_mutexattr_getrobust_np(3C)), the pthread_mutex_lock() and pthread_mutex_trylock() functions will fail
if:
EOWNERDEAD The last owner of this mutex died while holding the mutex. 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_np() 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_np() should not be called for the mutex, but it should be unlocked. Subsequent calls to
pthread_mutex_lock() and pthread_mutex_trylock() will fail to acquire the mutex with the error value ENOTRECOVER-
ABLE. If the owner who acquired the lock with EOWNERDEAD dies, the next owner will acquire the lock with EOWN-
ERDEAD.
ENOTRECOVERABLE The mutex trying to be acquired is protecting the state that has been left irrecoverable by the mutex's last owner,
who died while holding the lock. The mutex has not been acquired. This condition can occur when the lock was previ-
ously acquired with EOWNERDEAD, and the owner was not able to clean up the state and unlocked the mutex without
making the mutex consistent.
ENOMEM The limit on the number of simultaneously held mutexes has been exceeded.
See attributes(5) for descriptions of the following attributes:
+-----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+-----------------------------+-----------------------------+
|Interface Stability |Standard |
+-----------------------------+-----------------------------+
|MT-Level |MT-Safe |
+-----------------------------+-----------------------------+
pthread_mutex_init(3C), pthread_mutex_destroy(3C), pthread_mutex_consistent_np(3C), pthread_mutexattr_getrobust_np(3C), attributes(5),
standards(5)
In the current implementation of threads, pthread_mutex_lock(), pthread_mutex_unlock(), mutex_lock(), mutex_unlock(), pthread_mutex_try-
lock(), 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().
23 Mar 2005 pthread_mutex_lock(3C)