PTHREAD_MUTEX_LOCK

PROLOG

NAME

SYNOPSIS

#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

-

If the mutex type is PTHREAD_MUTEX_DEFAULT, the behavior of pthread_mutex_lock() may correspond to one of the three other standard mutex types as described in the table above. If it does not correspond to one of those three, the behavior is undefined for the cases marked -.

Where the table indicates recursive behavior, the mutex shall maintain the concept of a lock count. When a thread successfully acquires a mutex for the first time, the lock count shall be set to one. Every time a thread relocks this mutex, the lock count shall be incremented by one. Each time the thread unlocks the mutex, the lock count shall be decremented by one. When the lock count reaches zero, the mutex shall become available for other threads to acquire.

The pthread_mutex_trylock() function shall be equivalent 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 shall return immediately. If the mutex type is PTHREAD_MUTEX_RECURSIVE and the mutex is currently owned by the calling thread, the mutex lock count shall be incremented by one and the pthread_mutex_trylock() function shall immediately return success.

The pthread_mutex_unlock() function shall release 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 shall determine which thread shall acquire the mutex.

(In the case of PTHREAD_MUTEX_RECURSIVE mutexes, the mutex shall become 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 shall resume waiting for the mutex as if it was not interrupted.

If mutex is a robust mutex and the process containing the owning thread terminated while holding the mutex lock, a call to pthread_mutex_lock() shall return the error value [EOWNERDEAD]. If mutex is a robust mutex and the owning thread terminated while holding the mutex lock, a call to pthread_mutex_lock() may return the error value [EOWNERDEAD] even if the process in which the owning thread resides has not terminated. In these cases, the mutex is locked by the thread but the state it protects is marked as inconsistent. The application should ensure that the state is made consistent for reuse and when that is complete call pthread_mutex_consistent(). If the application is unable to recover the state, it should unlock the mutex without a prior call to pthread_mutex_consistent(), after which the mutex is marked permanently unusable.

If mutex does not refer to an initialized mutex object, the behavior of pthread_mutex_lock(), pthread_mutex_trylock(), and pthread_mutex_unlock() is undefined.  

RETURN VALUE

ERRORS

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.

ENOTRECOVERABLE

The state protected by the mutex is not recoverable.

EOWNERDEAD

The mutex is a robust mutex and the process containing the previous owning thread terminated while holding the mutex lock. The mutex lock shall be acquired by the calling thread and it is up to the new owner to make the state consistent.

The pthread_mutex_lock() function shall fail if:

EDEADLK

The mutex type is PTHREAD_MUTEX_ERRORCHECK and the current thread already owns the mutex.

The pthread_mutex_trylock() function shall fail if:

EBUSY

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

The pthread_mutex_unlock() function shall fail if:

EPERM

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

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

EOWNERDEAD

The mutex is a robust mutex and the previous owning thread terminated while holding the mutex lock. The mutex lock shall be acquired by the calling thread and it is up to the new owner to make the state consistent.

The pthread_mutex_lock() function may fail if:

EDEADLK

A deadlock condition was detected.

These functions shall not return an error code of [EINTR].

The following sections are informative.  

EXAMPLES

APPLICATION USAGE

RATIONALE

The mutex functions and the particular default settings of the mutex attributes have been motivated by the desire to not preclude fast, inlined implementations of mutex locking and unlocking.

Since most attributes only need to be checked when a thread is going to be blocked, the use of attributes does not slow the (common) mutex-locking case.

Likewise, while being able to extract the thread ID of the owner of a mutex might be desirable, it would require storing the current thread ID when each mutex is locked, and this could incur unacceptable levels of overhead. Similar arguments apply to a mutex_tryunlock operation.

For further rationale on the extended mutex types, see the Rationale (Informative) volume of POSIX.1-2017, Threads Extensions.

If an implementation detects that the value specified by the mutex argument does not refer to an initialized mutex object, it is recommended that the function should fail and report an [EINVAL] error.  

FUTURE DIRECTIONS

SEE ALSO

The Base Definitions volume of POSIX.1-2017, Section 4.12, Memory Synchronization, <pthread.h>  

COPYRIGHT

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

Index

PROLOG

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

EXAMPLES

APPLICATION USAGE

RATIONALE

FUTURE DIRECTIONS

SEE ALSO

COPYRIGHT