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.Dd "Jan 11, 2015" .Dt MTX 3C .Os .Sh NAME .Nm mtx , .Nm mtx_destroy , .Nm mtx_init , .Nm mtx_lock , .Nm mtx_timedlock , .Nm mtx_trylock , .Nm mtx_unlock .Nd C11 mutex operations .Sh SYNOPSIS n threads.h .Ft int .Fo mtx_init .Fa "mtx_t *mtx" .Fa "int type" .Fc .Ft void .Fo mtx_destroy .Fa "mtx_t *mtx" .Fc .Ft int .Fo mtx_lock .Fa "mtx_t *mtx" .Fc .Ft int .Fo mtx_timedlock .Fa "mtx_t *mtx" .Fa "const struct timespec *restrict ts" .Fc .Ft int .Fo mtx_trylock .Fa "mtx_t *mtx" .Fc .Ft int .Fo mtx_unlock .Fa "mtx_t *mtx" .Fc .Sh DESCRIPTION The .Sy mtx family of functions implement mutual exclusion locks (mutexes) and behave similarly to both POSIX threads and illumos threads; however, they have slightly different call signatures and return values. For more information, see .Xr threads 5 . Importantly, they do not allow for inter-process synchronization. .Ss Creating and Destroying Mutexes The .Fn mtx_init function initializes the mutex specified by .Fa mtx . The following types of mutexes are valid and may be specified by the .Fa type argument: l -tag -width Dv t Sy mtx_plain A simple, intra-process mutex. t Sy mtx_timed A simple, intra-process mutex, which allows timed locking operations. t Sy mtx_plain | mtx_recursive An intra-process mutex that may be acquired recursively by the same thread. It must be unlocked an equal number of times that it is locked. t Sy mtx_timed | mtx_recursive An intra-process mutex that supports timed locking operations and may be acquired recursively by the same thread. It must be unlocked an equal number of times that it is locked. .El For more information on the different kind of mutexes, see .Xr mutex_init 3C .

p The .Fn mtx_destroy function destroys the mutex pointed to by .Fa mtx . It is illegal for threads to be blocked waiting for .Fa mtx when .Fn mtx_destroy is called . .Ss Locking and Unlocking Mutexes The .Fn mtx_lock function attempts to lock the mutex .Fa mtx . When the function returns, it will be the sole owner of the mutex and must call .Fn mtx_unlock when it is done, or risk inducing a deadlock in the process. Other threads that make calls to .Fn mtx_lock after another thread has successfully completed its call to .Fn mtx_lock will block. When they finally return, then they will have obtained the mutex .Fa mtx .

p Unless a lock of type .Sy mtx_recursive was created, a thread calling .Fn mtx_lock when it already holds .Fa mtx will cause the thread to deadlock. Othewrise, the lock will be successfully taken again. However, a thread must call .Fn mtx_unlock for each time that it has acquried .Fa mtx .

p The .Fn mtx_trylock function will attempt to obtain the mutex pointed to by .Fa mtx . However, unlike .Fn mtx_lock , if .Fa mtx is locked, then it will not block and wait for .Fa mtx and instead return .Sy thrd_busy to indicate that the lock is currently held.

p The .Fn mtx_timedlock function attempts to obtain the mutex pointed to by .Fa mtx . If it is unable to obtain it, then it will block for a set amount of time dictated by .Fa ts . The timeout in .Fa ts is treated as an absolute time in UTC to block until, measured based on the .Sy CLOCK_REALTIME clock.

p The .Fn mtx_unlock function unlocks the mutex pointed to by .Fa mtx , which allows another thread the opportunity to obtain it. If any threads are actively blocking on the mutex, one of them will obtain it and be woken up. It is an error to call .Fn mtx_unlock on a mutex which the calling thread does not currently own. .Sh RETURN VALUES Upon successful completion, the function .Fn mtx_init returns .Sy thrd_success. If there was insufficient memory to create the thread, it instead returns .Sy thrd_nomem . If any other error occurred, it returns .Sy thrd_error .

p The functions .Fn mtx_lock , and .Fn mtx_unlock return .Sy thrd_success . If they were unable to successfully complete the operation, they instead return .Sy thrd_error .

p Upon sucessful completion, the .Fn mtx_timedlock function returns .Sy thrd_success . If the timeout is reached and the calling thread is unable to obtain the mutex, then .Sy thrd_timedout is returned . If any other error occurs, then .Sy thrd_error is returned.

p Upon successful completion, the .Fn mtx_trylock function returns .Sy thrd_success . If the thread was unable to obtain the mutex because another thread owns it, then it returns .Sy thrd_busy . Otherwise, an error will have occurred and .Sy thrd_error is returned. .Sh INTERFACE STABILITY .Sy Standard .Sh MT-LEVEL .Sy MT-Safe .Sh SEE ALSO .Xr mutex_init 3C , .Xr pthread_mutex_destroy 3C , .Xr pthread_mutex_init 3C , .Xr pthread_mutex_lock 3C , .Xr pthread_mutex_timedlock 3C , .Xr pthread_mutex_trylock 3C , .Xr pthread_mutex_unlock 3C , .Xr threads.h 3HEAD , .Xr attributes 5