illumos: manual page: mtx

MTX(3C) Standard C Library Functions MTX(3C)

NAME

mtx, mtx_destroy, mtx_init, mtx_lock, mtx_timedlock, mtx_trylock,
mtx_unlock - C11 mutex operations

SYNOPSIS

#include <threads.h>

int
mtx_init(mtx_t *mtx, int type);

void
mtx_destroy(mtx_t *mtx);

int
mtx_lock(mtx_t *mtx);

int
mtx_timedlock(mtx_t *mtx, const struct timespec *restrict ts);

int
mtx_trylock(mtx_t *mtx);

int
mtx_unlock(mtx_t *mtx);

DESCRIPTION

The 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 threads(7). Importantly, they do not allow for inter-
process synchronization.

Creating and Destroying Mutexes

The mtx_init() function initializes the mutex specified by mtx. The
following types of mutexes are valid and may be specified by the type
argument:

mtx_plain A simple, intra-process mutex.

mtx_timed A simple, intra-process mutex, which allows timed locking
operations.

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.

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.
For more information on the different kind of mutexes, see mutex_init(3C).

The mtx_destroy() function destroys the mutex pointed to by mtx. It is
illegal for threads to be blocked waiting for mtx when mtx_destroy() is
called .

Locking and Unlocking Mutexes

The mtx_lock() function attempts to lock the mutex mtx. When the function
returns, it will be the sole owner of the mutex and must call mtx_unlock()
when it is done, or risk inducing a deadlock in the process. Other threads
that make calls to mtx_lock() after another thread has successfully
completed its call to mtx_lock() will block. When they finally return,
then they will have obtained the mutex mtx.

Unless a lock of type mtx_recursive was created, a thread calling
mtx_lock() when it already holds mtx will cause the thread to deadlock.
Otherwise, the lock will be successfully taken again. However, a thread
must call mtx_unlock() for each time that it has acquired mtx.

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

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

The mtx_unlock() function unlocks the mutex pointed to by 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 mtx_unlock() on a mutex which the calling thread does not
currently own.

RETURN VALUES

Upon successful completion, the function mtx_init() returns thrd_success.
If there was insufficient memory to create the thread, it instead returns
thrd_nomem. If any other error occurred, it returns thrd_error.

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

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

Upon successful completion, the mtx_trylock() function returns
thrd_success. If the thread was unable to obtain the mutex because another
thread owns it, then it returns thrd_busy. Otherwise, an error will have
occurred and thrd_error is returned.

INTERFACE STABILITY

Standard

MT-LEVEL
MT-Safe