Tribblix: manual page: pthread_mutex

PTHREAD_MUTEX_LOCK(3C) Standard C Library Functions 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);

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

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().

November 11, 2008 PTHREAD_MUTEX_LOCK(3C)