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


See attributes(5) for descriptions of the following attributes:


+--------------------+-------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-------------------+
|Interface Stability | Committed |
+--------------------+-------------------+
|MT-Level | MT-Safe |
+--------------------+-------------------+
|Standard | See standards(5). |
+--------------------+-------------------+

SEE ALSO


pthread_mutex_consistent(3C), pthread_mutex_init(3C),
pthread_mutexattr_setprotocol(3C), pthread_mutexattr_setrobust(3C),
pthread_mutexattr_settype(3C), attributes(5), standards(5)

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)