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 main‐
tains 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 refer‐
enced 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.
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 may fail if:
EPERM 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 robust‐
ness attribute having the value PTHREAD_MUTEX_ROBUST_NP (see
pthread_mutexattr_getrobust_np(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)exec 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_np()
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_con‐
sistent_np() 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 previ‐
ously acquired with EOWNERDEAD, and the owner was
not able to clean up the state and unlocked the
mutex without making calling pthread_mutex_consis‐
tent_np().
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌─────────────────────────────┬─────────────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├─────────────────────────────┼─────────────────────────────┤
│Interface Stability │Standard │
├─────────────────────────────┼─────────────────────────────┤
│MT-Level │MT-Safe │
└─────────────────────────────┴─────────────────────────────┘
SEE ALSOpthread_mutex_consistent_np(3C), pthread_mutex_init(3C), pthread_mutex‐
attr_setprotocol(3C), pthread_mutexattr_setrobust_np(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().
SunOS 5.10 5 Jun 2007 pthread_mutex_lock(3C)
