pthread_mutex_lock(3C)													    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);

       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 returns immediately.

       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.

       If successful, the  pthread_mutex_lock() and  pthread_mutex_unlock() functions return  0. Otherwise, an error number is returned  to  indi-
       cate 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.

       The  pthread_mutex_lock() and pthread_mutex_trylock() functions will fail if:

       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.

       EAGAIN	       The mutex could not be acquired because the maximum number of  recursive locks for mutex has been exceeded.

       The  pthread_mutex_lock() function may fail if:

       EDEADLK	       The current thread already owns the mutex.

       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 symbol  _POSIX_THREAD_PRIO_INHERIT is defined and the
       mutex is initialized with the protocol attribute having the value PTHREAD_PRIO_INHERIT  and  the  robustness  attribute	having	the  value
       PTHREAD_MUTEX_ROBUST_NP	(see pthread_mutexattr_getrobust_np(3C)), the pthread_mutex_lock() and pthread_mutex_trylock() functions will fail
       if:

       EOWNERDEAD	       The last owner of this mutex died while holding the mutex. 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_consistent_np() should not be called for the mutex, but it should be unlocked.  Subsequent  calls	to
			       pthread_mutex_lock()  and  pthread_mutex_trylock() will fail to acquire the mutex with the error value ENOTRECOVER-
			       ABLE. If the owner who acquired the lock with EOWNERDEAD dies, the next owner will  acquire  the  lock  with  EOWN-
			       ERDEAD.

       ENOTRECOVERABLE	       The mutex trying to be acquired is protecting the state that has been left irrecoverable by the mutex's last owner,
			       who died while holding the lock. 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 the mutex consistent.

       ENOMEM		       The limit on the number of simultaneously held mutexes has been exceeded.

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

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Standard			   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |MT-Safe			   |
       +-----------------------------+-----------------------------+

       pthread_mutex_init(3C),	pthread_mutex_destroy(3C),  pthread_mutex_consistent_np(3C),  pthread_mutexattr_getrobust_np(3C),   attributes(5),
       standards(5)

       In  the	current  implementation of threads, pthread_mutex_lock(), pthread_mutex_unlock(), mutex_lock(), mutex_unlock(), pthread_mutex_try-
       lock(), 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().

								    23 Mar 2005 					    pthread_mutex_lock(3C)