NAME

       pthread_rwlock_rdlock,  pthread_rwlock_tryrdlock  -  lock  a  read-write  lock  object for
       reading

SYNOPSIS

       #include <pthread.h>

       int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
       int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);

DESCRIPTION

       The pthread_rwlock_rdlock() function shall apply  a  read  lock  to  the  read-write  lock
       referenced  by rwlock. The calling thread acquires the read lock if a writer does not hold
       the lock and there are no writers blocked on the lock.

       If the Thread Execution Scheduling option is supported, and the threads  involved  in  the
       lock are executing with the scheduling policies SCHED_FIFO or SCHED_RR, the calling thread
       shall not acquire the lock if a writer holds the lock or if writers  of  higher  or  equal
       priority are blocked on the lock; otherwise, the calling thread shall acquire the lock.

       If  the  Threads Execution Scheduling option is supported, and the threads involved in the
       lock are executing with the SCHED_SPORADIC scheduling policy, the calling thread shall not
       acquire  the lock if a writer holds the lock or if writers of higher or equal priority are
       blocked on the lock; otherwise, the calling thread shall acquire the lock.

       If the Thread Execution Scheduling option is not supported, it  is  implementation-defined
       whether  the  calling  thread  acquires  the lock when a writer does not hold the lock and
       there are writers blocked on the lock. If a writer holds  the  lock,  the  calling  thread
       shall  not  acquire  the  read  lock. If the read lock is not acquired, the calling thread
       shall block until it can acquire the lock.  The calling thread may deadlock if at the time
       the call is made it holds a write lock.

       A thread may hold multiple concurrent read locks on rwlock (that is, successfully call the
       pthread_rwlock_rdlock() function n times). If so, the application shall  ensure  that  the
       thread performs matching unlocks (that is, it calls the pthread_rwlock_unlock() function n
       times).

       The maximum number of simultaneous read locks that an  implementation  guarantees  can  be
       applied  to a read-write lock shall be implementation-defined. The pthread_rwlock_rdlock()
       function may fail if this maximum would be exceeded.

       The  pthread_rwlock_tryrdlock()  function  shall   apply   a   read   lock   as   in   the
       pthread_rwlock_rdlock()  function,  with the exception that the function shall fail if the
       equivalent pthread_rwlock_rdlock() call would have blocked the calling thread. In no  case
       shall  the  pthread_rwlock_tryrdlock()  function ever block; it always either acquires the
       lock or fails and returns immediately.

       Results are undefined if any of these functions are called  with  an  uninitialized  read-
       write lock.

       If  a  signal  is  delivered  to  a thread waiting for a read-write lock for reading, upon
       return from the signal handler the thread resumes waiting  for  the  read-write  lock  for
       reading as if it was not interrupted.

RETURN VALUE

       If successful, the pthread_rwlock_rdlock() function shall return zero; otherwise, an error
       number shall be returned to indicate the error.

       The pthread_rwlock_tryrdlock() function shall return zero if the lock for reading  on  the
       read-write  lock object referenced by rwlock is acquired. Otherwise, an error number shall
       be returned to indicate the error.

ERRORS

       The pthread_rwlock_tryrdlock() function shall fail if:

       EBUSY  The read-write lock could not be acquired for reading because a  writer  holds  the
              lock or a writer with the appropriate priority was blocked on it.

       The pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions may fail if:

       EINVAL The  value  specified  by  rwlock  does not refer to an initialized read-write lock
              object.

       EAGAIN The read lock could not be acquired because the maximum number of  read  locks  for
              rwlock has been exceeded.

       The pthread_rwlock_rdlock() function may fail if:

       EDEADLK
              The current thread already owns the read-write lock for writing.

       These functions shall not return an error code of [EINTR].

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       Applications  using  these functions may be subject to priority inversion, as discussed in
       the Base Definitions volume of IEEE Std 1003.1-2001, Section 3.285, Priority Inversion.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       pthread_rwlock_destroy() , pthread_rwlock_timedrdlock() ,  pthread_rwlock_timedwrlock()  ,
       pthread_rwlock_trywrlock()  , pthread_rwlock_unlock() , pthread_rwlock_wrlock() , the Base
       Definitions volume of IEEE Std 1003.1-2001, <pthread.h>

COPYRIGHT

       Portions of this text are reprinted and  reproduced  in  electronic  form  from  IEEE  Std
       1003.1,  2003  Edition,  Standard  for Information Technology -- Portable Operating System
       Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003  by
       the  Institute  of  Electrical  and  Electronics Engineers, Inc and The Open Group. In the
       event of any discrepancy between this version and the original IEEE  and  The  Open  Group
       Standard,  the  original  IEEE  and  The  Open Group Standard is the referee document. The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .