pthread_rwlock_rdlock

       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 func-
       tion  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.

       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  inver-
       sion,    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  Defini-
       tions volume of IEEE Std 1003.1-2001, <pthread.h>

COPYRIGHT
       Portions  of  this text are reprinted and reproduced in electronic form