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 .

IEEE/The Open Group                                   2003                              PTHREAD_RWLOCK_RDLOCK(P)