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PTHREAD_RWLOCK_RDLOCK(3P)  POSIX Programmer's Manual PTHREAD_RWLOCK_RDLOCK(3P)

PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the  corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.
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 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.
       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 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 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(3P)