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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 Thread 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:
       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
              A  deadlock condition was detected or 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 POSIX.1-2008, Sec-
       tion 3.287, Priority Inversion.
RATIONALE
       If an implementation detects that the value  specified  by  the  rwlock
       argument  to pthread_rwlock_rdlock() or pthread_rwlock_tryrdlock() does
       not refer to an initialized read-write lock object, it  is  recommended
       that the function should fail and report an [EINVAL] error.
FUTURE DIRECTIONS
       None.
SEE ALSO
       pthread_rwlock_destroy(), pthread_rwlock_timedrdlock(),
       pthread_rwlock_timedwrlock(), pthread_rwlock_trywrlock(),
       pthread_rwlock_unlock()
       The  Base  Definitions  volume of POSIX.1-2008, Section 3.287, Priority
       Inversion, Section 4.11, Memory Synchronization, <pthread.h>
COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) 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.unix.org/online.html .
       Any  typographical  or  formatting  errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files  to  man page format. To report such errors, see https://www.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013            PTHREAD_RWLOCK_RDLOCK(3P)