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



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

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
       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:

       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
       inversion, as discussed in the Base Definitions volume of POSIX.1-2017,
       Section 3.291, 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.1-2017, Section 3.291, Priority
       Inversion, Section 4.12, Memory Synchronization, <pthread.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       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.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017             PTHREAD_RWLOCK_RDLOCK(3)
