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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 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(3POSIX)