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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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PTHREAD...K_RDLOCK(3P) POSIX Programmer's Manual PTHREAD...K_RDLOCK(3P)
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.
pthread_rwlock_rdlock, pthread_rwlock_tryrdlock — lock a read-
write lock object for reading
#include <pthread.h>
int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);
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.
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.
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.
None.
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.
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.
None.
pthread_rwlock_destroy(3p), pthread_rwlock_timedrdlock(3p),
pthread_rwlock_timedwrlock(3p), pthread_rwlock_trywrlock(3p),
pthread_rwlock_unlock(3p)
The Base Definitions volume of POSIX.1‐2017, Section 3.291,
Priority Inversion, Section 4.12, Memory Synchronization,
pthread.h(0p)
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...K_RDLOCK(3P)
Pages that refer to this page: pthread.h(0p), pthread_rwlock_destroy(3p), pthread_rwlock_timedrdlock(3p), pthread_rwlock_timedwrlock(3p), pthread_rwlock_tryrdlock(3p), pthread_rwlock_trywrlock(3p), pthread_rwlock_unlock(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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