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PTHREAD_RWLOCK_RDLOCK(3P) POSIX Programmer's ManualPTHREAD_RWLOCK_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.1‐2008, Section 3.287, 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.1‐2008, Section 3.287, Priority
Inversion, Section 4.11, Memory Synchronization, pthread.h(0p)
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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 PTHREAD_RWLOCK_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)