Originální popis anglicky:
pthread_rwlock_rdlock, pthread_rwlock_tryrdlock - lock a read-write lock object
for reading
Návod, kniha: POSIX Programmer's Manual
#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 Threads 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:
- EINVAL
- The value specified by rwlock does not refer to an
initialized read-write lock object.
- 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
- 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
IEEE Std 1003.1-2001, Section 3.285, Priority Inversion.
None.
None.
pthread_rwlock_destroy() ,
pthread_rwlock_timedrdlock() ,
pthread_rwlock_timedwrlock() ,
pthread_rwlock_trywrlock() ,
pthread_rwlock_unlock() ,
pthread_rwlock_wrlock() , the Base
Definitions volume of IEEE Std 1003.1-2001,
<pthread.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 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
.
