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pthread_rwlock_rdlock(3) [freebsd man page]

PTHREAD_RWLOCK_RDLOCK(3)				   BSD Library Functions Manual 				  PTHREAD_RWLOCK_RDLOCK(3)

NAME
pthread_rwlock_rdlock, pthread_rwlock_tryrdlock -- acquire a read/write lock for reading LIBRARY
POSIX Threads Library (libpthread, -lpthread) SYNOPSIS
#include <pthread.h> int pthread_rwlock_rdlock(pthread_rwlock_t *lock); int pthread_rwlock_tryrdlock(pthread_rwlock_t *lock); DESCRIPTION
The pthread_rwlock_rdlock() function acquires a read lock on lock provided that lock is not presently held for writing and no writer threads are presently blocked on the lock. If the read lock cannot be immediately acquired, the calling thread blocks until it can acquire the lock. The pthread_rwlock_tryrdlock() function performs the same action, but does not block if the lock cannot be immediately obtained (i.e., the lock is held for writing or there are waiting writers). A thread may hold multiple concurrent read locks. If so, pthread_rwlock_unlock() must be called once for each lock obtained. The results of acquiring a read lock while the calling thread holds a write lock are undefined. IMPLEMENTATION NOTES
To prevent writer starvation, writers are favored over readers. RETURN VALUES
If successful, the pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions will return zero. Otherwise an error number will be returned to indicate the error. ERRORS
The pthread_rwlock_tryrdlock() function will fail if: [EBUSY] The lock could not be acquired because a writer holds the lock or was blocked on it. The pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions may fail if: [EAGAIN] The lock could not be acquired because the maximum number of read locks against lock has been exceeded. [EDEADLK] The current thread already owns lock for writing. [EINVAL] The value specified by lock is invalid. [ENOMEM] Insufficient memory exists to initialize the lock (applies to statically initialized locks only). SEE ALSO
pthread_rwlock_init(3), pthread_rwlock_trywrlock(3), pthread_rwlock_unlock(3), pthread_rwlock_wrlock(3) STANDARDS
The pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions are expected to conform to Version 2 of the Single UNIX Specification (``SUSv2''). HISTORY
The pthread_rwlock_rdlock() function first appeared in FreeBSD 3.0. BSD
August 4, 1998 BSD

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pthread_rwlock_rdlock(3C)                                                                                                pthread_rwlock_rdlock(3C)

NAME
pthread_rwlock_rdlock, pthread_rwlock_tryrdlock - lock or attempt to lock read-write lock object for reading SYNOPSIS
cc -mt [ flag... ] file... -lpthread [ library... ] #include <pthread.h> int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock); int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock); The pthread_rwlock_rdlock() function applies 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. The calling thread does 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 acquires the lock. If the read lock is not acquired, the calling thread blocks until it can acquire the lock. A thread can hold multiple concurrent read locks on rwlock (that is, successfully call the pthread_rwlock_rdlock() function n times). If so, the thread must perform matching unlocks (that is, it must call the pthread_rwlock_unlock() function n times). The maximum number of concurrent read locks that a thread can hold on one read-write lock is currently set at 100,000, though this number could change in a future release. There is no imposed limit on the number of different threads that can apply a read lock to one read-write lock. The pthread_rwlock_tryrdlock() function applies a read lock like the pthread_rwlock_rdlock() function, with the exception that the function fails if the equivalent pthread_rwlock_rdlock() call would have blocked the calling thread. In no case will the pthread_rwlock_tryrdlock() function ever bloc. 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 returns 0. Otherwise, an error number is returned to indicate the error. The pthread_rwlock_tryrdlock() function returns 0 if the lock for reading on the read-write lock object referenced by rwlock is acquired. Otherwise an error number is returned to indicate the error. The pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions will fail if: EAGAIN The read lock could not be acquired because the maximum number of read locks by the current thread for rwlock has been exceeded. The pthread_rwlock_rdlock() function will fail if: EDEADLK The current thread already owns the read-write lock for writing. The pthread_rwlock_tryrdlock() function will 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 pri- ority was blocked on it. See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Standard | +-----------------------------+-----------------------------+ |MT-Level |MT-Safe | +-----------------------------+-----------------------------+ pthread_rwlock_init(3C), pthread_rwlock_wrlock(3C), pthread_rwlockattr_init(3C), pthread_rwlock_unlock(3C), attributes(5), standards(5) 23 Mar 2005 pthread_rwlock_rdlock(3C)

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