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PTHREAD_RWLOCK_INIT(3) Library Functions Manual PTHREAD_RWLOCK_INIT(3)

pthread_rwlock_init, pthread_rwlock_destroy, pthread_rwlock_rdlock, pthread_rwlock_timedrdlock, pthread_rwlock_tryrdlock, pthread_rwlock_wrlock, pthread_rwlock_timedwrlock, pthread_rwlock_trywrlock, pthread_rwlock_unlock — operations on read/write locks

/* -lpthread */
#include <pthread.h>

int
pthread_rwlock_init(pthread_rwlock_t *lock, const pthread_rwlockattr_t *attr);

int
pthread_rwlock_destroy(pthread_rwlock_t *lock);

int
pthread_rwlock_rdlock(pthread_rwlock_t *lock);

int
pthread_rwlock_timedrdlock(pthread_rwlock_t *lock, const struct timespec *abstime);

int
pthread_rwlock_tryrdlock(pthread_rwlock_t *lock);

int
pthread_rwlock_wrlock(pthread_rwlock_t *lock);

int
pthread_rwlock_timedwrlock(pthread_rwlock_t *lock, const struct timespec *abstime);

int
pthread_rwlock_trywrlock(pthread_rwlock_t *lock);

int
pthread_rwlock_unlock(pthread_rwlock_t *lock);

The () function initializes a read/write lock, with attributes specified by attr. If attr is NULL, the default read/write lock attributes are used. The results of calling pthread_rwlock_init() with an already initialized lock are undefined.

The () function destroys a read/write lock previously created with pthread_rwlock_init().

The () 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 () function blocks until a write lock can be acquired against lock.

The () and () functions perform the same action, but will not wait beyond abstime to obtain the lock before returning.

The () and () functions perform the same action but do not block if the lock cannot be immediately obtained.

The () function releases the read/write lock previously obtained.

A thread may hold multiple concurrent read locks. If so, () must be called once for each lock obtained.

If successful, these functions return zero. Otherwise an error number will be returned to indicate the error.

pthread_rwlock_init() fails if:

[]
The system lacked the necessary resources (other than memory) to initialize the lock.
[]
Insufficient memory exists to initialize the lock.
[]
The caller does not have sufficient privilege to perform the operation.
[]
The system has detected an attempt to re-initialize the object referenced by lock, a previously initialized but not yet destroyed read/write lock.
[]
The value specified by attr is invalid.

Other functions fail if:

[]
The value specified by lock is invalid.
[]
Insufficient memory exists to initialize the lock (applies to statically initialized locks only).

pthread_rwlock_destroy() fails if:

[]
The system has detected an attempt to destroy the object referenced by lock while it is locked.

pthread_rwlock_tryrdlock() and pthread_rwlock_trywrlock() fail if:

[]
The lock could not be acquired without blocking.

pthread_rwlock_timedrdlock() and pthread_rwlock_timedwrlock() fail if:

[]
The time specified by abstime was reached before the lock could be obtained.

The pthread_rwlock_rdlock(), pthread_rwlock_timedrdlock(), and pthread_rwlock_tryrdlock() functions fail if:

[]
The lock could not be acquired because the maximum number of read locks against lock has been exceeded.
[]
The current thread already owns lock for writing.

The pthread_rwlock_wrlock(), pthread_rwlock_timedwrlock(), and pthread_rwlock_trywrlock() functions fail if:

[]
The calling thread already owns the read/write lock (for reading or writing).

The pthread_rwlock_unlock() fails if:

[]
The current thread does not own the read/write lock.

pthread_rwlockattr_init(3), pthreads(3)

These functions are expected to conform to Version 2 of the Single UNIX Specification (“SUSv2”).

pthread_rwlock_init(), pthread_rwlock_destroy(), pthread_rwlock_rdlock(), pthread_rwlock_tryrdlock(), pthread_rwlock_wrlock(), pthread_rwlock_trywrlock(), and pthread_rwlock_unlock() have been available since OpenBSD 2.5, and pthread_rwlock_timedrdlock() and pthread_rwlock_timedwrlock() since OpenBSD 4.8.

The PTHREAD_PROCESS_SHARED attribute is not supported.

July 8, 2025 OpenBSD-current