PTHREAD_RWLOCK_RDLOCK(3)

NAME
pthread_rwlock_rdlock, pthread_rwlock_timedrdlock, pthread_rwlock_tryrdlock - acquire a read/write lock for reading

LIBRARY
libpthread

SYNOPSIS
#include <pthread.h>
int
pthread_rwlock_rdlock(pthread_rwlock_t *lock);
int
pthread_rwlock_timedrdlock(pthread_rwlock_t * restrict lock, const struct timespec * restrict abstime);
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_timedrdlock() performs the same action, but will not wait beyond abstime to obtain the lock before returning.

The pthread_rwlock_tryrdlock() function performs the same action as pthread_rwlock_rdlock(), 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.

RETURN VALUES
If successful, the pthread_rwlock_rdlock(), pthread_rwlock_timedrdlock(), 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 shall fail if:

[EBUSY]
    The lock could not be acquired because a writer holds the lock or was blocked on it.

The pthread_rwlock_timedrdlock() function shall fail if:

[ETIMEDOUT]
    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 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.

SEE ALSO
pthread_rwlock_destroy(3), pthread_rwlock_init(3), pthread_rwlock_unlock(3), pthread_rwlock_wrlock(3)

STANDARDS
pthread_rwlock_rdlock(), pthread_rwlock_timedrdlock(), and pthread_rwlock_tryrdlock() conform to p1003.1-96.