pthread_rwlock_rdlock¶
Synopsis¶
#include <pthread.h>
int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);
Status¶
Implemented
Conformance¶
IEEE Std 1003.1-2017
Description¶
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 a writer holds the lock, the calling thread shall not acquire the read lock. Whether the calling thread acquires the lock when a writer does not hold the lock and there are writers blocked on the lock is implementation-defined. In Phoenix-RTOS the calling thread does not acquire the read lock while any writer is blocked on the lock, regardless of the priorities of the threads, so that waiting writers are not starved by a continuous stream of readers. 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. In Phoenix-RTOS the number of read locks is limited only by the range of size_t.
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.
Return value¶
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.
Errors¶
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.
POSIX deviations¶
No POSIX deviations discovered.