# pthread_rwlock_rdlock

## PROLOG



           This manual page is part of the POSIX Programmer's Manual.  The
           Linux implementation of this interface may differ (consult the
           corresponding Linux manual page for details of Linux behavior), or
           the interface may not be implemented on Linux.
    


## NAME



           pthread_rwlock_rdlock, pthread_rwlock_tryrdlock — lock a read-
           write lock object for reading
    


## SYNOPSIS



           #include <pthread.h>
    
           int pthread_rwlock_rdlock(pthread_rwlock_t *_rwlock_);
           int pthread_rwlock_tryrdlock(pthread_rwlock_t *_rwlock_);
    


## 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 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 Thread 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.
    


## 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]**.
    
           _The following sections are informative._
    


## EXAMPLES



           None.
    


## APPLICATION USAGE



           Applications using these functions may be subject to priority
           inversion, as discussed in the Base Definitions volume of
           POSIX.1‐2017, _Section 3.291_ , _Priority Inversion_.
    


## RATIONALE



           If an implementation detects that the value specified by the
           _rwlock_ argument to _pthread_rwlock_rdlock_() or
           _pthread_rwlock_tryrdlock_() does not refer to an initialized read-
           write lock object, it is recommended that the function should fail
           and report an **[EINVAL]** error.
    


## FUTURE DIRECTIONS



           None.
    


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