The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group

#### NAME

> pthread_rwlock_timedrdlock - lock a read-write lock for reading

#### SYNOPSIS

> ``#include <pthread.h>
> #include <time.h>
> int pthread_rwlock_timedrdlock(pthread_rwlock_t *restrict` rwlock`,
> const struct timespec *restrict` abstime`);
> ``

#### DESCRIPTION

> The pthread_rwlock_timedrdlock() function shall apply a read lock to the read-write lock referenced by rwlock as in the pthread_rwlock_rdlock() function. However, if the lock cannot be acquired without waiting for other threads to unlock the lock, this wait shall be terminated when the specified timeout expires. The timeout shall expire when the absolute time specified by abstime passes, as measured by the clock on which timeouts are based (that is, when the value of that clock equals or exceeds abstime), or if the absolute time specified by abstime has already been passed at the time of the call.

> The timeout shall be based on the CLOCK_REALTIME clock. The resolution of the timeout shall be the resolution of the CLOCK_REALTIME clock. The timespec data type is defined in the <time.h> header. Under no circumstances shall the function fail with a timeout if the lock can be acquired immediately. The validity of the abstime parameter need not be checked if the lock can be immediately acquired.

> If a signal that causes a signal handler to be executed is delivered to a thread blocked on a read-write lock via a call to pthread_rwlock_timedrdlock(), upon return from the signal handler the thread shall resume waiting for the lock as if it was not interrupted.

> The calling thread may deadlock if at the time the call is made it holds a write lock on rwlock. The results are undefined if this function is called with an uninitialized read-write lock.

#### RETURN VALUE

> The pthread_rwlock_timedrdlock() 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_timedrdlock() function shall fail if:

> [ETIMEDOUT]
>   The lock could not be acquired before the specified timeout expired.

> The pthread_rwlock_timedrdlock() function may fail if:

> [EAGAIN]
>   The read lock could not be acquired because the maximum number of read locks for lock would be exceeded.
> [EDEADLK]
>   A deadlock condition was detected or the calling thread already holds a write lock on rwlock.
> [EINVAL]
>   The abstime nanosecond value is less than zero or greater than or equal to 1000 million.

> This function shall not return an error code of [EINTR].

The following sections are informative.

#### EXAMPLES

> None.

#### APPLICATION USAGE

> Applications using this function may be subject to priority inversion, as discussed in XBD Priority Inversion.

#### RATIONALE

> If an implementation detects that the value specified by the rwlock argument to pthread_rwlock_timedrdlock() 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.

#### SEE ALSO

> pthread_rwlock_destroy, pthread_rwlock_rdlock, pthread_rwlock_timedwrlock, pthread_rwlock_trywrlock, pthread_rwlock_unlock

> XBD Priority Inversion, Memory Synchronization, <pthread.h>, <time.h>

#### CHANGE HISTORY

> First released in Issue 6. Derived from IEEE Std 1003.1j-2000.

> IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/102 is applied, updating the ERRORS section so that the [EDEADLK] error includes detection of a deadlock condition.

#### Issue 7

> The pthread_rwlock_timedrdlock() function is moved from the Timeouts option to the Base.

> The [EINVAL] error for an uninitialized read-write lock object is removed; this condition results in undefined behavior.

End of informative text.

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