NAME

pthread_rwlock_clockrdlock, pthread_rwlock_timedrdlock — lock a read-write lock for reading

SYNOPSIS

#include <pthread.h>

int pthread_rwlock_clockrdlock(pthread_rwlock_t *restrict rwlock,
       clockid_t clock_id, const struct timespec *restrict abstime);
int pthread_rwlock_timedrdlock(pthread_rwlock_t *restrict rwlock,
       const struct timespec *restrict abstime);


DESCRIPTION

The pthread_rwlock_clockrdlock() and pthread_rwlock_timedrdlock() functions 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.

For pthread_rwlock_timedrdlock(), the timeout shall be based on the CLOCK_REALTIME clock. For pthread_rwlock_clockrdlock(), the timeout shall be based on the clock specified by the clock_id argument. The resolution of the timeout shall be the resolution of the clock on which it is based. Implementations shall support passing CLOCK_REALTIME and CLOCK_MONOTONIC to pthread_rwlock_clockrdlock() as the clock_id argument.

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_clockrdlock() or 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 these functions are called with an uninitialized read-write lock.

RETURN VALUE

The pthread_rwlock_clockrdlock() and pthread_rwlock_timedrdlock() functions 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_clockrdlock() and pthread_rwlock_timedrdlock() functions shall fail if:

[ETIMEDOUT]

The lock could not be acquired before the specified timeout expired.

The pthread_rwlock_clockrdlock() and pthread_rwlock_timedrdlock() functions 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, or the pthread_rwlock_clockrdlock() function was passed an invalid or unsupported clock_id value.

These functions shall not return an error code of [EINTR].

EXAMPLES

None.

APPLICATION USAGE

Applications using these functions may be subject to priority inversion, as discussed in XBD 3.275 Priority Inversion.

RATIONALE

If an implementation detects that the value specified by the rwlock argument to pthread_rwlock_clockrdlock() or 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_clockwrlock, pthread_rwlock_destroy, pthread_rwlock_rdlock, pthread_rwlock_trywrlock, pthread_rwlock_unlock

XBD 3.275 Priority Inversion, 4.15.2 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.

Issue 8

Austin Group Defect 592 is applied, removing text relating to <time.h> from the SYNOPSIS and DESCRIPTION sections.

Austin Group Defects 1216 and 1472 are applied, adding pthread_rwlock_clockrdlock().