# 18.2.25. shmem_clear_lock

shmem_set_lock(3), shmem_clear_lock(3), shmem_test_lock(3) - Releases, locks, and tests a mutual exclusion memory lock.

## 18.2.25.1. SYNOPSIS

C or C++:

    #include <mpp/shmem.h>

    void shmem_clear_lock(volatile long *lock)

    void shmem_set_lock(volatile long *lock)

    int shmem_test_lock(volatile long *lock)

Fortran:

    INCLUDE "mpp/shmem.fh"

    INTEGER lock, SHMEM_TEST_LOCK

    CALL SHMEM_CLEAR_LOCK(lock)

    CALL SHMEM_SET_LOCK(lock)

    I = SHMEM_TEST_LOCK(lock)

## 18.2.25.2. DESCRIPTION

The shmem_set_lock routine sets a mutual exclusion lock after waiting for the lock to be freed by any other PE currently holding the lock. Waiting PEs are assured of getting the lock in a first-come, first-served manner.

The shmem_clear_lock routine releases a lock previously set by shmem_set_lock after ensuring that all local and remote stores initiated in the critical region are complete.

The shmem_test_lock function sets a mutual exclusion lock only if it is currently cleared. By using this function, a PE can avoid blocking on a set lock. If the lock is currently set, the function returns without waiting. These routines are appropriate for protecting a critical region from simultaneous update by multiple PEs. They accept the following arguments:

lock

  A symmetric data object that is a scalar variable or an array of length 1. This data object must be set to 0 on all processing elements (PEs) prior to the first use. lock must be of type integer. If you are using Fortran, it must be of default kind.

## 18.2.25.3. NOTES

The term symmetric data object is defined on intro_shmem(3).

## 18.2.25.4. RETURN VALUES

The shmem_test_lock function returns 0 if the lock was originally cleared and this call was able to set the lock. A value of 1 is returned if the lock had been set and the call returned without waiting to set the lock.

See also

intro_shmem(3)