---
name: "shmem_clear_lock"
section: "3"
platform: "linux"
package: "openmpi"
version: "5.0.11-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/shmem_clear_lock"
---

# shmem\_clear\_lock(3)

[*shmem\_set\_lock*(3)](/shmem_set_lock/3.md), [*shmem\_clear\_lock*(3)](/shmem_clear_lock/3.md), [*shmem\_test\_lock*(3)](/shmem_test_lock/3.md) - Releases, locks, and tests a mutual exclusion memory lock.

## 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)
```

## 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.

## NOTES

The term symmetric data object is defined on [*intro\_shmem*(3)](/intro_shmem/3.md).

## 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)](/intro_shmem/3.md)

## COPYRIGHT

2003-2026, The Open MPI Community