# 18.2.13. shmem_barrier

shmem_barrier - Performs a barrier operation on a subset of processing elements (PEs).

## 18.2.13.1. SYNOPSIS

C or C++:

    #include <mpp/shmem.h>

    void shmem_barrier(int PE_start, int logPE_stride, int PE_size,
      long *pSync)

Fortran:

    INCLUDE "mpp/shmem.fh"

    INTEGER PE_start, logPE_stride, PE_size
    INTEGER pSync(SHMEM_BARRIER_SYNC_SIZE)

    CALL SHMEM_BARRIER(PE_start, logPE_stride, PE_size, pSync)

## 18.2.13.2. DESCRIPTION

The shmem_barrier routine does not return until the subset of PEs specified by PE_start, logPE_stride and PE_size, has entered this routine at the same point of the execution path.

As with all SHMEM collective routines, each of these routines assumes that only PEs in the active set call the routine. If a PE not in the active set calls a SHMEM collective routine, undefined behavior results.

The arguments are as follows:

PE_start

  The lowest virtual PE number of the active set of PEs. PE_start must be of type integer. If you are using Fortran, it must be a default integer value.

logPE_stride

  The log (base 2) of the stride between consecutive virtual PE numbers in the active set. logPE_stride must be of type integer. If you are using Fortran, it must be a default integer value.

PE_size

  The number of PEs in the active set. PE_size must be of type integer. If you are using Fortran, it must be a default integer value.

pSync

  A symmetric work array. In C/C++, pSync must be of type int and size _SHMEM_BARRIER_SYNC_SIZE. In Fortran, pSync must be of type integer and size SHMEM_BARRIER_SYNC_SIZE. If you are using Fortran, it must be a default integer type. Every element of this array must be initialized to 0 before any of the PEs in the active set enter shmem_barrier the first time.

The values of arguments PE_start, logPE_stride, and PE_size must be equal on all PEs in the active set. The same work array must be passed in pSync to all PEs in the active set.

shmem_barrier ensures that all previously issued local stores and previously issued remote memory updates done by any of the PEs in the active set (by using SHMEM calls, for example shmem_put(3)) are complete before returning.

The same pSync array may be reused on consecutive calls to shmem_barrier if the same active PE set is used.

## 18.2.13.3. NOTES

The term symmetric is defined in intro_shmem(3).

If the pSync array is initialized at run time, be sure to use some type of synchronization, for example, a call to shmem_barrier_all(3), before calling shmem_barrier for the first time.

If the active set does not change, shmem_barrier can be called repeatedly with the same pSync array. No additional synchronization beyond that implied by shmem_barrier itself is necessary in this case.

## 18.2.13.4. EXAMPLES

C/C++ example:

    shmem_barrier(PE_start, logPE_stride, size, pSync);

Fortran example:

    INTEGER PSYNC(SHMEM_BARRIER_SYNC_SIZE)
    INTEGER PE_START, LOGPE_STRIDE, PE_SIZE, PSYNC
    DATA PSYNC /SHMEM_BARRIER_SYNC_SIZE*0/

    CALL SHMEM_BARRIER(PE_START, LOGPE_STRIDE, PE_SIZE, PSYNC)

See also

intro_shmem(3) shmem_barrier_all(3)