## shmem_int_cswap

#### Purpose

Performs an atomic conditional swap to a symmetric data object.

#### C syntax

    #include <shmem.h>

    int shmem_int_cswap(int *target, int cond, int value, int pe);

#### Parameters

INPUT
target
  The remotely accessible integer data object to be updated on the PE.
cond
  cond is compared to the target value. If cond and the target are equal, then value is swapped into the target. Otherwise, the target is unchanged. In either case, the old value of the target is returned as the function return value. cond must be of the same data type as target.
value
  Value to be atomically written to the target PE. Value is the same type as target.
pe
  An integer indicating the PE number on which target is to be updated.

#### Description

This conditional swap routine conditionally update a target data object on an arbitrary processing element (PE) and return prior contents of the data object in one atomic operation.

The function shmem_int_cswap performs an atomic conditional operation on a remotely accessible integer data object.

The atomic accessing of the shared variable is guaranteed only when that variable is updated solely using IBM openshmem functions.

#### Return values

This function returns the contents that had been at the target address on the remote PE prior to the swap is returned.

#### IBM NOTES

To utilize the hardware atomic operations support of the PERCS system, the variables of these operations should be aligned with their natural byte alignments. For example, an int variable should be 4-byte aligned; a long long variable should be 8-byte aligned. If the participating variables are not aligned, the operation is done in software with sub-optimal performance

#### C examples

    #include <stdlib.h>
    #include <stdio.h>

    #include <shmem.h>

    static int gIntValue_cswap;
    static int gIntValue_add;
    static int gIntValue_fadd;

    static long gLock;

    int main (int argc, char* argv[])
    {
        int total_tasks = -1;
        int my_task = -1;

        start_pes(0);

        total_tasks = _num_pes();

        if (total_tasks <= 0) {
            printf("FAILED\n");
            exit(1);
        } else {
            printf("number of pes is %d\n", total_tasks);
        }

        if (total_tasks < 2 || total_tasks % 2) {
            printf("FAILED: The number of pes should be an even number. (at least 2)\n");
            exit(1);
        }

        my_task = _my_pe();

        if (my_task < 0){
            printf("FAILED\n");
            exit(1);
        } else {
            printf("my pe id is %d\n", my_task);
        }

        gIntValue_cswap = -1;
        shmem_barrier_all();

        printf("remote cswap operations ...\n");

        if (my_task != 0) {
            int oldval = shmem_int_cswap(&gIntValue_cswap, -1, my_task, 0);
            if (oldval == -1) {
                printf("pe %d was the winner\n", my_task);
            } else {
                printf("pe %d was a loser\n", my_task);
            }
        }

        gIntValue_add = 0;
        shmem_barrier_all();

        printf("remote add operations ...\n");
        shmem_int_add(&gIntValue_add, 1, 0);

        gIntValue_fadd = 0;
        shmem_barrier_all();
        if (my_task == 0) {
            printf("initial value was 0, current value is %d after %d remote add operations\n",
                gIntValue_add, total_tasks);
        }

        printf("remote fadd operations ...\n");
        int oldVal = shmem_int_fadd(&gIntValue_fadd, 1, 0);
        printf("old value before remote fadd is %d\n", oldVal);

        shmem_barrier_all();

        printf("remote lock operations ...\n");
        shmem_set_lock(&gLock);
        printf("pe %d got the lock\n", my_task);
        shmem_clear_lock(&gLock);

        shmem_barrier_all();

        printf("PASSED\n");
        return 0;
    }

#### Related information

Subroutines: shmem_swap

OpenSHMEM API Index