The Great and Terrible implementation of MPI-2

function index


Test whether an RMA exposure epoch has completed
int MPI_Win_test(
  MPI_Win win,
  int *flag


[in] window object (handle)
[out] success flag (logical)


This is the nonblocking version of MPI_Win_wait.

It returns flag = true if MPI_WIN_WAIT would return, flag = false, otherwise. The effect of return of MPI_WIN_TEST with flag = true is the same as the effect of a return of MPI_WIN_WAIT. If flag = false is returned, then the call has no visible effect.

MPI_WIN_TEST should be invoked only where MPI_WIN_WAIT can be invoked. Once the call has returned flag = true, it must not be invoked anew, until the window is posted anew.

Assume that window win is associated with a "hidden" communicator wincomm, used for communication by the processes of win. The rules for matching of post and start calls and for matching complete and wait call can be derived from the rules for matching sends and receives, by considering the following (partial) model implementation.


{ MPI_WIN_POST(group,0,win)}
initiate a nonblocking send with tag tag0 to each process in group, using wincomm. No need to wait for the completion of these sends.
{ MPI_WIN_START(group,0,win)}
initiate a nonblocking receive with tag tag0 from each process in group, using wincomm. An RMA access to a window in target process i is delayed until the receive from i is completed.
initiate a nonblocking send with tag tag1 to each process in the group of the preceding start call. No need to wait for the completion of these sends.
{ MPI_WIN_WAIT(win)}
initiate a nonblocking receive with tag tag1 from each process in the group of the preceding post call. Wait for the completion of all receives.

No races can occur in a correct program: each of the sends matches a unique receive, and vice-versa.

Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as malloc or other non-MPICH runtime routines that are themselves not interrupt-safe.

Notes for Fortran

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr at the end of the argument list. ierr is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the call statement.

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.


All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

No error; MPI routine completed successfully.
Invalid MPI window object
Other error; use MPI_Error_string to get more information about this error code.
Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK).

See Also

MPI_Win_wait, MPI_Win_post

Example Code

The following sample code illustrates MPI_Win_test.

#include "mpi.h" 
#include "stdio.h"

/* tests put and get with post/start/complete/test on 2 processes */

#define SIZE1 10
#define SIZE2 20

int main(int argc, char *argv[]) 

    int rank, destrank, nprocs, A[SIZE2], B[SIZE2], i;
    MPI_Group comm_group, group;
    MPI_Win win;
    int errs = 0, flag;

    if (nprocs != 2) {
        printf("Run this program with 2 processes\n");fflush(stdout);

    MPI_Comm_group(MPI_COMM_WORLD, &comm_group);

    if (rank == 0) {
        for (i=0; i<SIZE2; i++) A[i] = B[i] = i;
        MPI_Win_create(NULL, 0, 1, MPI_INFO_NULL, MPI_COMM_WORLD, &win); 
        destrank = 1;
        MPI_Group_incl(comm_group, 1, &destrank, &group);
        MPI_Win_start(group, 0, win);
        for (i=0; i<SIZE1; i++)
            MPI_Put(A+i, 1, MPI_INT, 1, i, 1, MPI_INT, win); 
        for (i=0; i<SIZE1; i++)
            MPI_Get(B+i, 1, MPI_INT, 1, SIZE1+i, 1, MPI_INT, win);


        for (i=0; i<SIZE1; i++) 
            if (B[i] != (-4)*(i+SIZE1)) {
                printf("Get Error: B[i] is %d, should be %d\n", B[i], (-4)*(i+SIZE1));fflush(stdout);
    else {  /* rank=1 */
        for (i=0; i<SIZE2; i++) B[i] = (-4)*i;
        MPI_Win_create(B, SIZE2*sizeof(int), sizeof(int), MPI_INFO_NULL, MPI_COMM_WORLD, &win);
        destrank = 0;
        MPI_Group_incl(comm_group, 1, &destrank, &group);
        MPI_Win_post(group, 0, win);
        flag = 0;
        while (!flag)
            MPI_Win_test(win, &flag);
        for (i=0; i<SIZE1; i++) {
            if (B[i] != i) {
                printf("Put Error: B[i] is %d, should be %d\n", B[i], i);fflush(stdout);


    return errs;