The Great and Terrible implementation of MPI-2

function index


Completes an RMA exposure epoch begun with MPI_Win_post
int MPI_Win_wait(
  MPI_Win win


[in] window object (handle)


Completes an RMA exposure epoch started by a call to MPI_WIN_POST on win. This call matches calls to MPI_WIN_COMPLETE(win) issued by each of the origin processes that were granted access to the window during this epoch. The call to MPI_WIN_WAIT will block until all matching calls to MPI_WIN_COMPLETE have occurred. This guarantees that all these origin processes have completed their RMA accesses to the local window. When the call returns, all these RMA accesses will have completed at the target window.

Figure 4 illustrates the use of these four functions:


Figure 4: active target communication. Dashed arrows represent synchronizations and solid arrows represent data transfer.

Process 0 puts data in the windows of processes 1 and 2 and process 3 puts data in the window of process 2. Each start call lists the ranks of the processes whose windows will be accessed; each post call lists the ranks of the processes that access the local window. The figure illustrates a possible timing for the events, assuming strong synchronization; in a weak synchronization, the start, put or complete calls may occur ahead of the matching post calls.

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.

Example Code

The following sample code illustrates MPI_Win_wait.

#include "mpi.h"
#include "stdio.h"
/* tests put and get with post/start/complete/wait on 2 processes */
#define SIZE1 100
#define SIZE2 200
int main(int argc, char *argv[])
int rank, destrank, nprocs, *A, *B, i;
    MPI_Group comm_group, group;
    MPI_Win win;
int errs = 0;
if (nprocs != 2) {
        printf("Run this program with 2 processes\n");fflush(stdout);
    i = MPI_Alloc_mem(SIZE2 *
sizeof(int), MPI_INFO_NULL, &A);
if (i) {
        printf("Can't allocate memory in test program\n");fflush(stdout);
        MPI_Abort(MPI_COMM_WORLD, 1);
    i = MPI_Alloc_mem(SIZE2 *
sizeof(int), MPI_INFO_NULL, &B);
if (i) {
        printf("Can't allocate memory in test program\n");fflush(stdout);
        MPI_Abort(MPI_COMM_WORLD, 1);

    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);

        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;