int MPI_Win_start(
  MPI_Group group,
  int assert,
  MPI_Win win
);

Parameters

group

[in] group of target processes (handle)

assert

[in] Used to optimize this call; zero may be used as a default. See notes. (integer)

win

[in] window object (handle)

Remarks

Starts an RMA access epoch for win. RMA calls issued on win during this epoch must access only windows at processes in group. Each process in group must issue a matching call to MPI_WIN_POST. RMA accesses to each target window will be delayed, if necessary, until the target process executed the matching call to MPI_WIN_POST. MPI_WIN_START is allowed to block until the corresponding MPI_WIN_POST calls are executed, but is not required to.

The assert argument is used to indicate special conditions for the fence that an implementation may use to optimize the MPI_Win_start operation. The value zero is always correct. Other assertion values may be or'ed together. Assertions tha are valid for MPI_Win_start are:

MPI_MODE_NOCHECK

the matching calls to MPI_WIN_POST have already completed on all target processes when the call to MPI_WIN_START is made. The nocheck option can be specified in a start call if and only if it is specified in each matching post call. This is similar to the optimization of ready-send that may save a handshake when the handshake is implicit in the code. (However, ready-send is matched by a regular receive, whereas both start and post must specify the nocheck option.)

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

Errors

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.

MPI_SUCCESS

No error; MPI routine completed successfully.

MPI_ERR_WIN

Invalid MPI window object

MPI_ERR_OTHER

Other error; use MPI_Error_string to get more information about this error code.

Example Code

The following sample code illustrates MPI_Win_start.

#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;
 
    MPI_Init(&argc,&argv);
    MPI_Comm_size(MPI_COMM_WORLD,&nprocs);
    MPI_Comm_rank(MPI_COMM_WORLD,&rank);
    if (nprocs != 2) {
        printf("Run this program with 2 processes\n");fflush(stdout);
        MPI_Abort(MPI_COMM_WORLD,1);
    }
 
    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);
 
        MPI_Win_complete(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);
                errs++;
            }
    }
    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);
        MPI_Win_wait(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);
                errs++;
            }
        }
    }
 
    MPI_Group_free(&group);
    MPI_Group_free(&comm_group);
    MPI_Win_free(&win);
    MPI_Free_mem(A);
    MPI_Free_mem(B);
 
    MPI_Finalize();
    return errs;
}