The Great and Terrible implementation of MPI-2

function index


Waits for some given MPI Requests to complete
int MPI_Waitsome(
  int incount,
  MPI_Request array_of_requests[],
  int *outcount,
  int array_of_indices[],
  MPI_Status array_of_statuses[]


[in] length of array_of_requests (integer)
[in] array of requests (array of handles)
[out] number of completed requests (integer)
[out] array of indices of operations that completed (array of integers)
[out] array of status objects for operations that completed (array of Status). May be MPI_STATUSES_IGNORE.


Waits until at least one of the operations associated with active handles in the list have completed. Returns in outcount the number of requests from the list array_of_requests that have completed. Returns in the first outcount locations of the array array_of_indices the indices of these operations (index within the array array_of_requests; the array is indexed from zero in C and from one in Fortran). Returns in the first outcount locations of the array array_of_status the status for these completed operations. If a request that completed was allocated by a nonblocking communication call, then it is deallocated, and the associated handle is set to MPI_REQUEST_NULL.

If the list contains no active handles, then the call returns immediately with outcount = MPI_UNDEFINED.

When one or more of the communications completed by MPI_WAITSOME fails, then it is desirable to return specific information on each communication. The arguments outcount, array_of_indices and array_of_statuses will be adjusted to indicate completion of all communications that have succeeded or failed. The call will return the error code MPI_ERR_IN_STATUS and the error field of each status returned will be set to indicate success or to indicate the specific error that occurred. The call will return MPI_SUCCESS if no request resulted in an error, and will return another error code if it failed for other reasons (such as invalid arguments). In such cases, it will not update the error fields of the statuses.

The array of indicies are in the range 0 to incount - 1 for C and in the range 1 to incount for Fortran.

Null requests are ignored; if all requests are null, then the routine returns with outcount set to MPI_UNDEFINED.

While it is possible to list a request handle more than once in the array_of_requests, such an action is considered erroneous and may cause the program to unexecpectedly terminate or produce incorrect results.

MPI_Waitsome provides an interface much like the Unix select or poll calls and, in a high qualilty implementation, indicates all of the requests that have completed when MPI_Waitsome is called. However, MPI_Waitsome only guarantees that at least one request has completed; there is no guarantee that all completed requests will be returned, or that the entries in array_of_indices will be in increasing order. Also, requests that are completed while MPI_Waitsome is executing may or may not be returned, depending on the timing of the completion of the message.

Note on status for send operations

For send operations, the only use of status is for MPI_Test_cancelled or in the case that there is an error, in which case the MPI_ERROR field of status will be set.

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_Request. Either null or, in the case of a MPI_Start or MPI_Startall, not a persistent request.
Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK).
The actual error value is in the MPI_Status argument. This error class is returned only from the multiple-completion routines (MPI_Testall, MPI_Testany, MPI_Testsome, MPI_Waitall, MPI_Waitany, and MPI_Waitsome). The field MPI_ERROR in the status argument contains the error value or MPI_SUCCESS (no error and complete) or MPI_ERR_PENDING to indicate that the request has not completed.
The MPI Standard does not specify what the result of the multiple completion routines is when an error occurs. For example, in an MPI_WAITALL, does the routine wait for all requests to either fail or complete, or does it return immediately (with the MPI definition of immediately, which means independent of actions of other MPI processes)? MPICH has chosen to make the return immediate (alternately, local in MPI terms), and to use the error class MPI_ERR_PENDING (introduced in MPI 1.1) to indicate which requests have not completed. In most cases, only one request with an error will be detected in each call to an MPI routine that tests multiple requests. The requests that have not been processed (because an error occured in one of the requests) will have their MPI_ERROR field marked with MPI_ERR_PENDING.

Example Code

The following sample code illustrates MPI_Waitsome.

#include "mpi.h"
#include <stdio.h>

int main(int argc, char *argv[])
    int rank, size;
    int i, index[4], count, remaining;
    int buffer[400];
    MPI_Request request[4];
    MPI_Status status[4];

    MPI_Init(&argc, &argv);
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    if (size > 4)
        printf("Please run with 4 processes.\n");fflush(stdout);
        return 1;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    if (rank == 0)
        for (i=0; i<size * 100; i++)
            buffer[i] = i/100;
        for (i=0; i<size-1; i++)
            MPI_Isend(&buffer[i*100], 100, MPI_INT, i+1, 123, MPI_COMM_WORLD, &request[i]);
        remaining = size-1;
        while (remaining > 0)
            MPI_Waitsome(size-1, request, &count, index, status);
            if (count > 0)
                printf("%d sends completed\n", count);fflush(stdout);
                remaining = remaining - count;
        MPI_Recv(buffer, 100, MPI_INT, 0, 123, MPI_COMM_WORLD, &status[0]);
        printf("%d: buffer[0] = %d\n", rank, buffer[0]);fflush(stdout);

    return 0;