The Great and Terrible implementation of MPI-2

function index


Waits for any specified MPI Request to complete
int MPI_Waitany(
  int count,
  MPI_Request array_of_requests[],
  int *index,
  MPI_Status *status


[in] list length (integer)
[in/out] array of requests (array of handles)
[out] index of handle for operation that completed (integer). In the range 0 to count-1. In Fortran, the range is 1 to count.
[out] status object (Status). May be MPI_STATUS_IGNORE.


Blocks until one of the operations associated with the active requests in the array has completed. If more then one operation is enabled and can terminate, one is arbitrarily chosen. Returns in index the index of that request in the array and returns in status the status of the completing communication. (The array is indexed from zero in C, and from one in Fortran.) If the request was allocated by a nonblocking communication operation, then it is deallocated and the request handle is set to MPI_REQUEST_NULL.

The array_of_requests list may contain null or inactive handles. If the list contains no active handles (list has length zero or all entries are null or inactive), then the call returns immediately with index = MPI_UNDEFINED, and a empty status.

The execution of MPI_WAITANY(count, array_of_requests, index, status) has the same effect as the execution of MPI_WAIT(&array_of_requests[i], status), where i is the value returned by index (unless the value of index is MPI_UNDEFINED). MPI_WAITANY with an array containing one active entry is equivalent to MPI_WAIT.

If all of the requests are MPI_REQUEST_NULL, then index is returned as MPI_UNDEFINED, and status is returned as an empty status.

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.

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).

Example Code

The following sample code illustrates MPI_Waitany.

#include "mpi.h"
#include <stdio.h>

int main(int argc, char *argv[])
    int rank, size;
    int i, index;
    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]);
        for (i=0; i<size-1; i++)
            MPI_Waitany(size-1, request, &index, status);
        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;