The Great and Terrible implementation of MPI-2

function index


Waits for an MPI request to complete
int MPI_Wait(
  MPI_Request *request,
  MPI_Status *status


[in] request (handle)
[out] status object (Status). May be MPI_STATUS_IGNORE.


A call to MPI_WAIT returns when the operation identified by request is complete. If the communication object associated with this request was created by a nonblocking send or receive call, then the object is deallocated by the call to MPI_WAIT and the request handle is set to MPI_REQUEST_NULL. MPI_WAIT is a non-local operation.

The call returns, in status, information on the completed operation. The content of the status object for a receive operation can be accessed. The status object for a send operation may be queried by a call to MPI_TEST_CANCELLED.

One is allowed to call MPI_WAIT with a null or inactive request argument. In this case the operation returns immediately with empty status.

Advice to users.

Successful return of MPI_WAIT after a MPI_IBSEND implies that the user send buffer can be reused --- i.e., data has been sent out or copied into a buffer attached with MPI_BUFFER_ATTACH. Note that, at this point, we can no longer cancel the send. If a matching receive is never posted, then the buffer cannot be freed. This runs somewhat counter to the stated goal of MPI_CANCEL (always being able to free program space that was committed to the communication subsystem).

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.

The status argument must be declared as an array of size MPI_STATUS_SIZE, as in integer status(MPI_STATUS_SIZE).


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_Wait.

#include "mpi.h"
#include <stdio.h>
int main(int argc, char *argv[])
int myid, numprocs, left, right;
    int buffer[10], buffer2[10];
    MPI_Request request;
    MPI_Status status;
    MPI_Comm_size(MPI_COMM_WORLD, &numprocs);
    MPI_Comm_rank(MPI_COMM_WORLD, &myid);
    right = (myid + 1) % numprocs;
    left = myid - 1;
    if (left < 0)
        left = numprocs - 1;
    MPI_Irecv(buffer, 10, MPI_INT, left, 123, MPI_COMM_WORLD, &request);
    MPI_Send(buffer2, 10, MPI_INT, right, 123, MPI_COMM_WORLD);
    MPI_Wait(&request, &status);
    return 0;