The Great and Terrible implementation of MPI-2

function index


Blocking receive for a message
int MPI_Recv(
  void *buf,
  int count,
  MPI_Datatype datatype,
  int source,
  int tag,
  MPI_Comm comm,
  MPI_Status *status


[out] initial address of receive buffer (choice)
[in] maximum number of elements in receive buffer (integer)
[in] datatype of each receive buffer element (handle)
[in] rank of source (integer)
[in] message tag (integer)
[in] communicator (handle)
[out] status object (Status)


The receive buffer consists of the storage containing count consecutive elements of the type specified by datatype, starting at address buf. The length of the received message must be less than or equal to the length of the receive buffer. An overflow error occurs if all incoming data does not fit, without truncation, into the receive buffer.

If a message that is shorter than the receive buffer arrives, then only those locations corresponding to the (shorter) message are modified.

The count argument indicates the maximum length of a message; the actual length of the message can be determined with MPI_Get_count.

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 communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).
Invalid datatype argument. May be an uncommitted MPI_Datatype (see MPI_Type_commit).
Invalid count argument. Count arguments must be non-negative; a count of zero is often valid.
Invalid tag argument. Tags must be non-negative; tags in a receive (MPI_Recv, MPI_Irecv, MPI_Sendrecv, etc.) may also be MPI_ANY_TAG. The largest tag value is available through the the attribute MPI_TAG_UB.
Invalid source or destination rank. Ranks must be between zero and the size of the communicator minus one; ranks in a receive (MPI_Recv, MPI_Irecv, MPI_Sendrecv, etc.) may also be MPI_ANY_SOURCE.

Example Code

The following sample code illustrates MPI_Recv.

#include "mpi.h"
#include <stdio.h>
int main(int argc, char *argv[])
int rank, size, i;
int buffer[10];
    MPI_Status status;
    MPI_Init(&argc, &argv);
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
if (size < 2)
        printf("Please run with two processes.\n");fflush(stdout);
return 0;
    if (rank == 0)
for (i=0; i<10; i++)
            buffer[i] = i;
        MPI_Send(buffer, 10, MPI_INT, 1, 123, MPI_COMM_WORLD);
if (rank == 1)
        for (i=0; i<10; i++)
            buffer[i] = -1;
        MPI_Recv(buffer, 10, MPI_INT, 0, 123, MPI_COMM_WORLD, &status);
        for (i=0; i<10; i++)
            if (buffer[i] != i)
                printf("Error: buffer[%d] = %d but is expected to be %d\n", i, buffer[i], i);
    return 0;