The Great and Terrible implementation of MPI-2

function index


Determines the rank of the calling process in the communicator
int MPI_Comm_rank(
  MPI_Comm comm,
  int *rank


[in] communicator (handle)
[out] rank of the calling process in the group of comm (integer)


MPI_COMM_RANK indicates the rank of the process that calls it in the range from size-1, where size is the return value of MPI_COMM_SIZE.


This function is equivalent to accessing the communicator's group with MPI_COMM_GROUP (see above), computing the rank using MPI_GROUP_RANK, and then freeing the temporary group via MPI_GROUP_FREE. However, this function is so commonly used, that this shortcut was introduced. ( End of rationale.)

Advice to users.

This function gives the rank of the process in the particular communicator's group. It is useful, as noted above, in conjunction with MPI_COMM_SIZE.

Many programs will be written with the master-slave model, where one process (such as the rank-zero process) will play a supervisory role, and the other processes will serve as compute nodes. In this framework, the two preceding calls are useful for determining the roles of the various processes of a communicator.

Thread and Interrupt Safety

This routine is both thread- and interrupt-safe. This means that this routine may safely be used by multiple threads and from within a signal handler.

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 communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).

Example Code

The following sample code illustrates MPI_Comm_rank.

#include "mpi.h"
#include <stdio.h>
int main(int argc, char *argv[])
    int rank, nprocs;

    printf("Hello, world.  I am %d of %d\n", rank, nprocs);fflush(stdout);
return 0;