The Great and Terrible implementation of MPI-2

function index


Blocks until all processes in the communicator have reached this routine.
int MPI_Barrier(
  MPI_Comm comm

Input Parameter

[in] communicator (handle)


Blocks the caller until all processes in the communicator have called it; that is, the call returns at any process only after all members of the communicator have entered the call.

For MPI-2, comm may be an intercommunicator or an intracommunicator. If comm is an intercommunicator, the barrier is performed across all processes in the intercommunicator. In this case, all processes in the local group of the intercommunicator may exit the barrier when all of the processes in the remote group have entered the barrier.

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 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_Barrier.

#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;