The Great and Terrible implementation of MPI-2

function index


Begin an RMA access epoch at the target process.
int MPI_Win_lock(
  int lock_type,
  int rank,
  int assert,
  MPI_Win win


[in] Indicates whether other processes may access the target window at the same time (if MPI_LOCK_SHARED) or not (MPI_LOCK_EXCLUSIVE)
[in] rank of locked window (nonnegative integer)
[in] Used to optimize this call; zero may be used as a default. See notes. (integer)
[in] window object (handle)


Starts an RMA access epoch. Only the window at the process with rank rank can be accessed by RMA operations on win during that epoch.

The name of this routine is misleading. In particular, this routine need not block, except when the target process is the calling process.

Implementations may restrict the use of RMA communication that is synchronized by lock calls to windows in memory allocated by MPI_Alloc_mem. Locks can be used portably only in such memory.

The assert argument is used to indicate special conditions for the fence that an implementation may use to optimize the MPI_Win_fence operation. The value zero is always correct. Other assertion values may be or'ed together. Assertions that are valid for MPI_Win_fence are:

no other process holds, or will attempt to acquire a conflicting lock, while the caller holds the window lock. This is useful when mutual exclusion is achieved by other means, but the coherence operations that may be attached to the lock and unlock calls are still required.

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 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.
Invalid MPI window object

Example Code

The following sample code illustrates MPI_Win_lock.

#include "mpi.h" 
#include "stdio.h"
#include "stdlib.h"

/* tests passive target RMA on 2 processes */

#define SIZE1 100
#define SIZE2 200

int main(int argc, char *argv[]) 

    int rank, nprocs, A[SIZE2], B[SIZE2], i;
    MPI_Win win;
    int errs = 0;

    if (nprocs != 2) {
        printf("Run this program with 2 processes\n");fflush(stdout);

    if (rank == 0) {
        for (i=0; i<SIZE2; i++) A[i] = B[i] = i;
        MPI_Win_create(NULL, 0, 1, MPI_INFO_NULL, MPI_COMM_WORLD, &win); 

        for (i=0; i<SIZE1; i++) {
            MPI_Win_lock(MPI_LOCK_SHARED, 1, 0, win);
            MPI_Put(A+i, 1, MPI_INT, 1, i, 1, MPI_INT, win);
            MPI_Win_unlock(1, win);

        for (i=0; i<SIZE1; i++) {
            MPI_Win_lock(MPI_LOCK_SHARED, 1, 0, win);
            MPI_Get(B+i, 1, MPI_INT, 1, SIZE1+i, 1, MPI_INT, win);
            MPI_Win_unlock(1, win);


        for (i=0; i<SIZE1; i++) 
            if (B[i] != (-4)*(i+SIZE1)) {
                printf("Get Error: B[%d] is %d, should be %d\n", i, B[i], (-4)*(i+SIZE1));fflush(stdout);
    else {  /* rank=1 */
        for (i=0; i<SIZE2; i++) B[i] = (-4)*i;
        MPI_Win_create(B, SIZE2*sizeof(int), sizeof(int), MPI_INFO_NULL, MPI_COMM_WORLD, &win);

        for (i=0; i<SIZE1; i++) {
            if (B[i] != i) {
                printf("Put Error: B[%d] is %d, should be %d\n", i, B[i], i);

    return errs;