MPI_Win_shared_query give nullpointer with 1 process

[hakostra]

I'm using MPICH 4.2.1 on Linux and have the example program:

#include <stdio.h>
#include <mpi.h>

int main() {
    int myid, numprocs, ierr;
    int disp_unit;
    MPI_Aint winsize, nelems;
    MPI_Comm shmcomm;
    void *baseptr;
    MPI_Win win;

    // Initialize MPI
    ierr = MPI_Init(NULL, NULL);
    ierr = MPI_Comm_rank(MPI_COMM_WORLD, &myid);
    ierr = MPI_Comm_size(MPI_COMM_WORLD, &numprocs);
    ierr = MPI_Comm_split_type(MPI_COMM_WORLD, MPI_COMM_TYPE_SHARED, 0,
        MPI_INFO_NULL, &shmcomm);

    // Allocate shared memory
    winsize = 1024;

    ierr = MPI_Win_allocate_shared(winsize, 1, MPI_INFO_NULL, shmcomm,
        &baseptr, &win);
    printf("Allocated shared memory segment: %ld %d\n", winsize, ierr);
    printf("  Value of pointer: %p\n", baseptr);

    ierr = MPI_Allreduce(&winsize, &nelems, 1, MPI_AINT, MPI_SUM, shmcomm);
    printf("Allreduce: %ld %ld\n", winsize, nelems);

    ierr = MPI_Win_shared_query(win, MPI_PROC_NULL, &winsize, &disp_unit,
        &baseptr);
    printf("Got winsize: %ld %d\n", winsize, ierr);
    printf("  Value of pointer: %p\n", baseptr);

    // Clean up
    ierr = MPI_Win_free(&win);
    ierr = MPI_Comm_free(&shmcomm);
    ierr = MPI_Finalize();

    return 0;
}

When compiling mpicc shmem.c and running with two or more processes on a single node/workstation everything looks good:

$ mpirun -prepend-rank -n 2 ./a.out 
[0] Allocated shared memory segment: 1024 0
[0]   Value of pointer: 0x7e5818510000
[0] Allreduce: 1024 2048
[0] Got winsize: 1024 0
[0]   Value of pointer: 0x7e5818510000
[1] Allocated shared memory segment: 1024 0
[1]   Value of pointer: 0x7052b7629400
[1] Allreduce: 1024 2048
[1] Got winsize: 1024 0
[1]   Value of pointer: 0x7052b7629000

When running the program using a single process, the winsize and baseptr returned from MPI_Win_shared_query is null:

$ mpirun -prepend-rank -n 1 ./a.out 
[0] Allocated shared memory segment: 1024 0
[0]   Value of pointer: 0x561e2bf07560
[0] Allreduce: 1024 1024
[0] Got winsize: 0 0
[0]   Value of pointer: (nil)

I tried the example with Open MPI and Intel MPI and both works as expected (i.e. I get a non-null pointer and winsize), both with a single and multiple processes.

Could this be a bug in MPICH? Thanks for all comments and help.

[hakostra]

Small follow up:

1. Most recent code from github produce the same (nullpointer) result
2. MPICH 4.1.3 gives the expected result also running a single process:

$ mpirun -prepend-rank -n 1 ./a.out 
[0] Allocated shared memory segment: 1024 0
[0]   Value of pointer: 0x5cc2898d6260
[0] Allreduce: 1024 1024
[0] Got winsize: 1024 0
[0]   Value of pointer: 0x5cc2898d6260

[hzhou]

Thanks for reporting. Fix coming...