Hello, in my mpi program, I create 3 groups and communicators: forward_rank = (int *)malloc((numprocs-1) * sizeof(int)); backward_rank = (int *)malloc((numprocs-1) * sizeof(int)); comp_rank = (int *)malloc((numprocs-2) * sizeof(int)); for (i=0; i<numprocs-1; i++){ backward_rank[i] = i+1; forward_rank[i] = i; } for (i=0; i<numprocs-2; i++){ comp_rank[i] = i+1; } MPI_Group global_group, fgroup, bgroup, cgroup; // forward group, backward group, computation group MPI_Comm MPI_FOR_WORLD, MPI_BACK_WORLD, MPI_COMP_WORLD; // forward, backward, computation communicator MPI_Comm_group(MPI_COMM_WORLD, &global_group); MPI_Group_incl(global_group, numprocs-1, forward_rank, &bgroup); MPI_Comm_create(MPI_COMM_WORLD, bgroup, &MPI_BACK_WORLD); MPI_Group_incl(global_group, numprocs-1, backward_rank, &fgroup); MPI_Comm_create(MPI_COMM_WORLD, fgroup, &MPI_FOR_WORLD); MPI_Group_incl(global_group, numprocs-2, comp_rank, &cgroup); MPI_Comm_create(MPI_COMM_WORLD, cgroup, &MPI_COMP_WORLD); And I used them for my RMA operation So in my main function transfer new communicator to another c file as: udp_sub(win, &MPI_FOR_WORLD); image_sub(rank, win, strip_buff, &MPI_FOR_WORLD, &MPI_BACK_WORLD, &MPI_COMP_WORLD); in udp_sub and image_sub function: I called: MPI_Win_create(image_buff, 2*image_info->image_size*sizeof(uint16), sizeof(uint16), MPI_INFO_NULL, *MPI_FOR_WORLD, win); but I got the error when I execute them: MPI_Win_create(189): MPI_Win_create(base=0x7fe50c06c010, size=9216000, disp_unit=2, MPI_INFO_NULL, MPI_COMM_NULL, win=0x1a5e940) failed MPI_Win_create(116): Null communicator I already transfer the communicator address to it, but I don't understand why communicator is null. or I did in a wrong way? Any suggestions are appreciate. Thank you! -- Best Regards, Sufeng Niu ECASP lab, ECE department, Illinois Institute of Technology Tel: 312-731-7219