I can't tell for sure if this is a correct program, but multiple members of the MPI Forum suggested it is. If it is a correct program, it appears to expose a bug in MPICH, because the MPI_Waitall hangs. Thanks, Jeff $ mpicc -g -Wall -std=c99 cancel-sucks.c && mpiexec -n 4 ./a.out $ mpichversion MPICH Version: 3.2b1 MPICH Release date: unreleased development copy MPICH Device: ch3:nemesis MPICH configure: CC=gcc-4.9 CXX=g++-4.9 FC=gfortran-4.9 F77=gfortran-4.9 --enable-cxx --enable-fortran --enable-threads=runtime --enable-g=dbg --with-pm=hydra --prefix=/opt/mpich/dev/gcc/default --enable-wrapper-rpath --enable-static --enable-shared MPICH CC: gcc-4.9 -g -O2 MPICH CXX: g++-4.9 -g -O2 MPICH F77: gfortran-4.9 -g -O2 MPICH FC: gfortran-4.9 -g -O2 #include <stdio.h> #include <stdlib.h> #include <mpi.h> const int n=1000; int main(void) { MPI_Init(NULL,NULL); int size, rank; MPI_Comm_size(MPI_COMM_WORLD, &size); MPI_Comm_rank(MPI_COMM_WORLD, &rank); if (size<2) { printf("You must use 2 or more processes!\n"); MPI_Finalize(); exit(1); } MPI_Request reqs[2*n]; int target = (rank+1)%size; for (int i=0; i<n; i++) { MPI_Issend(NULL,0,MPI_BYTE,target,0,MPI_COMM_WORLD,&(reqs[i])); } srand((unsigned)(rank+MPI_Wtime())); int slot = rand()%n; printf("Cancelling send %d.\n", slot); MPI_Cancel(&reqs[slot]); #if 1 MPI_Barrier(MPI_COMM_WORLD); #endif int origin = (rank==0) ? (size-1) : (rank-1); for (int i=0; i<n; i++) { MPI_Irecv(NULL,0,MPI_BYTE,origin,0,MPI_COMM_WORLD,&(reqs[n+i])); } MPI_Status stats[2*n]; MPI_Waitall(2*n,reqs,stats); for (int i=0; i<n; i++) { int flag; MPI_Test_cancelled(&(stats[i]),&flag); if (flag) { printf("Status %d indicates cancel was successful.\n", i); } } MPI_Finalize(); return 0; } -- Jeff Hammond [email protected] http://jeffhammond.github.io/