The MPI standard does not specify that MPI_Comm = int and in fact OpenMPI uses a pointer value which lets the compiler do slightly more type checking. This type checking recently caused me trouble when building with-download-ml. There is a line in ml_comm.h which defines their communicator to be an int when ML_MPI is not defined. It was not immediately clear why this is not defined, but result is that the compiler chokes when building PETSc. This patch fixes the problem as long as sizeof(int) = sizeof(MPI_Comm), but this is *not* the case on x86_64 with OpenMPI. It's not clear to me how much of this is an upstream issue and how much is a configuration issue. Jed diff -r c074838b79ed src/ksp/pc/impls/ml/ml.c --- a/src/ksp/pc/impls/ml/ml.c Thu Mar 20 17:04:05 2008 -0500 +++ b/src/ksp/pc/impls/ml/ml.c Fri Mar 21 22:50:32 2008 +0100 @@ -815,7 +815,7 @@ PetscErrorCode MatWrapML_SHELL(ML_Operat MLcomm = mlmat->comm; ierr = PetscNew(Mat_MLShell,&shellctx);CHKERRQ(ierr); - ierr = MatCreateShell(MLcomm->USR_comm,m,n,PETSC_DETERMINE,PETSC_DETERMINE,shellctx,newmat);CHKERRQ(ierr); + ierr = MatCreateShell((MPI_Comm)MLcomm->USR_comm,m,n,PETSC_DETERMINE,PETSC_DETERMINE,shellctx,newmat);CHKERRQ(ierr); ierr = MatShellSetOperation(*newmat,MATOP_MULT,(void(*)(void))MatMult_ML);CHKERRQ(ierr); ierr = MatShellSetOperation(*newmat,MATOP_MULT_ADD,(void(*)(void))MatMultAdd_ML);CHKERRQ(ierr); shellctx->A = *newmat; @@ -844,7 +844,7 @@ PetscErrorCode MatWrapML_MPIAIJ(ML_Opera n = mlmat->invec_leng; if (m != n) SETERRQ2(PETSC_ERR_ARG_OUTOFRANGE,"m %d must equal to n %d",m,n); - ierr = MatCreate(mlmat->comm->USR_comm,&A);CHKERRQ(ierr); + ierr = MatCreate((MPI_Comm)mlmat->comm->USR_comm,&A);CHKERRQ(ierr); ierr = MatSetSizes(A,m,n,PETSC_DECIDE,PETSC_DECIDE);CHKERRQ(ierr); ierr = MatSetType(A,MATMPIAIJ);CHKERRQ(ierr); ierr = PetscMalloc3(m,PetscInt,&nnzA,m,PetscInt,&nnzB,m,PetscInt,&nnz);CHKERRQ(ierr);