NetCDF 4.10.1 does not build with MPICH

[eschnett]

I am building NetCDF 4.10.1 against MPICH for aarch64-linux-gnu. This fails because configure thinks that MPI_Comm_f2c is not available (although it is), and aarch64 is a 64-bit system.

The build error comes from

./libdispatch/dparallel.c:460:        #error "MPI_Comm_f2c unavailable on a 64-bit system: cannot safely convert Fortran comm handle"

I assume that the configure mis-detection is caused by the cmake lines

  CHECK_SYMBOL_EXISTS(MPI_Comm_f2c "mpi.h" HAVE_MPI_COMM_F2C)
  CHECK_SYMBOL_EXISTS(MPI_Info_f2c "mpi.h" HAVE_MPI_INFO_F2C)

which expect that there is a symbol MPI_Comm_f2c. MPICH provides no such symbol, it provides a macro instead.

This configure setting

CONFIGURE_OPTIONS="$CONFIGURE_OPTIONS ac_cv_func_MPI_Comm_f2c=yes ac_cv_func_MPI_Info_f2c=yes"

avoids the problem.

[eschnett]

This code in dparallel.c

    /* Only safe if sizeof(MPI_Comm) == sizeof(int) (MPI-1 only)
       See https://github.com/Unidata/netcdf-c/issues/3199 */
    #if SIZEOF_VOIDP != 4
        #error "MPI_Comm_f2c unavailable on a 64-bit system: cannot safely convert Fortran comm handle"
    #endif

is wrong in general. It is correct for OpenMPI, where MPI handles are pointers, but is wrong e.g. for MPICH where MPI handles are C integers. The condition should probably be something like sizeof(MPI_Comm) != sizeof(int) instead. However, even in this case it is not guaranteed that the internal representation of an MPI handle in C and in Fortran are the same.