pub unsafe extern "C" fn PMPI_Register_datarep(
datarep: *const c_char,
read_conversion_fn: MPI_Datarep_conversion_function,
write_conversion_fn: MPI_Datarep_conversion_function,
dtype_file_extent_fn: MPI_Datarep_extent_function,
extra_state: *mut c_void,
) -> c_int