Take function prototypes seriously

[Gnimuc]

MPI_Type_delete_attr_function is a function prototype, not a function pointer.

The type of type_delete_attr_fn is a function pointer(MPI_Type_delete_attr_function *). MPI_Type_delete_attr_function is a function prototype used for declaring this pointer.

int MPI_Type_create_keyval(MPI_Type_copy_attr_function *type_copy_attr_fn,
             MPI_Type_delete_attr_function *type_delete_attr_fn,
             int *type_keyval, void *extra_state)

This is the declaration of MPI_Type_delete_attr_function:

typedef int MPI_Type_delete_attr_function(MPI_Datatype datatype,
             int type_keyval, void *attribute_val, void *extra_state);

which is actually a function prototype and is generally used as a shortcut for declaring pointers.

A function pointer should be declared like this:

typedef int (MPI_Type_delete_attr_function*)(MPI_Datatype datatype,
             int type_keyval, void *attribute_val, void *extra_state);

So, the fix in #688 is not right. The root cause is that these "pointers" should not be defined as pointers(Ptr{Cvoid}).

[giordano]

Given the deluge of weird and inconsistent segfaults I'm still getting (see #684 (comment) and following), I'm somewhat glad #693 wasn't enough 😁