Question

I need help figuring out the correct data type for an assignment from a function call please.

I’m trying to get at the data in the content field of N_Vector u. Here’s what the documentation says about N_Vector:

The type N_Vector is defined as

N_Vector u;
tpedef struct _generic_N_Vector *N_Vector;
struct _generic_N_Vector {
  void *content;
  struct _generic_N_Vector_Ops *ops;
};

…

[The parallel NVECTOR module] defines the content field of N_Vector
to be a structure containing global and local lengths, a pointer to the
beginning of contiguous local data array, MPI communicator and flag.

struct _N_VectorContent_Parallel {
  long int local_length;
  long int global_length;
  booleantype own_data;
  realtype *data;
  MPI_Comm comm;
}

So I guess that means that content in _generic_N_Vector “points to” a structure of type _N_VectorContent_Parallel (right?).

Then I try to use a macro for accessing content. Here’s the documentation for NV_CONTENT_P.

v_cont=NV_CONTENT_P(v) sets v_cont to be a pointer to the N_Vector content
structure of type struct _N_VectorParallelContent.

Notice the different name of the struct!

What does that mean? What type do I declare v_cont to be?

I tried

N_Vector u;
...
_N_VectorParallelContent *v_cont1;
_N_VectorContent_Parallel *v_cont2;
v_cont1 = NV_CONTENT_P(u);
v_cont2 = NV_CONTENT_P(u);

but these declarations got the error “‘_N_VectorContent_Parallel’ undeclared…” or “‘_N_VectorParallelContent’ undeclared…”.

But it seems that these structures must be delcared already. I successfully declared (and used) u, of type N_Vector. And the docs seem to say that N_Vector contains one of those two structures (or maybe both).

So why the error message? What is the correct data type to declare for v_cont to receive data from NV_CONTENT_P?

I know this is a long, detailed question, but I don’t understand enough to whittle it down any more.
Thanks for your help.

Answer 1

I’m not familiar with this particular library, but it looks to me like the documentation is a little inconsistent.

Right after the blurb about NV_CONTENT_P(v), it says NV_CONTENT_P(v) is defined as:

#define NV_CONTENT_P(v) ( (N_VectorContent_Parallel)(v->content) )

So that version of the name is probably correct. I can’t see a definition for N_VectorContent_Parallel on that page, but it’s probably defined somewhere as something like struct _N_VectorContent_Parallel*. So, you can probably do:

N_VectorContent_Parallel v_cont1 = NV_CONTENT_P(u);

Remember that for structs, struct is part of the type name. This means that you’re getting errors in your example because you haven’t included struct:

// this is an unknown type
_N_VectorParallelContent *v_cont1; 

// this is a "struct _N_VectorParallelContent"
struct _N_VectorParallelContent *v_cont1; 

// But use this one, as it follows the macro
N_VectorContent_Parallel v_cont1;

---

If you want to see exactly what the preprocessor has done to your code, you can use gcc’s -E flag.

-E    Stop after the preprocessing stage; do not run the compiler proper. 
   The output is in  the form of preprocessed source code, which is sent to 
   the standard output.
   Input files which don't require preprocessing are ignored.

This is especially useful for seeing the results of macros and multiple complex header files.

---

Edit: From the source you’ve linked:

typedef struct _N_VectorContent_Parallel *N_VectorContent_Parallel;

This is a type definition that says that N_VectorContent_Parallel is the same as a struct _N_VectorContent_Parallel * (a pointer to a struct _N_VectorContent_Parallel), which means you can access v_cont1 using the -> syntax:

N_VectorContent_Parallel v_cont1;
printf("%d",v_cont1->local_length);

a->b is is shorthand for (*a).b – it’s just a cleaner-looking way of writing the dereference needed to accessing a member of a struct through a pointer to that struct. If that seems confusing, see my answer to this question.

Personally, I don’t like typedefs that hide pointers like this one, because it’s hard to tell by looking at the code whether you need to use a.b or a->b.