Header File Clarification

Hiyas, I am wondering what exactly does go into a header file?

From Wiki and a number of other pages (and assignments), I know programmers can break up larger programs into separate routines and such, and at the end everything can be put together to make the "master" program work

From my understanding just function prototypes go into it, is that really true?

For example,

Code:

In blahOther.c (library of functions)

//proper headers

struct node
{
  void* data;
  struct node *left;
  struct node *right;
}

typedef struct node tree;

int add(int a, int b)
{
  return (a+b);
}

In blahTest.h

#ifndef BLAHTEST 
#define BLAHTEST

extern  //do I need to include the struct??????

/* adds two numbers */
extern int add(int a, int b);

#endif /* blahtest_h */


In blahTest.c (contains main)

#include "blahTest.h"

...

int triple(int x)
{
  return x*add(a,b);
}

Is that the jest of it or is there something major I am missing?

Also if I am correct, and in my library of functions file (blahOther.c) I had some structs, specifically a binary tree (NOT BST), would I need to provide the blahText.h the typedefs for those structs?

The whole purpose of my blahTest.c is just to test my library of functions.

Of course all this is considering, that I am compiling the following way.

gcc -c blah.c

gcc -c blahTest.c

gcc blah.o blahTest.o -o test

As a general rule headers contain no active code or variables... just definitions, prototypes and typedefs... So yes, you've got the idea.

You don't need extern for for function prototypes.
You do need it for variables declared in a different file...
You only need to include the struct if one of your functions uses it in a different file.

Sweet, TY CommonTater