How to declare function pointer in header and c-file?

Question

I'm a little confused over how to declare a function pointer in a header file. I want to use it in main and a file called menus.c and declare it in menus.h I assume. We want to initialize to point to a certain function.

it looks like this:

void (*current_menu)(int);

What do we write in menus.c, menus.h and main?

Drew Dormann · Answer 1 · 2011-12-19T14:33:46.563

25

A function pointer is still a pointer, meaning it's still a variable.

If you want a variable to be visible from several source files, the simplest solution is to declare it extern in a header, with the definition elsewhere.

In a header:

extern void (*current_menu)(int);

In one source file:

void (*current_menu)(int) = &the_func_i_want;

edited Dec 19 '11 at 14:33

answered Dec 19 '11 at 14:28

Drew Dormann

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the ampersand isn't strictly necessary – Dave Dec 19 '11 at 16:27
1

That is true, @Dave. In C, it's a stylistic choice of mine for clarity. In C++ it's sometimes required in certain template contexts, so I just do it for consistency. – Drew Dormann Dec 19 '11 at 17:06
@DrewDormann I thought `extern` was redundant. Shouldn't it be globally visible without specifying `extern`? – Geremia Jun 11 '15 at 21:05
1

@Geremia [How do I use extern to share variables between source files in C?](http://stackoverflow.com/questions/1433204) – Drew Dormann Jun 11 '15 at 21:22

score 8 · Answer 2 · answered Dec 19 '11 at 14:34

It's often helpful to use typedef with function pointers, so you can name the type to something descriptive:

typedef void (*MenuFunction)(int);

Then you would have a global variable of this type, probably in menus.c, and declared (with extern) in menus.h:

static void my_first_menu_function(int x)
{
  printf("the menu function got %d\n", x);
}

MenuFunction current_menu = my_first_menu_function;

From main.c, you can then do:

#include "menu.h"

current_menu(4711);

to call whatever function is currently pointed at by current_menu.

Very true! `function` is also available if you have access to boost or C++11. — Drew Dormann, Dec 19 '11 at 15:12

score 0 · Answer 3 · answered Dec 19 '11 at 14:24

A pointer function itself does not have a function definition. It's nothing more than a pointer to a type, the type being specified by the return type of the function and the parameter list. What you need to do is define a function with the same parameter list and return type, then use your pointer function to hold that function's address. You can then call the function through the pointer.

How to declare function pointer in header and c-file?

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