C Header define order/position

Question

Currently I'm trying to include a library in a C header file. Here is what the original code looks like:

#ifndef SIMULATE_H
#define SIMULATE_H

#ifdef __cplusplus
extern "C" {
#endif

#define RAM_SIZE 16320
#define STRING_SIZE 10
#define BUFFER_SIZE 100
long final_hex[RAM_SIZE];
char zero[STRING_SIZE];
char stringValue[STRING_SIZE];
char buffer[BUFFER_SIZE];
char checksum[BUFFER_SIZE];

#ifdef __cplusplus
}
#endif

#endif /* SIMULATE_H */

What I want to include is "#include <math.h>". It should be very easy, but I have some doubts here:

1. Does this code mean it can be a header file for both C and C++:

   #ifdef __cplusplus
   extern "C" {
   #endif
   

2. When I include libraries here, shall I put before the "extern "C"" or after it? Does the order matter for them here?

Answer 1

I'm going to focus on another aspect of this code:

long final_hex[RAM_SIZE];

Global data structures need to be defined as extern in header files. Otherwise, each source file which includes the header will try to define its own copy of the structure, which will result in an error at link time (probably something along the lines of "multiple definitions of symbol final_hex").

Answer 2

First of all, you probably meant #include not #define .

extern "C" makes a function-name in C++ have 'C' linkage

If you inspect math.h you'll see that it has it's own

#ifdef __cplusplus
extern "C" {
#endif
//...
#ifdef __cplusplus
}
#endif

sections. It is better not to include external headers inside your extern "C" section.

Here is a good basic explanation about includes, but with regards to your example:

• there is no difference currently if you include before or after your extern "C" section as you are not using anything out of it in the header
• should you use something of external header, you should include it on top, before usage, otherwise you will get an error
• if you are not planning to use any defines or types out of math.h, nesting include is not necessary and creates useless dependency, which doesn't matter while project is small, but increases build times in large projects - you should rather include those files separately on top of you source file

Answer 3

I don't have the reputation to add a comment but I'm curious if you mean you want to add

#include <math.h>

rather than

#define <math.h>

I'll assume that you you meant #include

You include other libraries like math.h when you need to use something defined in that library such as a function or structure.

The preprocessor, when it comes to the #include statement will replace the #include line with the contents of the math.h file.

You should only include a header file when something in your file references something in the header file. The header file that you have posted the code for does not, as far as I can see, make reference to anything in math.h so you should not include it in that file.

Instead you should have your source file (*.c) that uses declarations in the simulate.h file and presumably math.h include both of those files.

#include "simulate.h"
#include <math.h>

int main(void) {
    double root = sqrt(25.0); /* from math.h */
    int n = STRING_SIZE;      /* from simulate.h */
}

As far as your first question is concerned, the #ifdef _cplusplus is just a conditional compilation directive to the preprocessor. If the variable _cplusplus is defined (using a C++ compiler) then it will add the code between the #ifdef / #endif segment; otherwise it will not.

If you do include the math.h file in the simulate.h file then its location shouldn't matter as far as I'm aware assuming that you don't later make reference in simulate.h to something in math.h before #include <math.h>