你如何定义头文件中的函数？

Question

The setup

If I have a program like this

A header file that declares my main library function, primary() and defines a short simple helper function, helper() .

/* primary_header.h */
#ifndef _PRIMARY_HEADER_H
#define _PRIMARY_HEADER_H

#include <stdio.h>

/* Forward declare the primary workhorse function */
void primary();

/* Also define a helper function */
void helper()
{
    printf("I'm a helper function and I helped!\n");
}
#endif /* _PRIMARY_HEADER_H */

---

The implementation file for my primary function that defines it.

/* primary_impl.c */
#include "primary_header.h"
#include <stdio.h>

/* Define the primary workhorse function */
void primary()
{
    /* do the main work */
    printf("I'm the primary function, I'm doin' work.\n");

    /* also get some help from the helper function */
    helper();
}

---

a main() file that tests the code by calling primary()

/* main.c */
#include "primary_header.h"

int main()
{
    /* just call the primary function */
    primary();
}

---

The Problem

Using

gcc main.c primary_impl.c

does not link because the primary_header.h file gets included twice and therefore there is an illegal double definition of the function helper() . What is the correct way to structure the source code for this project such that double definitions do not happen?

Answer 1

You almost never write a function inside a header file unless it is marked to always be inlined. Instead, you write the function in a .c file and copy the function's declaration (not definition) to the header file so it can be used elsewhere.

Answer 2

You should only write your function's prototype in the header file, the body of your function should be written in a .c file.

Do this :

primary_header.c

/* primary_header.h */
#ifndef PRIMARY_HEADER_H
#define PRIMARY_HEADER_H

#include <stdio.h>

/* Forward declare the primary workhorse function */
void primary(void);

/* Also define a helper function */
void helper(void);

#endif /* PRIMARY_HEADER_H */

primary_impl.c

/* primary_impl.c */
#include "primary_header.h"
#include <stdio.h>

/* Define the primary workhorse function */
void primary()
{
    /* do the main work */
    printf("I'm the primary function, I'm doin' work.\n");

    /* also get some help from the helper function */
    helper();
}

void helper()
{
    printf("I'm a helper function and I helped!\n");
}

Edit: change _PRIMARY_HEADER_H to PRIMARY_HEADER_H . As @Jonathan Leffler and @Pablo said, underscore names are reserved identifiers

Answer 3

You can define a function in header files if it's weak linkage like:

// test.h
__attribute__((weak)) int test() {
    static int s = 0;
    return s++;
}

// a.c
#include "test.h"
#include <stdio.h>
void a(){
    print("%d", test());
}

// b.c
#include "test.h"
#include <stdio.h>
void b(){
    print("%d", test());
}

// main.c
#include "test.h"
#include <stdio.h>
void a();
void b();

void main(){
    a();
    b();
    print("%d", test());
}

cc ac bc main.c won't raise multiple definitions error and the output should be 012 as expected, meaning ac , bc and main.c share the same test function. You can achieve the same result in c++ by using inline .

Moreover, weak linkage can also be used on variable definition, allowing you to define and initialize a global variable in header files without source files (similar to inline static in c++).

Note:

Weak symbols are not mentioned by the C or C++ language standards.

So be careful when using it in c. But in c++, inline and inline static are portable form c++11 and c++17.