How to correctly use the extern keyword in C

Question

extern changes the linkage. With the keyword, the function / variable is assumed to be available somewhere else and the resolving is deferred to the linker.

There's a difference between extern on functions and on variables.

For variables it doesn't instantiate the variable itself, i.e. doesn't allocate any memory. This needs to be done somewhere else. Thus it's important if you want to import the variable from somewhere else.

For functions, this only tells the compiler that linkage is extern. As this is the default (you use the keyword static to indicate that a function is not bound using extern linkage) you don't need to use it explicitly.

extern tells the compiler that this data is defined somewhere and will be connected with the linker.

With the help of the responses here and talking to a few friends here is the practical example of a use of extern.

Example 1 - to show a pitfall:

stdio.h:

int errno;

myCFile1.c:

#include <stdio.h>

// Code using errno...

myCFile2.c:

#include <stdio.h>

// Code using errno...

If myCFile1.o and myCFile2.o are linked, each of the c files have separate copies of errno. This is a problem as the same errno is supposed to be available in all linked files.

Example 2 - The fix.

stdio.h:

extern int errno;

stdio.c:

int errno;

myCFile1.c:

#include <stdio.h>

// Code using errno...

myCFile2.c:

#include <stdio.h>

// Code using errno...

Now if both myCFile1.o and MyCFile2.o are linked by the linker they will both point to the same errno. Thus, solving the implementation with extern.

It has already been stated that the extern keyword is redundant for functions.

As for variables shared across compilation units, you should declare them in a header file with the extern keyword, then define them in a single source file, without the extern keyword. The single source file should be the one sharing the header file's name, for best practice.

Many years later, I discover this question. After reading every answer and comment, I thought I could clarify a few details... This could be useful for people who get here through Google search.

The question is specifically about using extern functions, so I will ignore the use of extern with global variables.

Let's define 3 function prototypes:

// --------------------------------------
// Filename: "my_project.H"
extern int function_1(void);
static int function_2(void);
       int function_3(void);

The header file can be used by the main source code as follows:

// --------------------------------------
// Filename: "my_project.C"
#include "my_project.H"

void main(void) {
    int v1 = function_1();
    int v2 = function_2();
    int v3 = function_3();
}

int function_2(void) return 1234;

In order to compile and link, we must define function_2 in the same source code file where we call that function. The two other functions could be defined in different source code *.C or they may be located in any binary file (*.OBJ, *.LIB, *.DLL), for which we may not have the source code.

Let's include again the header my_project.H in a different *.C file to understand better the difference. In the same project, we add the following file:

// --------------------------------------
// Filename: "my_big_project_splitted.C"
#include "my_project.H"

void old_main_test(void){
    int v1 = function_1();
    int v2 = function_2();
    int v3 = function_3();
}

int function_2(void) return 5678;

int function_1(void) return 12;

int function_3(void) return 34;

Important features to notice:

• When a function is defined as static in a header file, the compiler/linker must find an instance of a function with that name in each module which uses that include file.

• A function which is part of the C library can be replaced in only one module by redefining a prototype with static only in that module. For example, replace any call to malloc and free to add memory leak detection feature.

• The specifier extern is not really needed for functions. When static is not found, a function is always assumed to be extern.

• However, extern is not the default for variables. Normally, any header file that defines variables to be visible across many modules needs to use extern. The only exception would be if a header file is guaranteed to be included from one and only one module.

  Many project managers would then require that such variable be placed at the beginning of the module, not inside any header file. Some large projects, such as the video game emulator "Mame" even require that such variables appears only above the first function using them.

In C, extern is implied for function prototypes, as a prototype declares a function which is defined somewhere else. In other words, a function prototype has external linkage by default; using extern is fine, but is redundant.

(If static linkage is required, the function must be declared as static both in its prototype and function header, and these should normally both be in the same .c file).

A very good article that I came about the extern keyword, along with the examples: http://www.geeksforgeeks.org/understanding-extern-keyword-in-c/

Though I do not agree that using extern in function declarations is redundant. This is supposed to be a compiler setting. So I recommend using the extern in the function declarations when it is needed.