Using keyword extern in C

In this post I will show why keyword extern is useful in large projects on C language with many files.

Let’s make a simple project. Create a file logger.h:

int max_callstack_depth = 3;

Then create file utils.h

#include <stdio.h>
void handle_error();

Here we only declare method handle_error.

Implement method in utils.c

#include "utils.h"
void handle_error() {
  int a = max_callstack_depth;
  printf("max_callstack_depth: %d\n", a);
}

Create file main.c

#include <stdio.h>
#include "logger.h"
#include "utils.h"
int main() {
  handle_error();
}

Let’s compile it:

gcc main.c logger.h utils.c -o main

You will get error:

This means that compiler failed to find our variable max_callstack_depth. To fix this we need to add line in file utils.c

extern int max_callstack_depth;

The whole content of utils.c is following

#include "utils.h"
extern int max_callstack_depth;
void handle_error() {
  int a = max_callstack_depth;
  printf("max_callstack_depth: %d\n", a);
}

The “extern” keyword is used to declare and define the external or global variables. Using keyword extern we signal compiler that variable max_callstack_depth is global and should be available across all files in project.