extern keyword

The extern storage class specifier is used to access a global variable defined outside the scope of a file.

extern storage class specifier can also be used during the function call, when the function is defined outside the scope of the file.

The keyword extern is relevant only when your project consists of multiple files and you need to access a variable defined in one file from another file.

• extern keyword is used to extend the visibility of a function or variable.

If we have a C project with two or more files, we might want use a global variable defined in another file from our current file.

main.c

#include <stdio.h>

void file1_func(void);

int mainPrivateData;

int main(){
    mainPrivateData = 100;
    printf("mainPrivateData = %d\n", mainPrivateData);

	file1_func();
	printf("mainPrivateData = %d\n", mainPrivateData);
    return 0;
}

file1.c

extern int mainPrivateData;

void file1_func(void){
	mainPrivateData = 900;
}

output

mainPrivateData = 100
mainPrivateData = 900

Here we can access and modify global variables from another file using the extern keyword.