Extern Storage Class in C

Visibility or linkage of a variable specifies if the variable is visible to other files in a multi-file program.

Extern modifiers control the visibility of a global variable in a multi-file project. Functions default to extern.

// main.c
int i = 10;

void foo(void);
int main(void) {
	foo(); // will print i = 10;
	return 0;
}

// foo.c
extern int i;
void foo(void) {
	printf("i = %d\n", i);
}

This way, foo.c can access i, which is defined and declared in main.c.

void foo(void) {
	extern int i;
	printf("i = %d\n", i);
}

extern can be used within a scope to define that we are using a global variable defined in another file.

// main.c
char text[255];
char text2[255][50];

// foo.c
extern char text[];
extern char text2[][50];

When using an array, a one-dimensional array doesn't need to specify its size cause the compiler already knows it. But for a multidimensional array the nested sizes need to be defined.

extern int add(int a, int b);
// and
int add(int a, int b);

Are effectively the same cause functions default to extern.