Declaration vs. Definition of a variable in C

main.c

global.c

#include <stdio.h>
#include <stdlib.h>

// Declaration
//  - what you need when using the variable
//  (name + type)
// Definition
//  - what you need when executing the program
//  (address + space + Declaration)
extern int x;
int y = 5;

int main(int argc, char* argv[]) {
    x = 20;
    printf("x = %d\n", x);
    printf("y = %d\n", y);
    return 0;
}

Quickstart guide for C pointers

Pointers, double pointers, pointer arithmetics, dereferencing, everything in a short and compact guide that you can reference anytime!