In the C variables and types post I introduced how to work with variables.

In this post I want to mention the difference between global and local variables.

A local variable is defined inside a function, and it’s only available inside that function.

Like this:

#include <stdio.h>

int main(void) {
  char j = 0;
  j += 10;
  printf("%u", j); //10
}

j is not available anywhere outside the main function.

A global variable is defined outside of any function, like this:

#include <stdio.h>

char i = 0;

int main(void) {
  i += 10;
  printf("%u", i); //10
}

A global variable can be accessed by any function in the program. Access is not limited to reading the value: the variable can be updated by any function.

Due to this, global variables are one way we have of sharing the same data between functions.

The main difference with local variables is that the memory allocated for variables is freed once the function ends.

Global variables are only freed when the program ends.