All the variables we’ve seen so far have been local variables – variables that are defined within a function. These variables are only visible within that function.

Consider this function:

int count(void) 
{
	int sum = 0;
	sum++;
	return sum;
}

Each time we call count, the sum variable is set back to 0, and the return value is 1. sum does not retain its value across function calls.

If we did want this function to keep track of how many times it had been called, we could store sum as a global variable. Global variables are declared outside any function, and are visible to any function in the same file:

int sum = 0;
int count(void) 
{
	sum++;
	return sum;
}

Now, sum does not get set back to 0 each time the function is called.

Global variables should be declared at the top of the file, near the function prototypes (but before any function implementation). If a global variable is declared in the middle of a file, some compilers will not allow you to refer to that variable in any function that comes before its declaration.