#External Variables
In C, we call variables declared inside of a function internal, and variables declared outside of all functions external. Internal variables are scoped to the function in which they are declared. External variables are normally scoped from their declaration to the end of the file in which they are defined.
To define a variable in one file, and use is somewhere else, you must declare it
in the second file with the extern keyword.
When declaring a variable with extern, storage is not allocated for it. That
only happens once, in the place where it is defined.
Example:
// in file 1
int max_count = 100;// in file 2
extern int max_count;
for (int i = 0; i < max_count; i++) {
...References:
The C Programming Language Section 4.3