there are 3 main uses for the static.

1. If you declare within a function:

It retains the value between function calls

2.If it is declared for a function name:

By default function is extern..so it will be visible from other files if the function declaration is as static..it is invisible for the outer files

3. Static for global variables:

By default we can use the global variables from outside files If it is static global..that variable is limitied to with in the file