If you create a function with a keyword static in front of it then that function can only be accessed from the same file. It cannot be called from any other file.
Example: While creating module for Linux Kernel, functions are normally declared as static, so there is no conflict with other modules that may have functions with the same name.
static function_name()
{
int bla; bla; bla;
}
{
int bla; bla; bla;
}