Why declare a variable or function static in C?

Question

I understand what static does, but not why we use it. Is it just for keeping the abstraction layer?

Answer 1

Static functions in C

In C, functions are global by default. The “static” keyword before a function name makes it static. For example, below function fun() is static.

static int fun(void)
{
  printf("I am a static function ");
}

Unlike global functions in C, access to static functions is restricted to the file where they are declared. Therefore, when we want to restrict access to functions, we make them static. Another reason for making functions static can be reuse of the same function name in other files.

For example, if we store following program in one file file1.c

/* Inside file1.c */
static void fun1(void)
{
  puts("fun1 called");
}

And store following program in another file file2.c

/* Iinside file2.c  */
int main(void)
{
  fun1(); 
  getchar();
  return 0;  
}

Now, if we compile the above code with command gcc file2.c file1.c, we get the error undefined reference to fun1. This is because fun1 is declared static in file1.c and cannot be used in file2.c. See also the explanation here, where the codes come from.