Why is this code parentheses on both sides of the function name?

If I hadn't pre-processed it, int (foo) (int *bar) is the same as int foo(int *bar).

I'm not sure there's an entire source code, Usually parentheses the function name to prevent the macro from expanding when the function and the macro have the same name

/* macro */
#define isdigit(c) ...

/* /* function */
int (isdigit)(intc) // use parentheses to distinguish from macros. Higher Priority
{
  return isdigit(c); // using macros
}