What do the parentheses around a function name mean?

In one of my project source files, I found this C function definition:

int (foo) (int *bar)
{
    return foo (bar);
}

Note: there is no asterisk next to foo, so it's not a function pointer. Or is it? What is going on here with the recursive call?

In the absence of any preprocessor stuff going on, foo's signature is equivalent to

int foo (int *bar)

The only context in which I've seen people putting seemingly unnecessary parentheses around function names is when there are both a function and a function-like macro with the same name, and the programmer wants to prevent macro expansion.

This practice may seem a little odd at first, but the C library sets a precedent by providing some macros and functions with identical names.

One such function/macro pair is isdigit(). The library might define it as follows:

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

/* the function */
int (isdigit)(int c) /* avoid the macro through the use of parentheses */
{
  return isdigit(c); /* use the macro */
}

Your function looks almost identical to the above, so I suspect this is what's going on in your code too.

The parantheses don't change the declaration - it's still just defining an ordinary function called foo.

The reason that they have been used is almost certainly because there is a function-like macro called foo defined:

#define foo(x) ...

Using (foo) in the function declaration prevents this macro from being expanded here. So what is likely happening is that a function foo() is being defined with its body being expanded from the function-like macro foo.

The parentheses are meaningless.
The code you show is nothing but an infinite recursion.

When defining a function pointer, you sometimes see strange parentheses that do mean something. But this isn't the case here.