Alternative (K&R) C syntax for function declaration versus prototypes

Question

The question you are asking is really two questions, not one. Most replies so far tried to cover the entire thing with a generic blanket "this is K&R style" answer, while in fact only a small part of it has anything to do with what is known as K&R style (unless you see the entire C language as "K&R-style" in one way or another :)

The first part is the strange syntax used in function definition

int func(p, p2)
void *p;
int  p2; /* <- optional in C89/90, but not in C99 */
{
  return 0;
}

This one is actually a K&R-style function definition. Other answer have covered this pretty well. And there's not much to it, actually. The syntax is deprecated, but still fully supported even in C99 (except for "no implicit int" rule in C99, meaning that in C99 you can't omit the declaration of p2).

The second part has little to do with K&R-style. I refer to the fact that the function can be called with "swapped" arguments, i.e. no parameter type checking takes place in such a call. This has very little to do with K&R-style definition per se, but it has everything to do with your function having no prototype. You see, in C when you declare a function like this

int foo();

it actually declares a function foo that takes an unspecified number of parameters of unknown type. You can call it as

foo(2, 3);

and as

j = foo(p, -3, "hello world");

ans so on (you get the idea);

Only the call with proper arguments will "work" (meaning that the others produce undefined behavior), but it is entirely up to you to ensure its correctness. The compiler is not required to diagnose the incorrect ones even if it somehow magically knows the correct parameter types and their total number.

Actually, this behavior is a feature of C language. A dangerous one, but a feature nevertheless. It allows you to do something like this

void foo(int i);
void bar(char *a, double b);
void baz(void);

int main()
{
  void (*fn[])() = { foo, bar, baz };
  fn[0](5);
  fn[1]("abc", 1.0);
  fn[2]();
}

i.e. mix different function types in a "polymorphic" array without any typecasts (variadic function types can't be used here though). Again, inherent dangers of this technique are quite obvious (I don't remember ever using it, but I can imagine where it can be useful), but that's C after all.

Finally, the bit that links the second part of the answer to the first. When you make a K&R-style function definition, it doesn't introduce a prototype for the function. As far as function type is concerned, your func definition declares func as

int func();

i.e. neither the types nor the total number of parameters are declared. In your original post you say "... it seems to specify is how many params it uses ...". Formally speaking, it doesn't! After your two-parameter K&R-style func definition you still can call func as

func(1, 2, 3, 4, "Hi!");

and there won't be any constraint violation in it. (Normally, a quality compiler will give you a warning).

Also, a sometimes overlooked fact is that

int f()
{
  return 0;
}

is also a K&R-style function definition that does not introduce a prototype. To make it "modern" you'd have to put an explicit void in the parameter list

int f(void)
{
  return 0;
}

Finally, contrary to a popular belief, both K&R-style function definitions and non-prototyped function declarations are fully supported in C99. The former has been deprecated since C89/90, if I remember correctly. C99 requires the function to be declared before the first use, but the declaration is not required to be a prototype. The confusion apparently stems from the popular terminological mix-up: many people call any function declaration "a prototype", while in fact "function declaration" is not the same thing as "prototype".

This is pretty old K&R C syntax (pre-dates ANSI/ISO C). Nowadays, you should not use it anymore (as you have already noticed its major disadvantage: the compiler won't check the types of arguments for you). The argument type actually defaults to int in your example.

At the time, this syntax was used, one sometimes would find functions like

foo(p, q) 
{
    return q + p;
}

which was actually a valid definition, as the types for foo, p, and q default to int.

This is simply an old syntax, that pre-dates the "ANSI C" syntax you might be more familiar with. It's called "K&R C", typically.

Compilers support it to be complete, and to be able to handle old code bases, of course.

That's a relic from when C had no prototypes for functions. Way back then, (I think) functions were assumed to return int and all its arguments were assumed to be int. There was no checking done on function parameters.

You're much better off using function prototypes in the current C language.
And you must use them in C99 (C89 still accepts the old syntax).

And C99 requires functions to be declared (possibly without a prototype). If you're writing a new function from scratch, you need to provide a declaration ... make it a prototype too: you lose nothing and gain extra checking from the compiler.

This is the original K&R syntax before C was standardized in 1989. C89 introduced function prototypes, borrowed from C++, and deprecated the K&R syntax. There is no reason to use it (and plenty of reasons not to) in new code.