That's K&R C, predating ANSI C. It's not that uncommon if you're looking at long-lived codebases. Rather it should look like this: int foo(x, y) int

That's K&R C, predating ANSI C. It's not that uncommon if you're looking at long-lived codebases. Rather it should look like this: int foo(x, y) int x; int y; { return x + y; } x and y would be assumed to be int in the absence of the type specifiers, so in this case they're optional.