I guessed the "one weird input parameter" would simply be the pointer to the structure to "return" multiple values, but it turns out I was wrong --- it is using the syntax of returning a structure by value, which depending upon the exact compiler, could be implemented in much the same way as an out-pointer parameter. i.e.

    struct Foo foo = return_foo();

becomes

    struct Foo foo;
    return_foo(&foo);

That said, the ability to pass, return, and assign structures by value seems to be one of the lesser-known features of C, so this serves as a good example.