What this macro return?

Question

I've seen using the return value of this macro, but from the definition I can't understnad which is the value that the execution returns.

   // packet parsing state machine helpers
    #define cursor_advance(_cursor, _len) \
      ({ void *_tmp = _cursor; _cursor += _len; _tmp; })

Answer 1

Macros don't return anything. Instead invocation of the macros are replaced inline with the body of the macro (with the macro arguments substituted).

So if you have the code

some_variable = cursor_advance(my_cursor, some_length);

then the preprocessor replaces that with

some_variable = ({ void *_tmp = my_cursor; my_cursor += some_length; _tmp; });

This in turn is an extension to the C language by the GCC C compiler called statement expression. It evaluates the statements between the ({ and }), and the result is the last expression. In the case of the example above the result is _tmp.

---

The whole expression

some_variable = ({ void *_tmp = my_cursor; my_cursor += some_length; _tmp; });

is equivalent to the statements

{
    void * _tmp = my_cursor;
    my_cursor += some_length;
    some_variable = _tmp;
}