通过基地址+ XX的偏移量和struct_name-> XX访问结构成员XX的区别

Question

I am using an open source library(Easy C State Machine). When I review the code, I found the struct is accessed by base address + offset of X, something like follows.

#define OFF_SET_OF_XX 1
#define GET_MEMBER(X, Y, Z) ((X)(*((X*)(((int*)(Y))+Z))))

typedef struct Foo {
int i;
int XX;
}Foo;

Foo *foo1 = alloc(...);
int j = GET_MEMBER(int, foo1, OFF_SET_OF_XX);

instead of:

Foo *foo1 = alloc(...);
int j = foo1->XX;

What's the difference between the above two? Any advantage of the former one?

The library is written in ANSI C. But it is said that it can only support two architecture: x86 and amd64. Since I want to use it on another arch, I reviewed the source code and find the only architecture-related code is the OFF_SET_OF_XXs.

Answer 1

First of all, I'd like to point out the left-most cast to type X is not necessary so you could simplify it to:

#define GET_MEMBER(X, Y, Z) (*((X*)(((int*)(Y))+Z)))

Still, I don't see the purpose of using a macro like this. It throws away type safety and the syntax is cumbersome.

The code that the compiler generates for the two different ways should be pretty similar, if not identical. But what compiler and platform is this library supposed to be compiled on? If there happened to be a performance gain from using GET_MEMBER in that compiler then that could be why they are using it.

EDIT 1: My guess is that the original author of the library did not know about structs and invented his own thing similar to structs using alloc, pointers, and macros. Now that he has made the library open source, others are trying to convert it to use structs but there is a lot of code to convert and they haven't finished yet.

Answer 2

The closest thing I have seen with a legitimate purpose (borderline, IMO) is the Linux kernel's container_of macro.

But for the example you give, there is absolutely no reason to do this other than "the author is a moron". It gains nothing in performance, sacrifices readability, and invites annoying bugs (eg, what happens if somebody decides to change i to a long someday?)