Question

I don’t know how to express my problem. The code is below, please tell me why the value of a got changed? In my opinion, the GET macro just returns the value of a, NOT the address of a…

#include <stdio.h>

#define GET(addr) (*(int *)(&addr))

int main()
{
    int a = 10;
    GET(a) = 20;
    printf("%d\n", a);
}

Answer 1

A result of dereferencing a pointer is an L-value, because it has an address; it certainly can be assigned.

Consider a simpler example: if p is an array or a pointer, you can certainly assign to p[0], as in

p[0] = 20;

Now recall that p[0] is the same as *p. Now expand your macro:

*((int*)&a) = 20;

This is equivalent to

(&a)[0] = 20;

which is certainly a valid (albeit somewhat unorthodox) assignment of the a itself.