Acessing struct fields within an assembly X64 function

Question

Is it possible to access directly struct fields within an assembly function? And how can I access via assembly an global variable?

In inline assembly on intel syntax I can do this:

 struct str
 {
   int a;
   int b;
 }
 int someGlobalVar;

 __declspec(naked)   void __fastcall func(str * r)
 {
    __asm
    {
       mov dword ptr [ecx].a, 2
       mov dword ptr [ecx].b,someGlobalVar
    }
}

How do I do this in a assembly x64 function (not inline), with ATT syntax (gcc), if it's not possible how do I do this in an inline function?

Answer 1

For this any many similar problems, the easiest solution is to write an example in C that does what you want, then use gcc -m64 -S ... to generate assembler source, and then use that source as a template for your own assembly code.

Consider the following example:

#include <stdio.h>

typedef struct
{
    int a;
    int b;
} S;

int foo(const S *s)
{
    int c = s->a + s->b;

    return c;
}

int main(void)
{
    S s = { 2, 2 };

    printf("foo(%d, %d) = %d\n", s.a, s.b, foo(&s));

    return 0;
}

If we generate asm using gcc -Wall -O1 -m64 -S foo.c -o foo.S we get the following for the "foo" function:

.globl _foo
_foo:
LFB3:
    pushq   %rbp
LCFI0:
    movq    %rsp, %rbp
LCFI1:
    movl    (%rdi), %eax
    addl    4(%rdi), %eax
    leave
    ret

As you can see, movl (%rdi), %eax gets the element a of the struct, and then addl 4(%rdi), %eax adds element b, and the function result is returned in %eax.