Accessing element in array with assembly

Question

Assume the following function:

void asmFunction(const int *first, ...) {
    __asm {
        xor eax, eax
        add eax, [first][0]
        ; ...
    }
}

It is being called this way:

int first[] =  { 0, 0, 0, 0, 0, 0, 0, 5, 7, 6, 2, 4, 3, 5 };
asmFunction2(first, ...);

As far as I understand, the second assembly line must add 0 element of array first to eax. However, a random value is being added. When debugging, first[0] equals 0, as it must be. What is the problem with the code?

I code in Visual Studio 2013 on 64-bit machine.

I'm not an expert, but shouldn't it be add eax, [first + 0]? With [] you dereference the first pointer/address. And for element 1: add eax, [first + 4], etc. — meaning-matters
– meaning-matters, Commented Dec 3, 2014 at 12:33
@meaning-matters As far as I know, [first][0] means the same as [first + 0], but I’m not sure. — Ivan Akulov
– Ivan Akulov, Commented Dec 3, 2014 at 13:26

Jester · Accepted Answer · 2014-12-03 12:38:15Z

1

That is a strange syntax and probably doesn't do what you want. If you disassemble the generated code you will most likely see something like add eax, [ebp+8]. The random value added should be the value of first, which is a pointer. You have effectively done eax += first. To get at the elements you need a level of indirection, that is eax += *first. For example this could work:

mov edx, [first]
add eax, [edx]

answered Dec 3, 2014 at 12:38

Jester

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2 Comments

Ivan Akulov Over a year ago

Interesting. Unfortunately, currently I’m unable to test it. Is the add eax, [[first]] syntax acceptable or wrong?

Jester Over a year ago

x86 doesn't support double indirection, so even if some assembler accepts that form, it's not going to work either.

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