Question

I’m new to asm and using string in.

1/I’ve got a string and I want to return the adress of the first caracter.
(I call the asm function in the C main).

I’ve tried :

movl  $str, %eax

According to me, it returns the string.

With movl $(str), %eax it returns the adress of the first caracter of the string ?
If I want to return the adress of the second character in the string, I don’t understand how I can do that in asm.

I diplay result in the C program like that :

printf("string : %s, adress : %d\n", function_asm(), function_asm()).

It returns me “string : programmation, adress : 134520852”
I think that “134520852” is not an adress and there is something I don’t understand.

2/I’ve seen there are %edi, %esi and functions for string but I can’t find a good (easy) tutorial using that. I’ve just understand that %edi is for index source and %esi for index destination…do you know some links about it ?

thanks in advance !
🙂

Answer 1

If you know how to get the address of the first character of a string into eax, the address of the second charcter can be obtained with a simple inc %eax (increment eax register).

I’m not sure why you think 134520852 is not the address of your string, especially if that string is indeed "programmation" – that would be prrof that the return from function_asm() was okay. Perhaps you would be happier printing it as a pointer, with %p.

And you’re basically correct about %edi and %esi though you have them the wrong way around. %esi is the source one and %edi is the destination one.