Question

GNU nano 2.2.4               File: argv.s                                     

.section .text

    .globl _start

_start:

    movl    %esp, %ebp

    movl    8(%ebp),%ecx
    movl    $4,%edx
    movl    $1,%ebx
    movl    $4,%eax
    int $0x80

    movl    $1,%eax
    movl    $0,%ebx
    int $0x80

let me preface this question by saying that i’m (obviously) new to asm and any random general advice not related to the topic is welcome…
I meant to simply read in the argv[1] from the bash terminal and print it back out. This was intended to be a test to ensure that I actually knew how to take arguments. If I enter an integer, like 2. The program prints 2O. capital ‘o’, not zero. why?

Answer 1

You need to understand that argv[1] is not an integer as you would like, but a string terminated null instead! A string is sequence of bytes ending with '\0'.

This means that when you pass the character 2 through the cmd-line to your app, this string in memory will look like [2][\0], which consists of only 2 bytes. The problem is that you specify the size in %edx as being 4, which is not true, and will make write() print another 2 bytes of memory garbage to your screen.

The right way to do what you want is implement the strlen() function on your application. This function iterates through the stream of bytes counting how many bytes there are before the character \0 shows up. Only then you’ll be able to print the correct string. Use the number returned by strlen() as %edx when you write().