Question

I just started down the Assembly road, and one of the first “Hello, World!” tutorials I found http://asm.sourceforge.net/intro/hello.html, gives a nice way of psudo-dynamicly getting the length of the string to enter into the system call.

section .data
msg db 'Hello, World!",0xa
len equ $ - msg

This works great in nasm, and everything assembles, links, and runs with out question.

The problem comes when I try to find a way to do the same thing in gas.

I understand that the $ in this case is a token that evaluates to the current assembly position http://www.csie.ntu.edu.tw/~comp03/nasm/nasmdoc3.html#section-3.5

Can this expression ($ – msg) be expressed in gas, or is this nasm exclusive syntactic sugar?

Answer 1

For x86, you can use “.” in the same way. e.g.

    .data

msg:
    .ascii "Hello, World!"
    .byte 0xa
    .equ len, . - msg

(Note: this is not necessarily true for other platforms. gas supports many platforms, and various aspects of syntax vary between them!)