How do you output assembly with gcc?

[iw0XoG1T]

This question really is can you output assembly for a C or C++ program using gcc?

And if so what dialect is it?

If it can be done with gcc could you give me a demonstration of what you enter in the terminal/command prompt?

If it cannot be done how are others looking at the assembly produced by a C or C++ programs? In particularly what are they using and where can I get it?

[Moschops]

The switch is -S

g++ -S sourceFile.cpp

[iw0XoG1T]

@Moschops thankyou

	.file	"main.c"
	.def	___main;	.scl	2;	.type	32;	.endef
	.section .rdata,"dr"
LC0:
	.ascii "hello world!\0"
	.text
	.globl	_main
	.def	_main;	.scl	2;	.type	32;	.endef
_main:
LFB6:
	.cfi_startproc
	pushl	%ebp
	.cfi_def_cfa_offset 8
	.cfi_offset 5, -8
	movl	%esp, %ebp
	.cfi_def_cfa_register 5
	andl	$-16, %esp
	subl	$16, %esp
	call	___main
	movl	$LC0, (%esp)
	call	_printf
	movl	$0, %eax
	leave
	.cfi_restore 5
	.cfi_def_cfa 4, 4
	ret
	.cfi_endproc
LFE6:
	.def	_printf;	.scl	2;	.type	32;	.endef
 

Would anyone know by looking what dialect of assembly this is?

[chrisname]

It's x86 assembly, AT&T syntax.

To convert into normal x86 syntax:
1. Remove the % from in front of the registers: movl $LC0, (%esp) => movl $LC0, (esp)
2. Remove the $ from in front of constants: movl $LC0, (esp) => movl LC0, (esp)
3. Change () to []: movl LC0, (esp) => movl LC0, [esp]
4. Reverse the order of operands: movl LC0, [esp] => movl [esp], LC0
5. Convert instruction size suffixes into prefixes: mov [esp], dword LC0.

GNU as uses AT&T syntax for compatibility with the original UNIX as. It can assemble normal (Intel) syntax code if you put the directives

.intel_syntax
.noprefix

before the code in Intel syntax, but gcc can't produce Intel syntax code, so if you're using embedded assembly in your C code, you either have to write in AT&T syntax, or set it to use AT&T syntax after a block of Intel syntax code with .att_syntax.

[iw0XoG1T]

Thank you chrismane but that is not what I am trying to do.

What I am trying to do is learn to do some simple programming in assembly.

But all I have been able to find for references is either old, or manuals. So I wanted to learn how to create some simple assembly and translate it to nasm. If I can do that I think I will have enough information to use the old references and make them work for me.

This tread is been very useful so far and any additional help would be appreciated.

[chrisname]

NASM uses x86 assembly as well, it just uses Intel syntax instead of AT&T. I just told you how to convert from AT&T (GNU as) syntax to Intel (NASM) syntax.

[iw0XoG1T]

@Chrisname
Just so you know I am pretty sure you saved me about two hours of googling and reading. So thanks again.

[iw0XoG1T]

in the effort to make this thread complete here is my hello world nasm program:

global _main
extern _printf
section .data 
    fmt_string: db 'hello, world', 0xA, 0

section .text
    _main:
    sub esp, 4
    lea eax, [fmt_string]
    mov [esp], eax
    call _printf
    add esp, 4
    ret

command prompt

nasm -felf32 main.asm
gcc main.o -otest2

gcc version 4.7.2 (GCC) (MinGW)
NASM version 2.10.06



Now I have a lot of reading to do.

[chrisname]

Did it work?

[iw0XoG1T]

yes