How to open a file in assembler and modify it? - linux

Question

I'm starting to learn Assembler and I'm working in Unix. I want to open a file and write 'Hello world' on it.
section .data
textoutput db 'Hello world!', 10
lentext equ $ - textoutput
filetoopen db 'hi.txt'
section .text
global _start
_start:
mov eax, 5 ;open
mov ebx, filetoopen
mov ecx, 2 ;read and write mode
int 80h
mov eax, 4
mov ebx, filetoopen ;I'm not sure what do i have to put here, what is the "file descriptor"?
mov ecx, textoutput
mov edx, lentext
mov eax, 1
mov ebx, 0
int 80h ; finish without errors
But when I compile it, it doesn't do anything. What am I doing wrong?
When I open a file where does the file descriptor value return to?

Answer 1

This is x86 Linux (x86 is not the only assembly language, and Linux is not the only Unix!)...
section .data
textoutput db 'Hello world!', 10
lentext equ $ - textoutput
filetoopen db 'hi.txt'
The filename string requires a 0-byte terminator: filetoopen db 'hi.txt', 0
section .text
global _start
_start:
mov eax, 5 ;open
mov ebx, filetoopen
mov ecx, 2 ;read and write mode
2 is the O_RDWR flag for the open syscall. If you want the file to be created if it doesn't already exist, you will need the O_CREAT flag as well; and if you specify O_CREAT, you need a third argument which is the permissions mode for the file. If you poke around in the C headers, you'll find that O_CREAT is defined as 0100 - beware of the leading zero: this is an octal constant! You can write octal constants in nasm using the o suffix.
So you need something like mov ecx, 0102o to get the right flags and mov edx, 0666o to set the permssions.
int 80h
The return code from a syscall is passed in eax. Here, this will be the file descriptor (if the open succeeded) or a small negative number, which is a negative errno code (e.g. -1 for EPERM). Note that the convention for returning error codes from a raw syscall is not quite the same as the C syscall wrappers (which generally return -1 and set errno in the case of an error)...
mov eax, 4
mov ebx, filetoopen ;I'm not sure what do i have to put here, what is the "file descriptor"?
...so here you need to mov ebx, eax first (to save the open result before eax is overwritten) then mov eax, 4. (You might want to think about checking that the result was positive first, and handling the failure to open in some way if it isn't.)
mov ecx, textoutput
mov edx, lentext
Missing int 80h here.
mov eax, 1
mov ebx, 0
int 80h ; finish without errors

Answer 2

Did you read the Linux Assembly HOWTO? It covers your question.
You can also compile some C code with gcc -S -fverbose-asm -O1 and look at the generated assembly. For example, with foo.c, run gcc -S -Wall -fverbose-asm -O1 foo.c (as a command in some terminal) then look (using some editor -perhaps GNU emacs) into the generated foo.s assembler file.
At last, I don't think it is worth bothering a lot about assembler. In 2020, a recent GCC compiler will surely generate better code than what you could write (if you invoke it with optimizations, at least -O2). See this draft report for more.

Answer 3

This is a x64 Linux sample
; Program to open and write to file
; Compile with:
; nasm -f elf64 -o writeToFile64.o writeToFile64.asm
; Link with:
; ld -m elf_x86_64 -o writeToFile64 writeToFile64.o
; Run with:
; ./writeToFile64
;==============================================================================
; Author : Rommel Samanez
;==============================================================================
global _start
%include 'basicFunctions.asm'
section .data
fileName: db "testFile.txt",0
fileFlags: dq 0102o ; create file + read and write mode
fileMode: dq 00600o ; user has read write permission
fileDescriptor: dq 0
section .rodata ; read only data section
msg1: db "Write this message to the test File.",0ah,0
msglen equ $ - msg1
msg2: db "File Descriptor=",0
section .text
_start:
mov rax,2 ; sys_open
mov rdi,fileName ; const char *filename
mov rsi,[fileFlags] ; int flags
mov rdx,[fileMode] ; int mode
syscall
mov [fileDescriptor],rax
mov rsi,msg2
call print
mov rax,[fileDescriptor]
call printnumber
call printnewline
; write a message to the created file
mov rax,1 ; sys_write
mov rdi,[fileDescriptor]
mov rsi,msg1
mov rdx,msglen
syscall
; close file Descriptor
mov rax,3 ; sys_close
mov rdi,[fileDescriptor]
syscall
call exit

Answer 4

It depends what assembler you are using and if you expect to be using the C runtime or not. In this case which appears to be the Hello World text example from rosettacode they are using nasm. Given you have a _start field you are not needing the C runtime so you assemble this to an elf object file and link it into a program:
nasm -felf hello.asm
ld hello.o -o hello
Now you can run the hello program.
A slightly more portable example that uses the C runtime to do the work rather than linux syscalls might look like the sample below. If you link this as described it can use printf to do the printing.
;;; helloworld.asm -
;;;
;;; NASM code for Windows using the C runtime library
;;;
;;; For windows - change printf to _printf and then:
;;; nasm -fwin32 helloworld.asm
;;; link -subsystem:console -out:helloworld.exe -nodefaultlib -entry:main
;;; helloworld.obj msvcrt.lib
;;; For gcc (linux, unix etc):
;;; nasm -felf helloworld.asm
;;; gcc -o helloworld helloworld.o
extern printf
section .data
message:
db 'Hello, World', 10, 0
section .text
global main
main:
push dword message ; push function parameters
call printf ; call C library function
add esp, 4 ; clean up the stack
mov eax, 0 ; exit code 0
ret
For information about file descriptors - read the open(2) manual page or look at wikipedia. It is how posix refers to an open i/o stream. In your case, stdout.