How to save a file in assembly language with nasm in Linux x86? - linux

Question

I'm just wondering how I can throw a buffer to a file. I know I can set up the registers like:
mov eax, 4
mov ebx, (file descriptor here)
mov ecx, myBuffer
mov edx, myBufferLen
int 80h
and then close the file with:
mov eax, 6
int 80h
but I'm not sure how to get the file descriptor. Someone told me that whenever you open a file, after calling the service dispatcher, the eax has the file descriptor. Whatever I try, it doesn't create a new file or save the current one.

Answer 1

mov eax, 5 ; __NR_open
mov ebx, filename ; zero terminated!
mov ecx, 2 ; O_WRITE? Check me on this!
mov edx, 777q ; permissions - Check me on this, too!
int 80h
; did we succeed?
cmp eax, -4096
ja exit
mov [descriptor], eax
; and so on...
;...
xor eax, eax ; claim no error
exit:
mov ebx, eax
neg ebx
mov eax, 1 ; __NR_exit
int 80h
I'm uncertain on the "magic numbers" for open flags and permissions, and too lazy to look them up right now (fs.h?). You may need to "or" the open flags with O_CREATE to create a new file(?). The trick at "exit:" negates the (negative) ERRNO (if any) so you can read it easily with "echo $?". Something like that...

Answer 2

Thanks to #gnometorule for the solution. So this is how you store a buffer into file:
in this case you are receiving the file name as a first parameter when you run the program.
section .text
global _start
_start:
pop ebx ; this is the amount of parameter received (in this case 2)
pop ebx ; this is the direction of our program
pop ebx ; this is the first parameter (our file name eg. "text.txt")
so we open it and move it to a buffer to edit or whatever we want with it.
textEdit:
;we open the file
mov eax, 5
push ebx ; we save the file name in the stack for later use
mov ecx,0
int 80h
; and then we move the information read to buffer
mov eax, 3
mov ebx, eax ; really dont know how this works, but it does.
mov ecx, fileBuffer
mov edx, fileBufferSize
int 80h
now fileBuffer (wich is a buffer defined in section .bss) has what we had in the file we opened. Here you can edit the information. When we want to save it back to the file it goes like this:
saveFile:
;open the file
mov eax, 5
pop ebx ; the file name was stored in the stack,
push ebx ; so we retrieve it and save it back for later use
mov ecx, 2 ; ecx has the access mode ("2"aloud us to read/write)
mov edx, $0666
int 80h
;now we are going to write to the file
mov ebx, eax ; eax has the file descriptor we opened. so we move it to ebx.
mov eax, 4 ; we are going to use the write service (#4)
mov ecx, fileBuffer ; ecx has the adress of what we want to write
mov edx, fileBufferSize ; edx has the amount of bytes we want to write
int 80h
; close the file
mov eax, 6
int 80h
jmp exit ; jump wherever you want, or end your program... etc.
This worked for me.