Linux Format forums

[pk_fox]

Hi all, I'm trying to write x number of lines to a text file using this code but it only writes 1 line ! any ideas as to what I am doing wrong


section .data
hello db 'Hello, world!',10 ; Our dear string
helloLen equ $ - hello ; Length of our dear string

section .bss
loopctr resd 1

section .text
global _start

_start:
mov dword [loopctr],10
pop ebx ; argc (argument count)
pop ebx ; argv[0] (argument 0, the program name)
pop ebx ; The first real arg, a filename

mov eax,8 ; The syscall number for creat() (we already have the filename in ebx)
mov ecx,00644Q ; Read/write permissions in octal (rw_rw_rw_)
int 80h ; Call the kernel
; Now we have a file descriptor in eax

test eax,eax ; Lets make sure the file descriptor is valid
js skipWrite ; If the file descriptor has the sign flag
; (which means it's less than 0) there was an oops,
; so skip the writing. Otherwise call the filewrite "procedure"
call fileWrite

skipWrite:
mov ebx,eax ; If there was an error, save the errno in ebx
mov eax,1 ; Put the exit syscall number in eax
int 80h ; Bail out

; proc fileWrite - write a string to a file
fileWrite:
mov ebx,eax ; sys_creat returned file descriptor into eax, now move into ebx
mov eax,4 ; sys_write
; ebx is already set up
mov ecx,hello ; We are putting the ADDRESS of hello in ecx
mov edx,helloLen ; This is the VALUE of helloLen because it's a constant (defined with equ)
jmp myloop

myloop:
dec dword [loopctr]
cmp dword [loopctr],0
je end
int 80h
jmp myloop


; endp fileWrite

end:
mov eax,6 ; sys_close (ebx already contains file descriptor)
int 80h
ret

[SheamusPatt]

Your problem is that a syscall, including 'write', returns a result in eax. So, you can't simply set the syscall number (in eax) and assume it's going to remain unchanged after the call, as you're doing in your write loop.

Set up the syscall number before each call in the 'myloop:' section, and it should work for you.
_________________
Sheamus
Ottawa, CANADA

[pk_fox]

Thanks Sheamus, I will give it a shot

[pk_fox]

[pk_fox]

Hi Sheamus, this is what I have now and it works fine, what I would like to do is add a line number to my output and can't for the life of me find a way of doing it ? any ideas as to how I can achieve this ?


section .data
hello db ' ) Hello, world!',10 ; Our dear string
helloLen equ $ - hello ; Length of our dear string

section .bss
loopctr resd 1
looplimit resd 1

section .text
global _start

_start:
mov dword [loopctr],0
mov dword [looplimit],1000
pop ebx ; argc (argument count)
pop ebx ; argv[0] (argument 0, the program name)
pop ebx ; The first real arg, a filename

mov eax,8 ; The syscall number for creat() (we already have the filename in ebx)
mov ecx,00644Q ; Read/write permissions in octal (rw_rw_rw_)
int 80h ; Call the kernel
; Now we have a file descriptor in eax

test eax,eax ; Lets make sure the file descriptor is valid
js skipWrite ; If the file descriptor has the sign flag
; (which means it's less than 0) there was an oops,
; so skip the writing. Otherwise call the filewrite "procedure"
call fileWrite

skipWrite:
mov ebx,eax ; If there was an error, save the errno in ebx
mov eax,1 ; Put the exit syscall number in eax
int 80h ; Bail out

; proc fileWrite - write a string to a file
fileWrite:
mov ebx,eax ; sys_creat returned file descriptor into eax, now move into ebx
mov eax,4 ; sys_write
; ebx is already set up
mov ecx,hello ; We are putting the ADDRESS of hello in ecx
mov edx,helloLen ; This is the VALUE of helloLen because it's a constant (defined with equ)
jmp myloop

myloop:
push eax
int 80h
pop eax
inc dword [loopctr]
push edx
mov edx,[loopctr]
cmp dword edx,[looplimit]
pop edx
je end
jmp myloop


; endp fileWrite

end:
mov eax,6 ; sys_close (ebx already contains file descriptor)
int 80h
ret

[M-Saunders]

What do you mean by line number here -- ie referring to which file?

M

[pk_fox]

[M-Saunders]

Ah. In that case you'll need a routine to convert a numeric value into its printable ASCII equivalent, and then print it. If you've got LXF156, the lib_print_number routine in library.asm does the trick. If not, I can post it here...

M

[pk_fox]

Thanks Mike , slight problem I've got the mag but not the dvd which has the code

[pk_fox]

[pk_fox]

[M-Saunders]

OK, you probably need to change the output stream. Put your current code into pastebin.com and I'll have a look at it!

M

[pk_fox]

[M-Saunders]

OK, here is a version with lib_print_number included from library.asm, and modified slightly to use the file descriptor. I've marked the additional lines with "XXX" in the comments:

http://pastebin.com/hNQjZE8R

This puts the number before each line in the file. Any questions, just shout!

M

[pk_fox]