How to read and display the contents of a text file in nasm? - linux

Question

I want to read and display the contents of a text file using nasm and Linux system calls. My text file is named "new.txt". I wrote the following code and am receiving no output on the terminal.
section .data
line db "This is George,",
db " and his first line.", 0xa, 0
len equ $ - line
line2 db "This is line number 2.", 0xa, 0
len2 equ $ - line2
filename: db 'ThisIsATestFile.txt', 0
section .bss
bssbuf: resb len ;any int will do here, even 0,
file: resb 4 ;since pointer is allocated anyway
global _start
section .text
_start:
; open file in read-only mode
mov eax, 5 ;sys_open file with fd in ebx
mov ebx, filename ;file to be opened
mov ecx, 0 ;O_RDONLY
int 80h
cmp eax, 0 ;check if fd in eax > 0 (ok)
jbe error ;can not open file
mov ebx, eax ;store new (!) fd of the same file
; read from file into bss data buffer
mov eax, 3 ;sys_read
mov ecx, bssbuf ;pointer to destination buffer
mov edx, len ;length of data to be read
int 80h
js error ;file is open but cannot be read
cmp eax, len ;check number of bytes read
jb close ;must close file first
; write bss data buffer to stderr
mov eax, 4 ;sys_write
push ebx ;save fd on stack for sys_close
mov ebx, 2 ;fd of stderr which is unbuffered
mov ecx, bssbuf ;pointer to buffer with data
mov edx, len ;length of data to be written
int 80h
pop ebx ;restore fd in ebx from stack
close:
mov eax, 6 ;sys_close file
int 80h
mov eax, 1 ;sys_exit
mov ebx, 0 ;ok
int 80h
error:
mov ebx, eax ;exit code
mov eax, 1 ;sys_exit
int 80h

Answer 1

When you write
jb close ;must close file first
you, in fact, are jumping to close, not calling it.
Once you reach close, you just exit after you close the file:
close:
mov eax, 6 ;sys_close file
int 80h
mov eax, 1 ;sys_exit
mov ebx, 0 ;ok
int 80h
Perhaps you want to either call close in some way (and then ret from close) or jump back to where you were to continue what you were doing?
Also, keep in mind that jbe is an unsigned comparison, so when you write:
cmp eax, 0 ;check if fd in eax > 0 (ok)
jbe error ;can not open file
You in fact will not detect negative numbers. Consider jle instead. (You can consult this handy resource about jumps to use.)