unable to read from file when user provides filename (x86 assembly program using nasm)

Question

I'm having a problem trying to open a file and read from it. The user is prompted for the file name.

The program compiles without error but doesn't show anything. When I hardcode the filename in the .data section it runs fine, but when I get the filename from the user it fails to read the file. Where am I doing wrong? I can't find any error.

Output using hardcoded name: welcome

Output when user enters the name:

Here is my code:

section .data
    promptUsr db 'enter a file name:',0xA,0xD
    lenPrompt equ $-promptUsr
    info db 1
    ;file_name db 'data.txt' (NOTE:harcoded name works when used)
section .bss
    fd_in resb 1
    buffer resb 7
    file_name resb 20  
section .text
    global _start
_start:

;prompt user to enter a file name
        mov eax,4   ;sys_write
        mov ebx,1   ;stdout
        mov ecx,promptUsr
        mov edx,lenPrompt
    int 0x80

;read filename  (NOTE:when user enters the same name 'data.txt',this is the output:���)
        mov eax,3   
        mov ebx,2   
        mov ecx,file_name   ;(NOTE:tried using 'dword[file_name]',doesnt work)
        mov edx,20
    int 0x80

;open file 
    mov eax,5
    mov ebx,file_name   ;(NOTE:also tried using 'dword[file_name]',doesnt work too)
    mov ecx,2           ;read n write
    mov edx,7777h   ;all file permissions
   int 0x80 
    mov [fd_in],eax 

;read 7 bytes of the file
    mov eax,3
    mov ebx,[fd_in]
    mov ecx,buffer  
    mov edx,7       
    int 0x80    

;close the file
    mov eax,6
   int 0x80
;print out what was read
    mov eax,4
    mov ebx,1
    mov ecx,buffer
    mov edx,7
   int 0x80
;end program
    mov eax,1
   int 0x80

Answer 1

To add to what Michael says... 1) fd_in is too small - make it resd 1 . 2) sys_read doesn't return a zero-terminated string, and sys_open wants one.

mov byte [ecx + eax - 1], 0

after the sys_read of the filename should zero-terminate it.

Answer 2

Looks like you're trying to read from STDERR :

mov eax,3   
mov ebx,2   
mov ecx,file_name   ;(NOTE:tried using 'dword[file_name]',doesnt work)
mov edx,20

That mov ebx,2 ought to be mov ebx,0 if you want to read from STDIN .

---

would also love to know how to check first if the file exists before opening it

If you use sys_open without the O_CREAT flag set, it should fail and return -1 if the file doesn't already exist. You could also use access ( syscall 33 ) or stat ( syscall 18 ) if you want to.