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Read And Write From And To A File Program Crashes

Started by c.2602, January 03, 2016, 09:08:16 AM

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c.2602

I need to read 8 byte at a time from a .txt file. For now, I just want to display that block to a destination .txt file. The problem is that my program is crashing and I cannot figure out at which point. It does get the first 8 byte segment and then crashes. It seems that it stops working at mov dl,[esi](i tried using ollydbg), but I am new to assembly so I am not sure. Can anyone please help or give me some suggestions/advice? I use masm. Here is the code:



.386
.model flat, stdcall
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
includelib msvcrt.lib
extern printf: proc
extern scanf: proc
extern fscanf: proc
extern fprintf: proc
extern fopen: proc
extern fclose: proc
extern exit: proc

;
public start

;data section
.data
  scanf_format db "%d",0
  printf_format db "%c",0

  ascii db 0
  type_read db "r",0
  type_write db "w", 0
  destination db "destinatie.txt",0
  fscanf_format db "%c",0
  fprintf_format db "%c",0
  start_msg db "The path to the source file :",0
  source dd 0
  source_format db "%s", 0
  pointer_source dd 0
  pointer_destination dd 0

  buffer db 64 dup(0)
  buffer_l equ 63

  step_nr dd 0
  step dd 0
  end_of_file dd 0
;the code section
.code

start:
    xor eax,eax
    ; The path to the source file
    push offset start_msg
   call printf
    add esp,4
    ; read the source file name
    push  offset source
    push offset source_format
    call scanf
    add esp,8
    ; open source file
    push offset type_read
    push offset source
    call fopen
    add esp,8
    mov pointer_source,eax


    ; create the destination file
    push offset type_write
    push offset destination
    call fopen
    mov pointer_destination,eax
    add esp,8



read_file:
       lea esi,buffer
       mov step,esi
       mov esi,step
       xor edi,edi
       mov edi,esi
       add edi,63
       dec esi
       for_0_63:
          push offset ascii
          push offset fscanf_format
          push pointer_source
          call fscanf
          add esp,12
          mov end_of_file,eax
          cmp end_of_file,0ffffffffH
          je write_file   
          xor ebx,ebx
          mov bl,ascii
          inc esi
          mov [esi],ebx
          cmp esi,edi
          jb for_0_63

       ;display the blocks
        mov end_of_file,eax
        mov step,esi
        inc step   
        xor edi,edi
        xor esi,esi
        lea ESI, buffer
        mov ecx,64
        dec esi

       write_file:
         inc esi
         xor edx,edx
         mov dl,[esi]
         ;push [esi]
         push edx
         push offset fprintf_format
         push pointer_destination
         call fprintf
         add esp,12
         cmp esi,[ecx]
         jne write_file
       cmp end_of_file,0ffffffffH
       jne read_file
jmp next


final :
         push 0
         call exit

next: push pointer_source
          call fclose
          add esp,4
          push pointer_destination
          call fclose
          add esp,4
          jmp final


end start

jj2007

First of all: Welcome to the Forum :icon14:

Your code crashes first here:
         xor edx,edx
         mov dl,[esi]  ; doesn't make sense
         ;push [esi]
         push ecx  ; needs to be preserved
         push edx
         push offset fprintf_format
         push pointer_destination
         call fprintf
         add esp,12
         pop ecx
         cmp esi,[ecx]  ; why is there a loop here?


There will be more errors. Check Tips, Tricks & Traps for some rules, in particular on preserving registers. And please, learn how to use the invoke macro. Pushing args "by hand" leads to buggy code.

c.2602

I tried making some modifications but now it does not work at all. Also I read the part about preserving registers, I understood why I should do it, but I do not really understand how to. Could you possibly be more specific on what should I do to make it work? I am really struggling with this.  :(

jj2007

That is register ecx preserving:
push ecx
invoke someapi, arg1, arg2, arg3
pop ecx


Btw where did you get the ideas for your code? It looks very, very old-style, almost nobody codes like that any more. if you want to learn assembler seriously, you should study the examples folder, e.g. \Masm32\examples\exampl07\fileio\ppfileio.asm

c.2602

We learned just the basics in class, like what data types are there, basic operations, to print and read. So I looked at an example and tried to adapt it to what I need to do. I will check the section, thanks.