Question

This is a homework if anyone’s curious.

The task is to have a C program create a 2D array and pass that array into assembly which it should be sorted using selection sort.

The error I’m having when trying to run it is segmentation fault. I’ve tried putting

push tesst
push sout
call printf

to print out “test” on the terminal window but I couldn’t figure out where it’s coming from. I also tried putting it right after “enter 0,0” and I still havent gotten a print out of test. So i’m completely out of ideas as to where i’m getting the segmentation fault.

Any assistance would be very much appreciated. 🙁 Due tomorrow night and i’ve been stuck at this for a couple of days. Someone shed some light please

-Chris

The codes will be below

my c program

#include <stdio.h>

int ssort(char * hi[], int x, int y);

int main(){

  int i, j;


  char hi[3][5] = {"Yoshi", "Annie", "Chris"};

  printf("Display Unsorted Strings\n");
  for(i = 0; i < 3; i++){
    for(j = 0; j < 5; j++){
      printf("%c", hi[i][j]);
    }
    printf("\n");
  }
  printf("It Got Here\n");
  ssort((char*)hi, 5, 3);
  printf("Sorted\n");
  for(i = 0; i < 3; i++){
    for(j = 0; j < 5; j++){
      printf("%c", hi[i][j]);
    }
    printf("\n");
  }
}

And my Assembly Code

extern    printf

segment   .data

tesst db  't','e','s','t',0
sout  db  "%s", 10, 0

segment   .text

  global  ssort

ssort:    
  enter   0,0

;;; for(i = 0; i < namecount; i++){
;;;   for(j = 1; j < namecount; j++){
;;;       if(array[i] < array[j])
;;;           do nothing
;;;       else
;;;           swap
;;;   }
;;; }


  mov esi,    [ebp+8] 
  mov edi,    esi
  add edi,    [ebp+8]
  mov ecx,    0   ;i
  mov edx,    0
  add edx,    1   ;j
  ;; [ebp+16] = namecount

iloop:
  push    esi
  cmp ecx,    [ebp+16]
  je  done_sorting
jloop:
  cmp edx,    [ebp+16]
  je  j_done

;;; compare here now
compare:
  mov al, [esi]
  mov bl, [edi]
  cmp al, bl
  jg  alisbigger
  jl  blisbigger
  inc esi
  inc edi
  jmp compare
alisbigger:   
  jmp swap
blisbigger:   
  jmp done

swap:
  mov ebx,    0
  mov ebx,    [ebp+12]
swap_loop:
  dec ebx

  mov al, [esi]
  mov bl, [edi]
  mov [esi],  bl
  mov [edi],  al

  inc esi
  inc edi

  cmp ebx,    0
  je  done
  jmp swap_loop

done:
  inc edx
  jmp jloop

j_done:
  add ecx, 1
  add esi, [ebp+12]
  mov edx, 1
  jmp iloop   


done_sorting:
  leave
  ret

Answer 1

Print-style debugging is okay in situations where there’s nothing else available but nothing beats getting down and dirty inside a source level debugger.

Step 1: Probably the easiest way is to first take out the call into assembly and run it, to ensure it’s not your C code causing the problem.

Step 2: Then, put the call back in and load it into gdb or your favorite source level debugger. Set a breakpoint on the first assembler instruction. When it breaks, examine the stack to ensure it’s what you expect.

Step 3: Then single step through the assembly until you find your problem.

That will be the best way for you to learn, and to quickly find and fix your problem.

---

I will tell you one thing I see as a potential problem. Just after the iloop label, you have push esi. I can’t see a pop (or other esp-modifying statement) anywhere in your code and, worse yet, that push is happening in a loop.

If you try to return from a function with the stack pointer different to what it was when you entered, you’re in for a world of pain. I may just have missed something but I’d start by looking there.