I can't figure out how to print arrays and in turn, I can't figure out how to swap elements in an array

Question

this is the c++ file:

#include <iostream>
using namespace std;

//This is the C prototype of the assembly function, it requires extern"C" to
//show the name is going to be decorated as _test and not the C++ way of
//doing things
extern"C"
{
    //char arrayReverse(char*, int);
    int numChars(char *, int);
    char swapChars(char *, int);
}

int main()
{
    const int SIZE = 7;
    char arr[SIZE] = { 'h', 'e', 'l', 'l', 'o', '1', '2' };

    int val1 = numChars(arr, SIZE);
    cout << "The number of elements is: " << val1 << endl;

    char val2 = swapChars(arr, SIZE);
    cout << "Swapped! " << endl;


    return 0;
}

and my swapChars() file:

    .686
    .model flat
    .code

    _swapChars PROC ; named _test because C automatically prepends an underscode, it is needed to interoperate
        push ebp
        mov ebp,esp ; stack pointer to ebp

        mov ebx,[ebp+8] ; address of first array element
        mov ecx,[ebp+12] ; number of elements in array
        mov ebp,0
        mov eax,0
        mov edx,ebx     ;move first to edx
        add edx, 7      ;last element in the array

    loopMe:
        cmp ebp, ecx        ;comparing iterator to total elements
        jge nextLoopMe
        mov eax, [ebx]      ;move 1st element into tmp eax
        mov ebx, [edx]      ;move last element into first
        mov edx, eax        ;move tmp into last
        push ebx            ;push first element onto stack
        add ebx, 1          ;first + 1
        sub edx, 1          ;last - 1
        add ebp, 1          ;increment
    jmp loopMe




        nextLoopMe:
            mov ebx,[ebp+8] ;find first element again  USING AS FFRAME POINTER AGAIN
            cmp ebx, ecx    ;comparing first element to number of elements
            je allDone

            pop ebx
            add ebx, 1
        jmp nextLoopMe

    allDone:    
        pop ebp
        ret
    _swapChars ENDP

    END 

This is supposed to take the value in arr[0] and swap that with arr[6], arr[1] with arr[5] etc. until the entire array is swapped and then display it. I don't know if any of the code I wrote does anything I want it to, but I'm looking for a way to see what is going on.

Is there a way I can make the asm code print something to the console while iterating through the loop?

Do brackets around the register ( [ebx] ) mean the value for the register?

When in loopMe:, the third line

mov eax, [ebx]

I get an exception "Exception thrown at 0x012125FC in assignment4.exe: 0xC0000005: Access violation reading location 0xCCCCCCCD."

Am I handling the swaps correctly?

Thanks for your time.

Answer 1

You do really need to learn to use the debugger to step thru this. That said, here are some problems I see.

add edx,7

will point edx past the end of your array. Just like arr[7] would in the C code. It should be add edx,6 to point edx to the last character.

It's error prone to change ebp in your the middle of your proc and I think you have an error there. You change it's value, but then expect [ebp+8] to reference the same data later.

You are not modifying the list correctly either. To move a char from one element to another you would do something like:

mov al, [ebx]     ; copy byte from address ebx to register al
mov [edx], al     ; copy byte in register al into address edx

The eax register is 32-bits and will copy 4 bytes at a time, not 1.

Answer 2

First of all, your code is not safe since you forget to add \\0 at the end of the char array. When you use function to process your char, or char string, it will initiate a memory leak. The size should be 8 and the last one in the array should be \\0.