Question

I am having a problem in my code and am unable to figure it out. I have three threads, thread 1 takes input of two numbers in hex, thread 2 and 3 exchange these first two digits with the last two and print the result.

Error message:

Run-Time Check Failure #2 – Stack around the variable ‘str’ was corrupted.

DWORD WINAPI changevalue( LPVOID lpParam )
{
    WaitForSingleObject(inThread,INFINITE); //Input thread
    printf("thread 1 and 2 running \n");

    int num = 0;
    num = (int)lpParam;
    int i = 0;

    char str[10] ={0};
    char a,b;
    _itoa(num,str,16);

    while (str[i] != NULL)
    {
        i++; //Get location of last char..
    }
    //Exchange first two digits with last two.
    a = str[0];
    b = str[1];

    str[0] = str[i-2];
    str[1] = str[i-1];
    str[i-2] = a;
    str[i-1] = b;
    printf("num=%s\n",str);
    //long numl=strtol(str,&stop,16);
    //printf("num = %x\n", numl);
    //We can also take input to a string then manuplate it, and 
    //then print it in form of a string only.
    //But int is used since it is mentioned in the statement.
    printf("thread 1 and 2 exit......\n ");
    return TRUE;
}

Answer 1

If lParam is 0, calling _itoa(num, str, 16) will result in the single-character string "0".

In that case, i will be 1, and str[i - 2] = a will write before the string, thus corrupting the stack.

More generally, values of lParam ranging from 0 to 15 (inclusive) will trigger the problem.