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np_123

Tue May 09, 2017 1:18 am


First element of array (rest are fine) is a non-value half the time after shuffle

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The problem I am having is that I'm getting weird output when I try to shuffle an array of 'strings'. 
When the array contains {"hi", "hola", "amigo"} half the time I get the same back, the other half this {"8y@", "hola", "amigo"} where @ is a weird character looking like a skinny rectangle.

The method giving problems is the shufflestring method below. I've omitted for clarity two interface methods that don't modify anything:


char **arr;
shufflestring(arr, sz); /*sz is just length of the array*/

/* Modern Fisher-Yates shuffle */
void shufflestring(char *a


the exact same code works if instead of char** (array of 'strings' is what it's supposed to be) it's double a
/*Example1*/

char *str


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Insectoid

Tue May 09, 2017 3:20 pm


RE:First element of array (rest are fine) is a non-value half the time after shuffle

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I notice your char** example uses & in the swap, while your char*[] example does not. It's a long time since I've worked with C, but when you get random-looking strings, it's due to a pointer error. a[j] is already a pointer and doesn't need to be dereferenced. The reason your int example works is you are swapping the data directly, whereas your first looks like you're trying to swap pointers.


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np_123

Tue May 09, 2017 4:57 pm


Re: First element of array (rest are fine) is a non-value half the time after shuffle

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yes, thanks, I've gotten rid of the & now, but the same problem still occurs with one exception: instead of a garbage string the first element ends up empty. And I do agree it's a pointer error of sorts.

I can't figure out how to test the values in an dynamic array (im compiling, debugging using VS) but I'm starting to think that maybe the 'array of strings'  that I wish to pass to the method isn't being created/stored properly. Reasoning: when I attempt to DebugOutputString(arr
void MIO_sort_sortstring(OOTaddr *sp)
{
	char **arr;
	OOTint sz;
	MyExecutorScan(sp, "AI", &arr, &sz);

	MIOSort_sortstring(arr, sz);
}


static void	MyExecutorScan (OOTaddr *sp, const char *param, ...)
{
   const char *pf = param;

   va_list curr_arg;

   va_start(curr_arg, param);

   while (*pf) 
   {
	switch(*pf)
	{
	    case 'r':
	        // Skip the function result stack position
	    	sp += ADDR_SIZE;
	    	break;

	    case 'A':
	    	{
	    	    OOTaddr *arg_arrayp = va_arg(curr_arg, OOTaddr *);

	    	    *arg_arrayp =  (OOTaddr) *sp;
	    	    sp += ADDR_SIZE*2;
	        }
	    	break;

	    case 'I':
	    	{
	    	    OOTint *arg_intp = va_arg(curr_arg, OOTint *);

	    	    *arg_intp = *((OOTint *) sp);
	    	    sp += INT_SIZE;
	    	}
	    	break;
	} // switch
	pf++;
    } // while
    va_end(curr_arg);
} // MyExecutorScan




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np_123

Tue May 09, 2017 9:23 pm


Re: First element of array (rest are fine) is a non-value half the time after shuffle

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I'm afraid it might have been a wild goose chase. The 'bug' seems to be not in my code, but in the program I'm extending (it's the OpenTuring project source, I doubt anyone's looked at that in years).

So what I gather the program is supposed to do is place/leave arguments on a stack frame which I later grab via pointer to the stack.

However, it appears to leave just the first element of the array on the stack at the and the rest I can't find.
How I came to that conclusion: DebugOutputString(*sp) prints hi  ; and *(sp+2) is 3. (given the input i mentioned in the first post) Offsetting the pointer via *(sp + x) with values from -2 to 5 I can't find any other meaningful data

I don't honestly know if there's anything I can do to fix it without going into a bunch of obscure code. Unless there's some pointer offsetting magic I'm missing


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np_123

Wed May 10, 2017 12:22 am


Re: First element of array (rest are fine) is a non-value half the time after shuffle

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and solved it - each further element of the array had an offset of 256.
as in, if 'hi' is at address x, hola is at x+256... meaning: *(sp) == "hi" and *(sp) + 256 == "hola"

I'm thinking the number 256 should be more than convenience, but doesn't ring any bells at the moment.

If anyone can confirm whether an offset of 256 (maybe it's a default string length/memory allocation?) can be assumed that'd be amazing