proble with an empty linked list queue

[diablo_mabbs]

hi iv got a problem every time i try to remove a pice of data from my queue when it is already empty it crashes cmd and i dont understand why i was wondering if anyone could help me

is is menu driven here are the relevent sections

case 2:
			strcmp(szReg,leave(&taxiQueue));
			if(szReg == NULL)
			{
				printf("there are no taxis in the queue to leave");
			}
			else
			{
			 printf("\nTaxi %s was removed from the queue",szReg);
			}
			break;

and the function

char *leave(TaxiQueue *tq)
{
	Taxi **front = &tq->pFront;
	Taxi **back  = &tq->pBack;

	char *reg = malloc(sizeof(char) * 20);

	// Test if the queue is empty
	if((*front) == NULL && (*back) == NULL)
	{
		// Nothing in queue
		return NULL;
	}
	else if((*front) == (*back))
	{
		// The front and back are the same
		strcpy(reg,(*front)->szReg);
		// Delete the taxi
		free((*front));
		// Set the front and back to null
		(*front) = NULL;
		(*back) = NULL;

		tq->iLength--;

		return reg;
	}
	else
	{
		strcpy(reg,(*front)->szReg);
		// Remove from the front
		(*front) = (*front)->pNext;
		free((*front)->pPrev);
		(*front)->pPrev = NULL;

		tq->iLength--;

		return reg;
	}

	return NULL;
}

i think iv got a case of looking at it so long my brain imagines that theres nothing wrong lol

[tabstop]

If you call strcmp on a NULL pointer, bad things will (and did!) happen.

[quzah]

Don't use then ask if it's OK to use. In C, it's not easier to ask forgiveness than it is to ask permission.


Quzah.

[claudiu]

Don't use then ask if it's OK to use. In C, it's not easier to ask forgiveness than it is to ask permission.


Quzah.

lol, good one quzah!

[diablo_mabbs]

Don't use then ask if it's OK to use. In C, it's not easier to ask forgiveness than it is to ask permission.


Quzah.

what ?

is that ment to help me ?

[quzah]

if( ptr != NULL ) /* something's being pointed at... */
{
    *ptr = blahblah;
}

Do that, instead of:

*ptr = blahblah;
if( ptr != NULL )
    ...yay?

Basically, before you ever start fiddling with what is being pointed at, you need to make sure you're actually pointing AT something.


Quzah.