Thread: Seg fault

Hi.

I am getting a Seg fault and I don't know why.

Basically, in blackjack, I have a hand of cards and I am trying to sum up the hand:

Code:

struct card_t {
	unsigned short int card;  /* Ace-King (1-13) */
	unsigned short int suit;  /* Clubs,Diamonds,Hearts,Spades (1-4) */
};

int main(void) {
  ....

  printf("mycards score = %d\ncompscards score = %d\n", sum_up_hand( mycards ), sum_up_hand( compscards ) );

 ......
}

unsigned short int 
sum_up_hand( struct card_t *hand ) {

	int i = 0;
	unsigned short int total = 0;
	while( (hand + i) != NULL ) {
		total += value_of_card( (hand + i)->card );
		i++;
	}
	
	return total;

}

unsigned short int
value_of_card( unsigned short int n ) {

	int i, value = 0;
	for(i = 0; i < 10; i++) {
		/* card ace corresponds to i=0, card 2 corresponds to i=1, and so on until i=9*/
		value = ( n == i ) ? i + 1 : 0;
	}

	switch( n ) {
	
		case 10: //jack
		  value = 10;
		  break;
		case 11: // queen
		  value = 10;
		  break;
		case 12:  //king
		  value = 10;
		  break;

	}

	return value;

}

Above is just the necessary code, well hopefully. Compiles ok, I go to run it and receive a Seg fault. Any idea? I know I'm trying to access something which doesn't belong to me, but I don't know what.

Hmm I'm not sure about the for loop. Seems unnecessary to loop more than is needed. There must be a way to stop when it comes to an uninitialised value?

Code:

for(i=0;i<52;i++) {
		total += value_of_card( (hand + i)->card );
}

Wont I get garbage for 'total', as memory is filled with garbage initially? I suppose I could set every value to -1 and loop until it hits a -1? (And remove the unsigned obviously). What would you suggest?

About the %d, you're right; I couldn't find a flag for short, so assuming there isn't one? Will I just cast (int) to the argument then in that case?

I guess what you mean was

Code:

while( *(hand + i) != NULL ) // same as hand[i] != NULL

Better way to implement your function would be to add an extra argument telling the number of element to check

Code:

unsigned short int 
sum_up_hand(struct card_t *hand, int nbOfCards )

There's also different ways of doing this...


And your function value_of_card seems overly complicated. Also, i don't know if you realized that this loop

Code:

for(i = 0; i < 10; i++)
{
	value = ( n == i ) ? i + 1 : 0;
}

was in fact equivalent to this

Code:

value = (n == 9) ? 10 : 0;
i = 10;

As for this

Code:

switch( n ) {
	
		case 10: //jack
		  value = 10;
		  break;
		case 11: // queen
		  value = 10;
		  break;
		case 12:  //king
		  value = 10;
		  break;

	}

could be replaced with this

Code:

if (n >= 10)
	value = 10;

And by the way, the format specifier for unsigned variable is %u, whatever it's an unsigned char, short or int. But i'm not sure this would have lead to incorrect output, since an unsigned short promoted to an (signed/unsigned) int wouldn't have mess with the sign bit of the int, in the case int is of larger size than short. But i'm not 100% sure.

Oh and by the way there's a difference between an "uninitialised value" an the NULL value. If one variable has the NULL value it is because it has been initialised to the NULL value.

Sorry, I did make a fair amount of foolish mistakes. New functions:

Code:

unsigned short int 
sum_up_hand( struct card_t *hand , unsigned short int no_of_cards ) {

	int i;
	unsigned short int total = 0;

	for(i = 0 ;i < no_of_cards;i++) {
		total += value_of_card( (hand + i)->card );
	}
	
	return total;

}

unsigned short int
value_of_card( unsigned short int n ) {

	int value = 0;
	if( n < 10 ) {
		value = n+1;
	}

	if( n >= 10 )
		value = 10;

	return value;

}

Code:

while( *(hand + i) != NULL ) // same as hand[i] != NULL

That is indeed what i meant. D'oh.

Code:

for(i = 0; i < 10; i++)
{
	value = ( n == i ) ? i + 1 : 0;
}

Yes, i see it now, thanks. Pesky 0.

%hd is for short.

Thanks, I missed that.

Thank you foxman and Salem, much appreciated.

Well i wasn't talking about cast, but more about promotion, because, like you mentionned, the called function doesn't know the type of the argument so i tought C was maybe putting in place the char->short->int and float->double promotion in this case.

Because i'm sure you already saw those kind of code working perfectly well

Code:

char c = 'a';
unsigned int i = 0xFF0000FF;

printf("ASCII code of 'a' is %d, and i have %u things", c, i);

But that's it, i don't know exactly how are working function with varying number of additional arguments (even if i have some ideas). I looked a bit on the net but found nothing really explicit. I still found this, who let me believe i'm right. That's it, promotion is happening in those case, for variable of type char, short (to int) or float (to double).

What i meaned is that there is promotion BEFORE the function (the one with the variable arguments list) is called, and it's the "promoted" version of the values who are passed to the function. The compiler does know the type of the arguments you are passing . But the called function won't know.

Well, for things to be clear, what i mean is that if you write something like this

Code:

char c = 'a';
printf("%d", c);

well the compiler will add some instructions (promoting char to int) so it's exactly like you wrote this

Code:

char c = 'a';
printf("%d", (int) c);

how would it know you want it converted to an int?

oh ok, I guess I was wrong.
thanks Dave_Sinkula.

Sorry to digress, but I completed the blackjack game. Any advice/suggestions/improvements would be appreciated. Thanks