p && *p !='$' I don't understand why p is so important to this test!<Resolved>

[voidflux]

Hello everyone, I was looking at some code, and this throws me off, i have no idea why if I delete p and just leave *p!='$'; it will crash the whole program. here's the code:

void tokenize(char **name, int &n, char * buffer)
{
	char *p = strtok(buffer, "\n");
	for(n = 0;  p && *p !='$'; n++)
	{
		name[n] = p;
		p = strtok(NULL, "\n");
	}
}

here's some info about the values being passed:

const int NAME_LENGTH = 20;
const int MAX_NUM_NAMES = 25;
const int BUFFER_LENGTH = MAX_NUM_NAMES* (NAME_LENGTH + 1);
void input(char * buffer);
void tokenize(char **name, int & numNames, char* buffer);
void print(char **name, int numNames);
void sort(char ** name, int numNames);

int main(void)
{
	char * name[MAX_NUM_NAMES];
	char buffer[BUFFER_LENGTH+1];
	int numNames;
	input(buffer);
	tokenize(name,numNames,buffer);
	print(name,numNames);
	sort(name,numNames);
	print(name,numNames);
	return 0;
}

If someone could explain to me what exactly p && *p !='$'; evalutes that would be great!
also, is it a good idea to use char ** or is there a better way to appraoch this? This isn't my code, its code out of a book and i've never seen it put this way thats why i'm so concerned!
-Thanks

[sunburnt]

if (p && *p != '$') { // ....

the first part is equivalent to if (p != NULL). this is necessary because if p is a bad pointer, then you don't want to touch it, because you'll end up overwriting something else, resulting in a crash at best.

[Zaei]

Correct. p && *p != '$' first checks that p is not NULL. If it IS NULL, the && MUST be false, and so the second statement (*p) is never executed.

Z.

[voidflux]

Thanks for the replies!
I figured thats what it did, it just suprised me that the program would crash for not checking a NULL pointer. Because if i allocate memory using the new keyword and forget to check if *p = NULL the program will still run fine. Thats what threw me off! thanks again!

[CornedBee]

That's only because failures of new are rare. strtok however is guaranteed to return NULL sooner or later, so failing to check there is guaranteed to crash the app.

You must know that the app doesn't crash simply because you forget to check the pointer, the app crashes because you access a NULL pointer, which you must not do. The check only prevents you from doing so, but in cases where you're unlikely to do so the app might run 10000 times before it crashes (because, due to strange circumstances involving UFOs and unknown quantum phenomena, the operation that usually succeeds decided to fail).