Thread: Question about casting (relating to qsort and pointers to structures)

Hi everyone,

I've managed to write a working program in response to Exercise 6.4 in K&R (2nd Ed, p. 143) ('Write a program that prints the distinct words in its input sorted into decreasing order of frequency of occurrence. Precede each word by its count.') but only after much difficulty in getting qsort to work with my comparison function. Here's the code with the irrelevant bits cut out:

Code:

typedef struct wordc {
	char *word;
	unsigned int count;
} Word;

...

int treecount(Treeptr); /* Treeptr is a pointer to a node in a binary tree */

int main (void)
{
	unsigned int nwords;

	...

	nwords = treecount(root); /* count no. of words in binary tree */

	...

	Word *freqlist[nwords-1];

	...

	qsort(freqlist, nwords, sizeof(Word *), wcountcmp);
	
	...

	return 0;	
}

int wcountcmp (const void *w1, const void *w2)
{
	Word *nw1 = *(Word * const *)w1;
	Word *nw2 = *(Word * const *)w2;
	
	if (nw1->count == nw2->count) {
		return 0;
	}
	
	return nw1->count < nw2->count ? -1 : 1;	
}

The program worked after modifying wcountcmp() based on information from this page: http://c-faq.com/lib/qsort2.html.

However, I still don't understand the method of casting i.e. of the form:

struct mystruct *sp1 = *(struct mystruct * const *)p1;

On the right side, what do the asterisks after 'mystruct' and 'const' do? And what about the asterisk outside the brackets i.e. before (struct ... ?

Any help much appreciated.

Mats,
Thanks for the reply — bit clearer now! Bear with me here:

1. Doesn't w1 need to be a pointer to Word (as opposed to a pointer to pointer of Word)?
2. In terms of syntax, to cast w1 to a const pointer to pointer of Word, why isn't it just:
   
   Code:

   Word *nw1 = (Word const **)w1;

Originally Posted by kuda

Mats,
Thanks for the reply — bit clearer now! Bear with me here:

1. Doesn't w1 need to be a pointer to Word (as opposed to a pointer to pointer of Word)?
2. In terms of syntax, to cast w1 to a const pointer to pointer of Word, why isn't it just:
   
   Code:

   Word *nw1 = (Word const **)w1;

1. You're forgetting the dereference -- the * in front. After the dereference, you must have a pointer to Word, so before the dereference, you must have a pointer to pointer of Word.

2. Same thing: you've forgotten the * in front. If you think of the * in front and the first * in parentheses as "cancelling", then you've got what you expect, I think.

I'm assuming that the reason for the oddness is that w1 is in fact a pointer to a pointer, so any cast on the right that doesn't have two * in it can't be right. Since we want nw1 to have one less level of indirection, we have to dereference (hence the * in front), and we want nw1 to point to something that can't be changed, hence the "const" inside one level, where it will be in the right place once we actually do the dereference. Hence nw1 "sees" Word const * (a pointer to a constant Word).