Bug in new K&R?

[mcdaniel at uicsrd.csrd.uiuc.edu]

I just got the Second Edition of "The C Programming Language" by
Kernighan & Richie.  (The "New Testament"? :-) )

I'm puzzled by the example in section 5.11, "Pointers to Functions",
pp. 119--120.  It's a generic quicksort which sorts using an array of
void pointers to the data.  This "qsort" is passed a comparison
function.
	void qsort(void *lineptr[], int left, int right,
		   int (*comp)(void *, void*));

"lineptr" is an array of lines to sort.  "strcmp" is the normal string
comparison function, and "numcmp" compares strings numerically (the
results of atof() are compared):
	char *lineptr[MAXLINES];
	int numcmp(char *, char *);

The problem is that "qsort" is called thus:
	qsort((void **) lineptr, 0, nlines-1,
	  (int (*)(void*,void*))(numeric ? numcmp : strcmp));

Are those two casts portable---is it guaranteed that "void *" and
"char *" have the same internal representation?  Is it likely that
they would under any "reasonable" implementation (whatever THAT
means)?

Whether or not it's legal, the type punning above is obscene.  I would
use the following declarations:
	void *lineptr[MAXLINES];
	int numcmp(void *, void *);
	int my_strcmp(void *, void *);

If I'm careful to store them as "void *"s and use them as "char *"s,
there are no storage or use type problems.

-- 
Tim, the Bizarre and Oddly-Dressed Enchanter
Center for Supercomputing Research and Development
at the University of Illinois at Urbana-Champaign

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