From: torek@horse.ee.lbl.gov (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: How to print an integer as binary?
Date: 6 Nov 1992 11:02:15 GMT
Organization: Lawrence Berkeley Laboratory, Berkeley
Reply-To: torek@horse.ee.lbl.gov (Chris Torek)
NNTP-Posting-Host: 128.3.112.15

In article <1dbpe8INNcop@frigate.doc.ic.ac.uk> anb@doc.ic.ac.uk
(A N Burton) writes:
>The program appearing below gives three possible [ways to print a value
 in binary]

Arthur Rubin has already pointed out that, on some machines, there is a
`most negative' number (e.g., -2147483648) which, when negated,
overflows and remains negative.  Method 1 recurses forever if overflow
is undetected.  Method 2 may or may not work, depending on whether
right shift of negative signed integers is arithmetic or logical.
(ANSI C leaves the effect of such shifts implementation-defined.)

Methods 2 and 3 have a more interesting feature.  They perform leading
zero suppression---method 2 must do so since it only terminates when
the value goes to zero---but neither one checks to see if the entire
value is 0.  In this case, method 2 produces no output, but method 3
loops forever!

Method 3 also makes the assumption that there are 8 bits per `char'.
This can be fixed easily by using CHAR_BIT from <limits.h>.

As a general solution, the following will produce a (signed) value in
any base in [2..36].  The caller must supply a sufficiently-large
buffer; ltostr returns a pointer somewhere into this buffer, or NULL
for overflow or error.

The code for the corresponding ultostr() function should be obvious.

/*
 * Copyright (c) 1992 The Regents of the University of California.
 * All rights reserved.
 *
 * This software was developed by the Computer Systems Engineering group
 * at Lawrence Berkeley Laboratory.
 *
 * All advertising materials mentioning features or use of this software
 * must display the following acknowledgement:
 *	This product includes software developed by the University of
 *	California, Lawrence Berkeley Laboratories.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *	This product includes software developed by the University of
 *	California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * Convert a value (of type long) to a printable representation.
 */
char *
ltostr(long v, int base, char *buf, int size)
{
	register char *p;
	unsigned long u;
	int neg;
	static const char digits[] = "0123456789abcdefghijklmnopqrstuvwyxz";

	/*
	 * Verify arguments.  Base must be between 2 and 36 (we do not
	 * do unary).  We need at least two characters: the smallest
	 * value becomes {'0', '\0'}.
	 */
	if (base < 2 || base > 36 || size < 2)
		return (NULL);

	/*
	 * Build the output backwards in the buffer provided.  Remember
	 * whether a leading sign is needed; use unsigned to avoid overflow.
	 */
	p = buf + size;
	*--p = '\0';			/* at least 1 byte remains */
	if (v < 0) {
		neg = 1;
		u = -(unsigned long)v;
	} else {
		neg = 0;
		u = v;
	}
	/* Loop invariant: there is always room for another digit. */
	for (;;) {
		*--p = digits[u % base];
		u /= base;
		if (u == 0)		/* done */
			break;
		/* Need another digit... check for buffer overrun. */
		if (p == buf)
			return (NULL);
	}
	if (neg) {
		/* We may have exactly filled the buffer, so check first. */
		if (p == buf)
			return (NULL);
		*--p = '-';
	}
	return (p);
}
-- 
In-Real-Life: Chris Torek, Lawrence Berkeley Lab CSE/EE (+1 510 486 5427)
Berkeley, CA		Domain:	torek@ee.lbl.gov
Newsgroups: comp.lang.c