This is xnu-8019. See this file in:
/*
 * Copyright (c) 2010 Apple Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 *
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
 * unlawful or unlicensed copies of an Apple operating system, or to
 * circumvent, violate, or enable the circumvention or violation of, any
 * terms of an Apple operating system software license agreement.
 *
 * Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this file.
 *
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
 */

#include "string.h"

static const char hex[] = "0123456789abcdef";

static int
_mach_strlen(const char *str)
{
	const char *p;
	for (p = str; p; p++) {
		if (*p == '\0') {
			return (int)(p - str);
		}
	}
	/* NOTREACHED */
	return 0;
}

static void
_mach_hex(char **buffer, int *length, unsigned long long n)
{
	char buf[32];
	char *cp = buf + sizeof(buf);

	if (n) {
		*--cp = '\0';
		while (n) {
			*--cp = hex[n & 0xf];
			n >>= 4;
		}

		int width = _mach_strlen(cp);
		while (width > 0 && length > 0) {
			*(*buffer)++ = *cp++;
			(*length)--;
			width--;
		}
	}
}

int
_mach_vsnprintf(char *buffer, int length, const char *fmt, va_list ap)
{
	int width, max = length;
	char *out_ptr = buffer;

	// we only ever write n-1 bytes so we can put a \0 at the end
	length--;
	while (length > 0 && *fmt) {
		if (*fmt == '\0') {
			break;
		}
		if (*fmt != '%') {
			*(out_ptr++) = *(fmt++);
			length--;
			continue;
		}
		fmt++;
		// only going to support a specific subset of sprintf flags
		// namely %s, %x, with no padding modifiers
		switch (*fmt++) {
		case 's':
		{
			char *cp = va_arg(ap, char*);
			width = _mach_strlen(cp);
			while (width > 0 && length > 0) {
				*(out_ptr++) = *(cp++);
				width--;
				length--;
			}
			break;
		}
		case 'x':
		{
			_mach_hex(&out_ptr, &length, va_arg(ap, unsigned int));
			break;
		}
		}
	}
	if (max > 0) {
		*out_ptr = '\0';
	}
	return max - (length + 1); /* don't include the final NULL in the return value */
}

int
_mach_snprintf(char *buffer, int length, const char *fmt, ...)
{
	int ret;
	va_list ap;
	va_start(ap, fmt);
	ret = _mach_vsnprintf(buffer, length, fmt, ap);
	va_end(ap);
	return ret;
}