This is xnu-8019. See this file in:
/*
 * Copyright (c) 2003-2010 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@
 */
/*
 * @OSF_FREE_COPYRIGHT@
 *
 */

#include <mach/mach.h>
#include <mach/mach_init.h>
#include <stdarg.h>
#include "string.h"

int (*vprintf_stderr_func)(const char *format, va_list ap);

#define __STDERR_FILENO 2
int write(int fd, const char* cbuf, int nbyte);

/* This function allows the writing of a mach error message to an
 * application-controllable output method, the default being to
 * use printf if no other method is specified by the application.
 *
 * To override, set the global function pointer vprintf_stderr to
 * a function which takes the same parameters as vprintf.
 */

__private_extern__ int
fprintf_stderr(const char *format, ...)
{
	va_list args;
	int retval;

	va_start(args, format);
	if (vprintf_stderr_func == NULL) {
		char buffer[1024];
		retval = _mach_vsnprintf(buffer, sizeof(buffer), format, args);
		write(__STDERR_FILENO, buffer, retval);
	} else {
		retval = (*vprintf_stderr_func)(format, args);
	}
	va_end(args);

	return retval;
}