This is xnu-8019. See this file in:
/*===---- stdarg.h - Variable argument handling ----------------------------===
 *
 * Copyright (c) 2008 Eli Friedman
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 *
 *===-----------------------------------------------------------------------===
 */

#ifndef __STDARG_H
#define __STDARG_H

#ifndef _VA_LIST
typedef __builtin_va_list va_list;
#define _VA_LIST
#endif
#define va_start(ap, param) __builtin_va_start(ap, param)
#define va_end(ap)          __builtin_va_end(ap)
#define va_arg(ap, type)    __builtin_va_arg(ap, type)

/* GCC always defines __va_copy, but does not define va_copy unless in c99 mode
 * or -ansi is not specified, since it was not part of C90.
 */
#define __va_copy(d,s) __builtin_va_copy(d,s)

#if __STDC_VERSION__ >= 199900L || __cplusplus >= 201103L || !defined(__STRICT_ANSI__)
#define va_copy(dest, src)  __builtin_va_copy(dest, src)
#endif

/* Hack required to make standard headers work, at least on Ubuntu */
#define __GNUC_VA_LIST 1
typedef __builtin_va_list __gnuc_va_list;

#endif /* __STDARG_H */