llvm/clang/lib/Headers/__stdarg_va_arg.h

/*===---- __stdarg_va_arg.h - Definitions of va_start, va_arg, va_end-------===
 *
 * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 * See https://llvm.org/LICENSE.txt for license information.
 * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 *
 *===-----------------------------------------------------------------------===
 */

#ifndef va_arg

#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
/* C23 does not require the second parameter for va_start. */
#define va_start(ap, ...) __builtin_va_start(ap, 0)
#else
/* Versions before C23 do require the second parameter. */
#define va_start(ap, param) __builtin_va_start(ap, param)
#endif
#define va_end(ap) __builtin_va_end(ap)
#define va_arg(ap, type) __builtin_va_arg(ap, type)

#endif