This is xnu-8019. See this file in:
/*
 * Copyright (c) 2003-2012 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@
 */
#ifndef _FD_SET
#define _FD_SET

#include <machine/types.h> /* __int32_t and uintptr_t */
#if !KERNEL
#include <Availability.h>
#endif

/*
 * Select uses bit masks of file descriptors in longs.  These macros
 * manipulate such bit fields (the filesystem macros use chars).  The
 * extra protection here is to permit application redefinition above
 * the default size.
 */
#ifdef FD_SETSIZE
#define __DARWIN_FD_SETSIZE     FD_SETSIZE
#else /* !FD_SETSIZE */
#define __DARWIN_FD_SETSIZE     1024
#endif /* FD_SETSIZE */
#define __DARWIN_NBBY           8                               /* bits in a byte */
#define __DARWIN_NFDBITS        (sizeof(__int32_t) * __DARWIN_NBBY) /* bits per mask */
#define __DARWIN_howmany(x, y)  ((((x) % (y)) == 0) ? ((x) / (y)) : (((x) / (y)) + 1)) /* # y's == x bits? */

__BEGIN_DECLS
typedef struct fd_set {
	__int32_t       fds_bits[__DARWIN_howmany(__DARWIN_FD_SETSIZE, __DARWIN_NFDBITS)];
} fd_set;

#if !KERNEL
int __darwin_check_fd_set_overflow(int, const void *, int) __API_AVAILABLE(macosx(10.16), ios(14.0), tvos(14.0), watchos(7.0));
#endif
__END_DECLS

#if !KERNEL
__header_always_inline int
__darwin_check_fd_set(int _a, const void *_b)
{
#ifdef __clang__
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wunguarded-availability-new"
#endif
	if ((uintptr_t)&__darwin_check_fd_set_overflow != (uintptr_t) 0) {
#if defined(_DARWIN_UNLIMITED_SELECT) || defined(_DARWIN_C_SOURCE)
		return __darwin_check_fd_set_overflow(_a, _b, 1);
#else
		return __darwin_check_fd_set_overflow(_a, _b, 0);
#endif
	} else {
		return 1;
	}
#ifdef __clang__
#pragma clang diagnostic pop
#endif
}

/* This inline avoids argument side-effect issues with FD_ISSET() */
__header_always_inline int
__darwin_fd_isset(int _fd, const struct fd_set *_p)
{
	if (__darwin_check_fd_set(_fd, (const void *) _p)) {
		return _p->fds_bits[(unsigned long)_fd / __DARWIN_NFDBITS] & ((__int32_t)(((unsigned long)1) << ((unsigned long)_fd % __DARWIN_NFDBITS)));
	}

	return 0;
}

__header_always_inline void
__darwin_fd_set(int _fd, struct fd_set *const _p)
{
	if (__darwin_check_fd_set(_fd, (const void *) _p)) {
		(_p->fds_bits[(unsigned long)_fd / __DARWIN_NFDBITS] |= ((__int32_t)(((unsigned long)1) << ((unsigned long)_fd % __DARWIN_NFDBITS))));
	}
}

__header_always_inline void
__darwin_fd_clr(int _fd, struct fd_set *const _p)
{
	if (__darwin_check_fd_set(_fd, (const void *) _p)) {
		(_p->fds_bits[(unsigned long)_fd / __DARWIN_NFDBITS] &= ~((__int32_t)(((unsigned long)1) << ((unsigned long)_fd % __DARWIN_NFDBITS))));
	}
}

#else /* KERNEL */

__header_always_inline int
__darwin_fd_isset(int _fd, const struct fd_set *_p)
{
	return _p->fds_bits[(unsigned long)_fd / __DARWIN_NFDBITS] & ((__int32_t)(((unsigned long)1) << ((unsigned long)_fd % __DARWIN_NFDBITS)));
}

__header_always_inline void
__darwin_fd_set(int _fd, struct fd_set *const _p)
{
	(_p->fds_bits[(unsigned long)_fd / __DARWIN_NFDBITS] |= ((__int32_t)(((unsigned long)1) << ((unsigned long)_fd % __DARWIN_NFDBITS))));
}

__header_always_inline void
__darwin_fd_clr(int _fd, struct fd_set *const _p)
{
	(_p->fds_bits[(unsigned long)_fd / __DARWIN_NFDBITS] &= ~((__int32_t)(((unsigned long)1) << ((unsigned long)_fd % __DARWIN_NFDBITS))));
}
#endif /* KERNEL */

#define __DARWIN_FD_SET(n, p)   __darwin_fd_set((n), (p))
#define __DARWIN_FD_CLR(n, p)   __darwin_fd_clr((n), (p))
#define __DARWIN_FD_ISSET(n, p) __darwin_fd_isset((n), (p))

#if __GNUC__ > 3 || __GNUC__ == 3 && __GNUC_MINOR__ >= 3
/*
 * Use the built-in bzero function instead of the library version so that
 * we do not pollute the namespace or introduce prototype warnings.
 */
#define __DARWIN_FD_ZERO(p)     __builtin_bzero(p, sizeof(*(p)))
#else
#define __DARWIN_FD_ZERO(p)     bzero(p, sizeof(*(p)))
#endif

#define __DARWIN_FD_COPY(f, t)  bcopy(f, t, sizeof(*(f)))
#endif /* _FD_SET */