✏ EDIT: byteswap.h

/* Macros and inline functions to swap the order of bytes in integer values.
   Copyright (C) 1997-2018 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <http://www.gnu.org/licenses/>.  */

#if !defined _BYTESWAP_H && !defined _NETINET_IN_H && !defined _ENDIAN_H
# error "Never use <bits/byteswap.h> directly; include <byteswap.h> instead."
#endif

#ifndef _BITS_BYTESWAP_H
#define _BITS_BYTESWAP_H 1

#include <features.h>
#include <bits/types.h>

/* Swap bytes in 16-bit value.  */
#define __bswap_constant_16(x)					\
  ((__uint16_t) ((((x) >> 8) & 0xff) | (((x) & 0xff) << 8)))

static __inline __uint16_t
__bswap_16 (__uint16_t __bsx)
{
#if __GNUC_PREREQ (4, 8)
  return __builtin_bswap16 (__bsx);
#else
  return __bswap_constant_16 (__bsx);
#endif
}

/* Swap bytes in 32-bit value.  */
#define __bswap_constant_32(x)					\
  ((((x) & 0xff000000u) >> 24) | (((x) & 0x00ff0000u) >> 8)	\
   | (((x) & 0x0000ff00u) << 8) | (((x) & 0x000000ffu) << 24))

static __inline __uint32_t
__bswap_32 (__uint32_t __bsx)
{
#if __GNUC_PREREQ (4, 3)
  return __builtin_bswap32 (__bsx);
#else
  return __bswap_constant_32 (__bsx);
#endif
}

/* Swap bytes in 64-bit value.  */
#define __bswap_constant_64(x)			\
  ((((x) & 0xff00000000000000ull) >> 56)	\
   | (((x) & 0x00ff000000000000ull) >> 40)	\
   | (((x) & 0x0000ff0000000000ull) >> 24)	\
   | (((x) & 0x000000ff00000000ull) >> 8)	\
   | (((x) & 0x00000000ff000000ull) << 8)	\
   | (((x) & 0x0000000000ff0000ull) << 24)	\
   | (((x) & 0x000000000000ff00ull) << 40)	\
   | (((x) & 0x00000000000000ffull) << 56))

__extension__ static __inline __uint64_t
__bswap_64 (__uint64_t __bsx)
{
#if __GNUC_PREREQ (4, 3)
  return __builtin_bswap64 (__bsx);
#else
  return __bswap_constant_64 (__bsx);
#endif
}

#endif /* _BITS_BYTESWAP_H */

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