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diff --git a/gl/byteswap.in.h b/gl/byteswap.in.h
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+++ b/gl/byteswap.in.h
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+/* byteswap.h - Byte swapping
+   Copyright (C) 2005, 2007, 2009-2021 Free Software Foundation, Inc.
+   Written by Oskar Liljeblad <oskar@osk.mine.nu>, 2005.
+
+   This file 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.
+
+   This file 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 this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+#ifndef _GL_BYTESWAP_H
+#define _GL_BYTESWAP_H
+
+/* Given an unsigned 16-bit argument X, return the value corresponding to
+   X with reversed byte order.  */
+#define bswap_16(x) ((((x) & 0x00FF) << 8) | \
+                     (((x) & 0xFF00) >> 8))
+
+/* Given an unsigned 32-bit argument X, return the value corresponding to
+   X with reversed byte order.  */
+#define bswap_32(x) ((((x) & 0x000000FF) << 24) | \
+                     (((x) & 0x0000FF00) << 8) | \
+                     (((x) & 0x00FF0000) >> 8) | \
+                     (((x) & 0xFF000000) >> 24))
+
+/* Given an unsigned 64-bit argument X, return the value corresponding to
+   X with reversed byte order.  */
+#define bswap_64(x) ((((x) & 0x00000000000000FFULL) << 56) | \
+                     (((x) & 0x000000000000FF00ULL) << 40) | \
+                     (((x) & 0x0000000000FF0000ULL) << 24) | \
+                     (((x) & 0x00000000FF000000ULL) << 8) | \
+                     (((x) & 0x000000FF00000000ULL) >> 8) | \
+                     (((x) & 0x0000FF0000000000ULL) >> 24) | \
+                     (((x) & 0x00FF000000000000ULL) >> 40) | \
+                     (((x) & 0xFF00000000000000ULL) >> 56))
+
+#endif /* _GL_BYTESWAP_H */