/*      $NetBSD: swab.c,v 1.20 2022/12/28 15:34:19 riastradh Exp $      */


/*-
 * Copyright (c) 2022 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

#include <sys/cdefs.h>
#if defined(LIBC_SCCS) && !defined(lint)
__RCSID("$NetBSD: swab.c,v 1.20 2022/12/28 15:34:19 riastradh Exp $");
#endif

#include <assert.h>
#include <unistd.h>

void
swab(const void *__restrict src, void *__restrict dst, ssize_t nbytes)
{
        const char *p = src;
        char *q = dst;
        size_t n;

        _DIAGASSERT(src != NULL);
        _DIAGASSERT(dst != NULL);

        if (nbytes <= 1)
                return;

        /*
         * Round nbytes/2 down; we ignore the last byte altogether if
         * nbytes is odd.  This way, if the destination array is
         * uninitialized, sanitizers will recognize the last byte as
         * still uninitialized and flag it as undefined.
         */
        for (n = nbytes/2; n --> 0; p += 2, q += 2) {
                /*
                 * According to POSIX (2018), the behaviour is
                 * undefined if src and dst overlap.  However, there
                 * are uses in-tree (xsrc/external/mit/xfwp/dist/io.c)
                 * that rely on swab(ptr, ptr, n) to do the swabbing
                 * in-place.  So make sure this works if src == dst.
                 */
                const char p0 = p[0], p1 = p[1];

                q[0] = p1;
                q[1] = p0;
        }
}