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      1 /*	$NetBSD: swab.c,v 1.20 2022/12/28 15:34:19 riastradh Exp $	*/
      2 
      3 
      4 /*-
      5  * Copyright (c) 2022 The NetBSD Foundation, Inc.
      6  * All rights reserved.
      7  *
      8  * Redistribution and use in source and binary forms, with or without
      9  * modification, are permitted provided that the following conditions
     10  * are met:
     11  * 1. Redistributions of source code must retain the above copyright
     12  *    notice, this list of conditions and the following disclaimer.
     13  * 2. Redistributions in binary form must reproduce the above copyright
     14  *    notice, this list of conditions and the following disclaimer in the
     15  *    documentation and/or other materials provided with the distribution.
     16  *
     17  * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
     18  * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
     19  * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
     20  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
     21  * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
     22  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
     23  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
     24  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
     25  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
     26  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
     27  * POSSIBILITY OF SUCH DAMAGE.
     28  */
     29 
     30 #include <sys/cdefs.h>
     31 #if defined(LIBC_SCCS) && !defined(lint)
     32 __RCSID("$NetBSD: swab.c,v 1.20 2022/12/28 15:34:19 riastradh Exp $");
     33 #endif
     34 
     35 #include <assert.h>
     36 #include <unistd.h>
     37 
     38 void
     39 swab(const void *__restrict src, void *__restrict dst, ssize_t nbytes)
     40 {
     41 	const char *p = src;
     42 	char *q = dst;
     43 	size_t n;
     44 
     45 	_DIAGASSERT(src != NULL);
     46 	_DIAGASSERT(dst != NULL);
     47 
     48 	if (nbytes <= 1)
     49 		return;
     50 
     51 	/*
     52 	 * Round nbytes/2 down; we ignore the last byte altogether if
     53 	 * nbytes is odd.  This way, if the destination array is
     54 	 * uninitialized, sanitizers will recognize the last byte as
     55 	 * still uninitialized and flag it as undefined.
     56 	 */
     57 	for (n = nbytes/2; n --> 0; p += 2, q += 2) {
     58 		/*
     59 		 * According to POSIX (2018), the behaviour is
     60 		 * undefined if src and dst overlap.  However, there
     61 		 * are uses in-tree (xsrc/external/mit/xfwp/dist/io.c)
     62 		 * that rely on swab(ptr, ptr, n) to do the swabbing
     63 		 * in-place.  So make sure this works if src == dst.
     64 		 */
     65 		const char p0 = p[0], p1 = p[1];
     66 
     67 		q[0] = p1;
     68 		q[1] = p0;
     69 	}
     70 }
     71