1/* Copyright (C) 1991, 1993, 1996, 1997, 1999, 2000, 2003, 2004 Free 2 Software Foundation, Inc. 3 4 Based on strlen implementation by Torbjorn Granlund (tege@sics.se), 5 with help from Dan Sahlin (dan@sics.se) and 6 commentary by Jim Blandy (jimb@ai.mit.edu); 7 adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu), 8 and implemented by Roland McGrath (roland@ai.mit.edu). 9 10NOTE: The canonical source of this file is maintained with the GNU C Library. 11Bugs can be reported to bug-glibc@prep.ai.mit.edu. 12 13This program is free software; you can redistribute it and/or modify it 14under the terms of the GNU General Public License as published by the 15Free Software Foundation; either version 2, or (at your option) any 16later version. 17 18This program is distributed in the hope that it will be useful, 19but WITHOUT ANY WARRANTY; without even the implied warranty of 20MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 21GNU General Public License for more details. 22 23You should have received a copy of the GNU General Public License 24along with this program; if not, write to the Free Software Foundation, 25Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */ 26#include <sys/cdefs.h> 27__RCSID("$NetBSD: memchr.c,v 1.2 2016/05/17 14:00:09 christos Exp $"); 28 29 30#ifdef HAVE_CONFIG_H 31# include <config.h> 32#endif 33 34#include <string.h> 35 36#include <stddef.h> 37 38#if defined _LIBC 39# include <memcopy.h> 40#else 41# define reg_char char 42#endif 43 44#include <limits.h> 45 46#if HAVE_BP_SYM_H || defined _LIBC 47# include <bp-sym.h> 48#else 49# define BP_SYM(sym) sym 50#endif 51 52#undef memchr 53#undef __memchr 54 55/* Search no more than N bytes of S for C. */ 56void * 57__memchr (void const *s, int c_in, size_t n) 58{ 59 const unsigned char *char_ptr; 60 const unsigned long int *longword_ptr; 61 unsigned long int longword, magic_bits, charmask; 62 unsigned reg_char c; 63 int i; 64 65 c = (unsigned char) c_in; 66 67 /* Handle the first few characters by reading one character at a time. 68 Do this until CHAR_PTR is aligned on a longword boundary. */ 69 for (char_ptr = (const unsigned char *) s; 70 n > 0 && (size_t) char_ptr % sizeof longword != 0; 71 --n, ++char_ptr) 72 if (*char_ptr == c) 73 return (void *) char_ptr; 74 75 /* All these elucidatory comments refer to 4-byte longwords, 76 but the theory applies equally well to any size longwords. */ 77 78 longword_ptr = (const unsigned long int *) char_ptr; 79 80 /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits 81 the "holes." Note that there is a hole just to the left of 82 each byte, with an extra at the end: 83 84 bits: 01111110 11111110 11111110 11111111 85 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD 86 87 The 1-bits make sure that carries propagate to the next 0-bit. 88 The 0-bits provide holes for carries to fall into. */ 89 90 /* Set MAGIC_BITS to be this pattern of 1 and 0 bits. 91 Set CHARMASK to be a longword, each of whose bytes is C. */ 92 93 magic_bits = 0xfefefefe; 94 charmask = c | (c << 8); 95 charmask |= charmask << 16; 96#if 0xffffffffU < ULONG_MAX 97 magic_bits |= magic_bits << 32; 98 charmask |= charmask << 32; 99 if (8 < sizeof longword) 100 for (i = 64; i < sizeof longword * 8; i *= 2) 101 { 102 magic_bits |= magic_bits << i; 103 charmask |= charmask << i; 104 } 105#endif 106 magic_bits = (ULONG_MAX >> 1) & (magic_bits | 1); 107 108 /* Instead of the traditional loop which tests each character, 109 we will test a longword at a time. The tricky part is testing 110 if *any of the four* bytes in the longword in question are zero. */ 111 while (n >= sizeof longword) 112 { 113 /* We tentatively exit the loop if adding MAGIC_BITS to 114 LONGWORD fails to change any of the hole bits of LONGWORD. 115 116 1) Is this safe? Will it catch all the zero bytes? 117 Suppose there is a byte with all zeros. Any carry bits 118 propagating from its left will fall into the hole at its 119 least significant bit and stop. Since there will be no 120 carry from its most significant bit, the LSB of the 121 byte to the left will be unchanged, and the zero will be 122 detected. 123 124 2) Is this worthwhile? Will it ignore everything except 125 zero bytes? Suppose every byte of LONGWORD has a bit set 126 somewhere. There will be a carry into bit 8. If bit 8 127 is set, this will carry into bit 16. If bit 8 is clear, 128 one of bits 9-15 must be set, so there will be a carry 129 into bit 16. Similarly, there will be a carry into bit 130 24. If one of bits 24-30 is set, there will be a carry 131 into bit 31, so all of the hole bits will be changed. 132 133 The one misfire occurs when bits 24-30 are clear and bit 134 31 is set; in this case, the hole at bit 31 is not 135 changed. If we had access to the processor carry flag, 136 we could close this loophole by putting the fourth hole 137 at bit 32! 138 139 So it ignores everything except 128's, when they're aligned 140 properly. 141 142 3) But wait! Aren't we looking for C, not zero? 143 Good point. So what we do is XOR LONGWORD with a longword, 144 each of whose bytes is C. This turns each byte that is C 145 into a zero. */ 146 147 longword = *longword_ptr++ ^ charmask; 148 149 /* Add MAGIC_BITS to LONGWORD. */ 150 if ((((longword + magic_bits) 151 152 /* Set those bits that were unchanged by the addition. */ 153 ^ ~longword) 154 155 /* Look at only the hole bits. If any of the hole bits 156 are unchanged, most likely one of the bytes was a 157 zero. */ 158 & ~magic_bits) != 0) 159 { 160 /* Which of the bytes was C? If none of them were, it was 161 a misfire; continue the search. */ 162 163 const unsigned char *cp = (const unsigned char *) (longword_ptr - 1); 164 165 if (cp[0] == c) 166 return (void *) cp; 167 if (cp[1] == c) 168 return (void *) &cp[1]; 169 if (cp[2] == c) 170 return (void *) &cp[2]; 171 if (cp[3] == c) 172 return (void *) &cp[3]; 173 if (4 < sizeof longword && cp[4] == c) 174 return (void *) &cp[4]; 175 if (5 < sizeof longword && cp[5] == c) 176 return (void *) &cp[5]; 177 if (6 < sizeof longword && cp[6] == c) 178 return (void *) &cp[6]; 179 if (7 < sizeof longword && cp[7] == c) 180 return (void *) &cp[7]; 181 if (8 < sizeof longword) 182 for (i = 8; i < sizeof longword; i++) 183 if (cp[i] == c) 184 return (void *) &cp[i]; 185 } 186 187 n -= sizeof longword; 188 } 189 190 char_ptr = (const unsigned char *) longword_ptr; 191 192 while (n-- > 0) 193 { 194 if (*char_ptr == c) 195 return (void *) char_ptr; 196 else 197 ++char_ptr; 198 } 199 200 return 0; 201} 202#ifdef weak_alias 203weak_alias (__memchr, BP_SYM (memchr)) 204#endif 205