1 /* $NetBSD: memchr.c,v 1.1.1.1 2016/01/10 21:36:18 christos Exp $ */ 2 3 /* Copyright (C) 1991, 1993, 1997, 1999 Free Software Foundation, Inc. 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 10 NOTE: The canonical source of this file is maintained with the GNU C Library. 11 Bugs can be reported to bug-glibc@prep.ai.mit.edu. 12 13 This program is free software; you can redistribute it and/or modify it 14 under the terms of the GNU General Public License as published by the 15 Free Software Foundation; either version 2, or (at your option) any 16 later version. 17 18 This program is distributed in the hope that it will be useful, 19 but WITHOUT ANY WARRANTY; without even the implied warranty of 20 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 21 GNU General Public License for more details. 22 23 You should have received a copy of the GNU General Public License 24 along with this program; if not, write to the Free Software 25 Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, 26 USA. */ 27 28 #ifdef HAVE_CONFIG_H 29 # include <config.h> 30 #endif 31 32 #undef __ptr_t 33 #if defined (__cplusplus) || (defined (__STDC__) && __STDC__) 34 # define __ptr_t void * 35 #else /* Not C++ or ANSI C. */ 36 # define __ptr_t char * 37 #endif /* C++ or ANSI C. */ 38 39 #if defined (_LIBC) 40 # include <string.h> 41 #endif 42 43 #if defined (HAVE_LIMITS_H) || defined (_LIBC) 44 # include <limits.h> 45 #endif 46 47 #define LONG_MAX_32_BITS 2147483647 48 49 #ifndef LONG_MAX 50 # define LONG_MAX LONG_MAX_32_BITS 51 #endif 52 53 #include <sys/types.h> 54 55 56 /* Search no more than N bytes of S for C. */ 57 58 __ptr_t 59 memchr (s, c, n) 60 const __ptr_t s; 61 int c; 62 size_t n; 63 { 64 const unsigned char *char_ptr; 65 const unsigned long int *longword_ptr; 66 unsigned long int longword, magic_bits, charmask; 67 68 c = (unsigned char) c; 69 70 /* Handle the first few characters by reading one character at a time. 71 Do this until CHAR_PTR is aligned on a longword boundary. */ 72 for (char_ptr = (const unsigned char *) s; 73 n > 0 && ((unsigned long int) char_ptr 74 & (sizeof (longword) - 1)) != 0; 75 --n, ++char_ptr) 76 if (*char_ptr == c) 77 return (__ptr_t) char_ptr; 78 79 /* All these elucidatory comments refer to 4-byte longwords, 80 but the theory applies equally well to 8-byte longwords. */ 81 82 longword_ptr = (unsigned long int *) char_ptr; 83 84 /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits 85 the "holes." Note that there is a hole just to the left of 86 each byte, with an extra at the end: 87 88 bits: 01111110 11111110 11111110 11111111 89 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD 90 91 The 1-bits make sure that carries propagate to the next 0-bit. 92 The 0-bits provide holes for carries to fall into. */ 93 94 if (sizeof (longword) != 4 && sizeof (longword) != 8) 95 abort (); 96 97 #if LONG_MAX <= LONG_MAX_32_BITS 98 magic_bits = 0x7efefeff; 99 #else 100 magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff; 101 #endif 102 103 /* Set up a longword, each of whose bytes is C. */ 104 charmask = c | (c << 8); 105 charmask |= charmask << 16; 106 #if LONG_MAX > LONG_MAX_32_BITS 107 charmask |= charmask << 32; 108 #endif 109 110 /* Instead of the traditional loop which tests each character, 111 we will test a longword at a time. The tricky part is testing 112 if *any of the four* bytes in the longword in question are zero. */ 113 while (n >= sizeof (longword)) 114 { 115 /* We tentatively exit the loop if adding MAGIC_BITS to 116 LONGWORD fails to change any of the hole bits of LONGWORD. 117 118 1) Is this safe? Will it catch all the zero bytes? 119 Suppose there is a byte with all zeros. Any carry bits 120 propagating from its left will fall into the hole at its 121 least significant bit and stop. Since there will be no 122 carry from its most significant bit, the LSB of the 123 byte to the left will be unchanged, and the zero will be 124 detected. 125 126 2) Is this worthwhile? Will it ignore everything except 127 zero bytes? Suppose every byte of LONGWORD has a bit set 128 somewhere. There will be a carry into bit 8. If bit 8 129 is set, this will carry into bit 16. If bit 8 is clear, 130 one of bits 9-15 must be set, so there will be a carry 131 into bit 16. Similarly, there will be a carry into bit 132 24. If one of bits 24-30 is set, there will be a carry 133 into bit 31, so all of the hole bits will be changed. 134 135 The one misfire occurs when bits 24-30 are clear and bit 136 31 is set; in this case, the hole at bit 31 is not 137 changed. If we had access to the processor carry flag, 138 we could close this loophole by putting the fourth hole 139 at bit 32! 140 141 So it ignores everything except 128's, when they're aligned 142 properly. 143 144 3) But wait! Aren't we looking for C, not zero? 145 Good point. So what we do is XOR LONGWORD with a longword, 146 each of whose bytes is C. This turns each byte that is C 147 into a zero. */ 148 149 longword = *longword_ptr++ ^ charmask; 150 151 /* Add MAGIC_BITS to LONGWORD. */ 152 if ((((longword + magic_bits) 153 154 /* Set those bits that were unchanged by the addition. */ 155 ^ ~longword) 156 157 /* Look at only the hole bits. If any of the hole bits 158 are unchanged, most likely one of the bytes was a 159 zero. */ 160 & ~magic_bits) != 0) 161 { 162 /* Which of the bytes was C? If none of them were, it was 163 a misfire; continue the search. */ 164 165 const unsigned char *cp = (const unsigned char *) (longword_ptr - 1); 166 167 if (cp[0] == c) 168 return (__ptr_t) cp; 169 if (cp[1] == c) 170 return (__ptr_t) &cp[1]; 171 if (cp[2] == c) 172 return (__ptr_t) &cp[2]; 173 if (cp[3] == c) 174 return (__ptr_t) &cp[3]; 175 #if LONG_MAX > 2147483647 176 if (cp[4] == c) 177 return (__ptr_t) &cp[4]; 178 if (cp[5] == c) 179 return (__ptr_t) &cp[5]; 180 if (cp[6] == c) 181 return (__ptr_t) &cp[6]; 182 if (cp[7] == c) 183 return (__ptr_t) &cp[7]; 184 #endif 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 (__ptr_t) char_ptr; 196 else 197 ++char_ptr; 198 } 199 200 return 0; 201 } 202