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1 /* Copyright (C) 1991, 1993, 1996, 1997, 1999, 2000, 2003, 2004, 2006, |
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2 2008 Free Software Foundation, Inc. |
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3 |
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4 Based on strlen implementation by Torbjorn Granlund (tege@sics.se), |
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5 with help from Dan Sahlin (dan@sics.se) and |
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6 commentary by Jim Blandy (jimb@ai.mit.edu); |
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7 adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu), |
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8 and implemented in glibc by Roland McGrath (roland@ai.mit.edu). |
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9 Extension to memchr2 implemented by Eric Blake (ebb9@byu.net). |
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10 |
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11 This program is free software: you can redistribute it and/or modify it |
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12 under the terms of the GNU General Public License as published by the |
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13 Free Software Foundation; either version 3 of the License, or any |
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14 later version. |
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15 |
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16 This program is distributed in the hope that it will be useful, |
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17 but WITHOUT ANY WARRANTY; without even the implied warranty of |
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18 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
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19 GNU General Public License for more details. |
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20 |
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21 You should have received a copy of the GNU General Public License |
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22 along with this program. If not, see <http://www.gnu.org/licenses/>. */ |
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23 |
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24 #include <config.h> |
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25 |
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26 #include "memchr2.h" |
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27 |
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28 #include <limits.h> |
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29 #include <stdint.h> |
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30 #include <string.h> |
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31 |
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32 /* Return the first address of either C1 or C2 (treated as unsigned |
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33 char) that occurs within N bytes of the memory region S. If |
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34 neither byte appears, return NULL. */ |
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35 void * |
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36 memchr2 (void const *s, int c1_in, int c2_in, size_t n) |
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37 { |
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38 const unsigned char *char_ptr; |
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39 const uintmax_t *longword_ptr; |
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40 uintmax_t longword1; |
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41 uintmax_t longword2; |
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42 uintmax_t magic_bits; |
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43 uintmax_t charmask1; |
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44 uintmax_t charmask2; |
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45 unsigned char c1; |
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46 unsigned char c2; |
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47 int i; |
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48 |
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49 c1 = (unsigned char) c1_in; |
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50 c2 = (unsigned char) c2_in; |
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51 |
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52 if (c1 == c2) |
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53 return memchr (s, c1, n); |
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54 |
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55 /* Handle the first few characters by reading one character at a time. |
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56 Do this until CHAR_PTR is aligned on a longword boundary. */ |
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57 for (char_ptr = (const unsigned char *) s; |
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58 n > 0 && (size_t) char_ptr % sizeof longword1 != 0; |
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59 --n, ++char_ptr) |
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60 if (*char_ptr == c1 || *char_ptr == c2) |
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61 return (void *) char_ptr; |
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62 |
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63 /* All these elucidatory comments refer to 4-byte longwords, |
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64 but the theory applies equally well to any size longwords. */ |
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65 |
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66 longword_ptr = (const uintmax_t *) char_ptr; |
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67 |
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68 /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits |
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69 the "holes." Note that there is a hole just to the left of |
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70 each byte, with an extra at the end: |
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71 |
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72 bits: 01111110 11111110 11111110 11111111 |
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73 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD |
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74 |
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75 The 1-bits make sure that carries propagate to the next 0-bit. |
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76 The 0-bits provide holes for carries to fall into. */ |
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77 |
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78 /* Set MAGIC_BITS to be this pattern of 1 and 0 bits. |
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79 Set CHARMASK to be a longword, each of whose bytes is C. */ |
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80 |
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81 magic_bits = 0xfefefefe; |
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82 charmask1 = c1 | (c1 << 8); |
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83 charmask2 = c2 | (c2 << 8); |
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84 charmask1 |= charmask1 << 16; |
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85 charmask2 |= charmask2 << 16; |
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86 #if 0xffffffffU < UINTMAX_MAX |
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87 magic_bits |= magic_bits << 32; |
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88 charmask1 |= charmask1 << 32; |
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89 charmask2 |= charmask2 << 32; |
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90 if (8 < sizeof longword1) |
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91 for (i = 64; i < sizeof longword1 * 8; i *= 2) |
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92 { |
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93 magic_bits |= magic_bits << i; |
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94 charmask1 |= charmask1 << i; |
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95 charmask2 |= charmask2 << i; |
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96 } |
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97 #endif |
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98 magic_bits = (UINTMAX_MAX >> 1) & (magic_bits | 1); |
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99 |
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100 /* Instead of the traditional loop which tests each character, |
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101 we will test a longword at a time. The tricky part is testing |
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102 if *any of the four* bytes in the longword in question are zero. */ |
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103 while (n >= sizeof longword1) |
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104 { |
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105 /* We tentatively exit the loop if adding MAGIC_BITS to |
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106 LONGWORD fails to change any of the hole bits of LONGWORD. |
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107 |
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108 1) Is this safe? Will it catch all the zero bytes? |
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109 Suppose there is a byte with all zeros. Any carry bits |
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110 propagating from its left will fall into the hole at its |
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111 least significant bit and stop. Since there will be no |
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112 carry from its most significant bit, the LSB of the |
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113 byte to the left will be unchanged, and the zero will be |
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114 detected. |
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115 |
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116 2) Is this worthwhile? Will it ignore everything except |
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117 zero bytes? Suppose every byte of LONGWORD has a bit set |
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118 somewhere. There will be a carry into bit 8. If bit 8 |
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119 is set, this will carry into bit 16. If bit 8 is clear, |
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120 one of bits 9-15 must be set, so there will be a carry |
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121 into bit 16. Similarly, there will be a carry into bit |
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122 24. If one of bits 24-30 is set, there will be a carry |
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123 into bit 31, so all of the hole bits will be changed. |
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124 |
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125 The one misfire occurs when bits 24-30 are clear and bit |
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126 31 is set; in this case, the hole at bit 31 is not |
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127 changed. If we had access to the processor carry flag, |
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128 we could close this loophole by putting the fourth hole |
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129 at bit 32! |
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130 |
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131 So it ignores everything except 128's, when they're aligned |
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132 properly. |
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133 |
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134 3) But wait! Aren't we looking for C, not zero? |
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135 Good point. So what we do is XOR LONGWORD with a longword, |
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136 each of whose bytes is C. This turns each byte that is C |
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137 into a zero. */ |
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138 |
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139 longword1 = *longword_ptr ^ charmask1; |
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140 longword2 = *longword_ptr++ ^ charmask2; |
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141 |
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142 /* Add MAGIC_BITS to LONGWORD. */ |
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143 if ((((longword1 + magic_bits) |
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144 |
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145 /* Set those bits that were unchanged by the addition. */ |
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146 ^ ~longword1) |
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147 |
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148 /* Look at only the hole bits. If any of the hole bits |
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149 are unchanged, most likely one of the bytes was a |
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150 zero. */ |
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151 & ~magic_bits) != 0 |
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152 || (((longword2 + magic_bits) ^ ~longword2) & ~magic_bits) != 0) |
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153 { |
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154 /* Which of the bytes was C? If none of them were, it was |
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155 a misfire; continue the search. */ |
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156 |
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157 const unsigned char *cp = (const unsigned char *) (longword_ptr - 1); |
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158 |
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159 if (cp[0] == c1 || cp[0] == c2) |
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160 return (void *) cp; |
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161 if (cp[1] == c1 || cp[1] == c2) |
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162 return (void *) &cp[1]; |
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163 if (cp[2] == c1 || cp[2] == c2) |
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164 return (void *) &cp[2]; |
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165 if (cp[3] == c1 || cp[3] == c2) |
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166 return (void *) &cp[3]; |
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167 if (4 < sizeof longword1 && (cp[4] == c1 || cp[4] == c2)) |
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168 return (void *) &cp[4]; |
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169 if (5 < sizeof longword1 && (cp[5] == c1 || cp[5] == c2)) |
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170 return (void *) &cp[5]; |
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171 if (6 < sizeof longword1 && (cp[6] == c1 || cp[6] == c2)) |
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172 return (void *) &cp[6]; |
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173 if (7 < sizeof longword1 && (cp[7] == c1 || cp[7] == c2)) |
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174 return (void *) &cp[7]; |
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175 if (8 < sizeof longword1) |
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176 for (i = 8; i < sizeof longword1; i++) |
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177 if (cp[i] == c1 || cp[i] == c2) |
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178 return (void *) &cp[i]; |
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179 } |
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180 |
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181 n -= sizeof longword1; |
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182 } |
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183 |
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184 char_ptr = (const unsigned char *) longword_ptr; |
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185 |
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186 while (n-- > 0) |
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187 { |
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188 if (*char_ptr == c1 || *char_ptr == c2) |
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189 return (void *) char_ptr; |
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190 ++char_ptr; |
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191 } |
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192 |
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193 return 0; |
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194 } |
