1 | /* |
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2 | FUNCTION |
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3 | <<strncmp>>---character string compare |
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4 | |
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5 | INDEX |
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6 | strncmp |
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7 | |
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8 | SYNOPSIS |
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9 | #include <string.h> |
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10 | int strncmp(const char *<[a]>, const char * <[b]>, size_t <[length]>); |
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11 | |
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12 | DESCRIPTION |
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13 | <<strncmp>> compares up to <[length]> characters |
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14 | from the string at <[a]> to the string at <[b]>. |
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15 | |
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16 | RETURNS |
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17 | If <<*<[a]>>> sorts lexicographically after <<*<[b]>>>, |
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18 | <<strncmp>> returns a number greater than zero. If the two |
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19 | strings are equivalent, <<strncmp>> returns zero. If <<*<[a]>>> |
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20 | sorts lexicographically before <<*<[b]>>>, <<strncmp>> returns a |
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21 | number less than zero. |
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22 | |
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23 | PORTABILITY |
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24 | <<strncmp>> is ANSI C. |
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25 | |
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26 | <<strncmp>> requires no supporting OS subroutines. |
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27 | |
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28 | QUICKREF |
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29 | strncmp ansi pure |
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30 | */ |
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31 | |
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32 | #include <string.h> |
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33 | #include <limits.h> |
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34 | |
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35 | /* Nonzero if either X or Y is not aligned on a "long" boundary. */ |
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36 | #define UNALIGNED(X, Y) \ |
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37 | (((long)X & (sizeof (long) - 1)) | ((long)Y & (sizeof (long) - 1))) |
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38 | |
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39 | /* DETECTNULL returns nonzero if (long)X contains a NULL byte. */ |
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40 | #if LONG_MAX == 2147483647L |
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41 | #define DETECTNULL(X) (((X) - 0x01010101) & ~(X) & 0x80808080) |
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42 | #else |
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43 | #if LONG_MAX == 9223372036854775807L |
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44 | #define DETECTNULL(X) (((X) - 0x0101010101010101) & ~(X) & 0x8080808080808080) |
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45 | #else |
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46 | #error long int is not a 32bit or 64bit type. |
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47 | #endif |
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48 | #endif |
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49 | |
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50 | #ifndef DETECTNULL |
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51 | #error long int is not a 32bit or 64bit byte |
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52 | #endif |
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53 | |
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54 | int |
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55 | strncmp (const char *s1, |
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56 | const char *s2, |
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57 | size_t n) |
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58 | { |
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59 | #if defined(PREFER_SIZE_OVER_SPEED) || defined(__OPTIMIZE_SIZE__) |
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60 | if (n == 0) |
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61 | return 0; |
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62 | |
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63 | while (n-- != 0 && *s1 == *s2) |
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64 | { |
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65 | if (n == 0 || *s1 == '\0') |
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66 | break; |
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67 | s1++; |
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68 | s2++; |
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69 | } |
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70 | |
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71 | return (*(unsigned char *) s1) - (*(unsigned char *) s2); |
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72 | #else |
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73 | unsigned long *a1; |
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74 | unsigned long *a2; |
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75 | |
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76 | if (n == 0) |
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77 | return 0; |
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78 | |
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79 | /* If s1 or s2 are unaligned, then compare bytes. */ |
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80 | if (!UNALIGNED (s1, s2)) |
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81 | { |
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82 | /* If s1 and s2 are word-aligned, compare them a word at a time. */ |
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83 | a1 = (unsigned long*)s1; |
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84 | a2 = (unsigned long*)s2; |
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85 | while (n >= sizeof (long) && *a1 == *a2) |
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86 | { |
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87 | n -= sizeof (long); |
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88 | |
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89 | /* If we've run out of bytes or hit a null, return zero |
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90 | since we already know *a1 == *a2. */ |
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91 | if (n == 0 || DETECTNULL (*a1)) |
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92 | return 0; |
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93 | |
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94 | a1++; |
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95 | a2++; |
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96 | } |
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97 | |
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98 | /* A difference was detected in last few bytes of s1, so search bytewise */ |
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99 | s1 = (char*)a1; |
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100 | s2 = (char*)a2; |
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101 | } |
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102 | |
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103 | while (n-- > 0 && *s1 == *s2) |
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104 | { |
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105 | /* If we've run out of bytes or hit a null, return zero |
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106 | since we already know *s1 == *s2. */ |
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107 | if (n == 0 || *s1 == '\0') |
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108 | return 0; |
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109 | s1++; |
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110 | s2++; |
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111 | } |
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112 | return (*(unsigned char *) s1) - (*(unsigned char *) s2); |
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113 | #endif /* not PREFER_SIZE_OVER_SPEED */ |
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114 | } |
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