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  1. /*
  2. FUNCTION
  3.         <<strncmp>>---character string compare
  4.        
  5. INDEX
  6.         strncmp
  7.  
  8. ANSI_SYNOPSIS
  9.         #include <string.h>
  10.         int strncmp(const char *<[a]>, const char * <[b]>, size_t <[length]>);
  11.  
  12. TRAD_SYNOPSIS
  13.         #include <string.h>
  14.         int strncmp(<[a]>, <[b]>, <[length]>)
  15.         char *<[a]>;
  16.         char *<[b]>;
  17.         size_t <[length]>
  18.  
  19. DESCRIPTION
  20.         <<strncmp>> compares up to <[length]> characters
  21.         from the string at <[a]> to the string at <[b]>.
  22.  
  23. RETURNS
  24.         If <<*<[a]>>> sorts lexicographically after <<*<[b]>>>,
  25.         <<strncmp>> returns a number greater than zero.  If the two
  26.         strings are equivalent, <<strncmp>> returns zero.  If <<*<[a]>>>
  27.         sorts lexicographically before <<*<[b]>>>, <<strncmp>> returns a
  28.         number less than zero.
  29.  
  30. PORTABILITY
  31. <<strncmp>> is ANSI C.
  32.  
  33. <<strncmp>> requires no supporting OS subroutines.
  34.  
  35. QUICKREF
  36.         strncmp ansi pure
  37. */
  38.  
  39. #include <string.h>
  40. #include <limits.h>
  41.  
  42. /* Nonzero if either X or Y is not aligned on a "long" boundary.  */
  43. #define UNALIGNED(X, Y) \
  44.   (((long)X & (sizeof (long) - 1)) | ((long)Y & (sizeof (long) - 1)))
  45.  
  46. /* DETECTNULL returns nonzero if (long)X contains a NULL byte. */
  47. #if LONG_MAX == 2147483647L
  48. #define DETECTNULL(X) (((X) - 0x01010101) & ~(X) & 0x80808080)
  49. #else
  50. #if LONG_MAX == 9223372036854775807L
  51. #define DETECTNULL(X) (((X) - 0x0101010101010101) & ~(X) & 0x8080808080808080)
  52. #else
  53. #error long int is not a 32bit or 64bit type.
  54. #endif
  55. #endif
  56.  
  57. #ifndef DETECTNULL
  58. #error long int is not a 32bit or 64bit byte
  59. #endif
  60.  
  61. int
  62. _DEFUN (strncmp, (s1, s2, n),
  63.         _CONST char *s1 _AND
  64.         _CONST char *s2 _AND
  65.         size_t n)
  66. {
  67. #if defined(PREFER_SIZE_OVER_SPEED) || defined(__OPTIMIZE_SIZE__)
  68.   if (n == 0)
  69.     return 0;
  70.  
  71.   while (n-- != 0 && *s1 == *s2)
  72.     {
  73.       if (n == 0 || *s1 == '\0')
  74.         break;
  75.       s1++;
  76.       s2++;
  77.     }
  78.  
  79.   return (*(unsigned char *) s1) - (*(unsigned char *) s2);
  80. #else
  81.   unsigned long *a1;
  82.   unsigned long *a2;
  83.  
  84.   if (n == 0)
  85.     return 0;
  86.  
  87.   /* If s1 or s2 are unaligned, then compare bytes. */
  88.   if (!UNALIGNED (s1, s2))
  89.     {
  90.       /* If s1 and s2 are word-aligned, compare them a word at a time. */
  91.       a1 = (unsigned long*)s1;
  92.       a2 = (unsigned long*)s2;
  93.       while (n >= sizeof (long) && *a1 == *a2)
  94.         {
  95.           n -= sizeof (long);
  96.  
  97.           /* If we've run out of bytes or hit a null, return zero
  98.              since we already know *a1 == *a2.  */
  99.           if (n == 0 || DETECTNULL (*a1))
  100.             return 0;
  101.  
  102.           a1++;
  103.           a2++;
  104.         }
  105.  
  106.       /* A difference was detected in last few bytes of s1, so search bytewise */
  107.       s1 = (char*)a1;
  108.       s2 = (char*)a2;
  109.     }
  110.  
  111.   while (n-- > 0 && *s1 == *s2)
  112.     {
  113.       /* If we've run out of bytes or hit a null, return zero
  114.          since we already know *s1 == *s2.  */
  115.       if (n == 0 || *s1 == '\0')
  116.         return 0;
  117.       s1++;
  118.       s2++;
  119.     }
  120.   return (*(unsigned char *) s1) - (*(unsigned char *) s2);
  121. #endif /* not PREFER_SIZE_OVER_SPEED */
  122. }
  123.