newlib/libc/string/strncmp.c

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