newlib/libc/string/memrchr.c

   1 /*
   2 FUNCTION
   3         <<memrchr>>---reverse search for character in memory
   4
   5 INDEX
   6         memrchr
   7
   8 SYNOPSIS
   9         #include <string.h>
  10         void *memrchr(const void *<[src]>, int <[c]>, size_t <[length]>);
  11
  12 DESCRIPTION
  13         This function searches memory starting at <[length]> bytes
  14         beyond <<*<[src]>>> backwards for the character <[c]>.
  15         The search only ends with the first occurrence of <[c]>; in
  16         particular, <<NUL>> does not terminate the search.
  17
  18 RETURNS
  19         If the character <[c]> is found within <[length]> characters
  20         of <<*<[src]>>>, a pointer to the character is returned. If
  21         <[c]> is not found, then <<NULL>> is returned.
  22
  23 PORTABILITY
  24 <<memrchr>> is a GNU extension.
  25
  26 <<memrchr>> requires no supporting OS subroutines.
  27
  28 QUICKREF
  29         memrchr
  30 */
  31
  32 #include <_ansi.h>
  33 #include <string.h>
  34 #include <limits.h>
  35
  36 /* Nonzero if X is not aligned on a "long" boundary.  */
  37 #define UNALIGNED(X) ((long)(X + 1) & (sizeof (long) - 1))
  38
  39 /* How many bytes are loaded each iteration of the word copy loop.  */
  40 #define LBLOCKSIZE (sizeof (long))
  41
  42 /* Threshhold for punting to the bytewise iterator.  */
  43 #define TOO_SMALL(LEN)  ((LEN) < LBLOCKSIZE)
  44
  45 #if LONG_MAX == 2147483647L
  46 #define DETECTNULL(X) (((X) - 0x01010101) & ~(X) & 0x80808080)
  47 #else
  48 #if LONG_MAX == 9223372036854775807L
  49 /* Nonzero if X (a long int) contains a NULL byte. */
  50 #define DETECTNULL(X) (((X) - 0x0101010101010101) & ~(X) & 0x8080808080808080)
  51 #else
  52 #error long int is not a 32bit or 64bit type.
  53 #endif
  54 #endif
  55
  56 #ifndef DETECTNULL
  57 #error long int is not a 32bit or 64bit byte
  58 #endif
  59
  60 /* DETECTCHAR returns nonzero if (long)X contains the byte used
  61    to fill (long)MASK. */
  62 #define DETECTCHAR(X,MASK) (DETECTNULL(X ^ MASK))
  63
  64 void *
  65 memrchr (const void *src_void,
  66         int c,
  67         size_t length)
  68 {
  69   const unsigned char *src = (const unsigned char *) src_void + length - 1;
  70   unsigned char d = c;
  71
  72 #if !defined(PREFER_SIZE_OVER_SPEED) && !defined(__OPTIMIZE_SIZE__)
  73   unsigned long *asrc;
  74   unsigned long  mask;
  75   unsigned int i;
  76
  77   while (UNALIGNED (src))
  78     {
  79       if (!length--)
  80         return NULL;
  81       if (*src == d)
  82         return (void *) src;
  83       src--;
  84     }
  85
  86   if (!TOO_SMALL (length))
  87     {
  88       /* If we get this far, we know that length is large and src is
  89          word-aligned. */
  90       /* The fast code reads the source one word at a time and only
  91          performs the bytewise search on word-sized segments if they
  92          contain the search character, which is detected by XORing
  93          the word-sized segment with a word-sized block of the search
  94          character and then detecting for the presence of NUL in the
  95          result.  */
  96       asrc = (unsigned long *) (src - LBLOCKSIZE + 1);
  97       mask = d << 8 | d;
  98       mask = mask << 16 | mask;
  99       for (i = 32; i < LBLOCKSIZE * 8; i <<= 1)
 100         mask = (mask << i) | mask;
 101
 102       while (length >= LBLOCKSIZE)
 103         {
 104           if (DETECTCHAR (*asrc, mask))
 105             break;
 106           length -= LBLOCKSIZE;
 107           asrc--;
 108         }
 109
 110       /* If there are fewer than LBLOCKSIZE characters left,
 111          then we resort to the bytewise loop.  */
 112
 113       src = (unsigned char *) asrc + LBLOCKSIZE - 1;
 114     }
 115
 116 #endif /* not PREFER_SIZE_OVER_SPEED */
 117
 118   while (length--)
 119     {
 120       if (*src == d)
 121         return (void *) src;
 122       src--;
 123     }
 124
 125   return NULL;
 126 }