newlib/libc/string/strncat.c

   1 /*
   2 FUNCTION
   3         <<strncat>>---concatenate strings
   4
   5 INDEX
   6         strncat
   7
   8 SYNOPSIS
   9         #include <string.h>
  10         char *strncat(char *restrict <[dst]>, const char *restrict <[src]>,
  11                       size_t <[length]>);
  12
  13 DESCRIPTION
  14         <<strncat>> appends not more than <[length]> characters from
  15         the string pointed to by <[src]> (including the terminating
  16         null character) to the end of the string pointed to by
  17         <[dst]>.  The initial character of <[src]> overwrites the null
  18         character at the end of <[dst]>.  A terminating null character
  19         is always appended to the result
  20
  21 WARNINGS
  22         Note that a null is always appended, so that if the copy is
  23         limited by the <[length]> argument, the number of characters
  24         appended to <[dst]> is <<n + 1>>.
  25
  26 RETURNS
  27         This function returns the initial value of <[dst]>
  28
  29 PORTABILITY
  30 <<strncat>> is ANSI C.
  31
  32 <<strncat>> requires no supporting OS subroutines.
  33
  34 QUICKREF
  35         strncat ansi pure
  36 */
  37
  38 #include <string.h>
  39 #include <limits.h>
  40
  41 /* Nonzero if X is aligned on a "long" boundary.  */
  42 #define ALIGNED(X) \
  43   (((long)X & (sizeof (long) - 1)) == 0)
  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 char *
  61 strncat (char *__restrict s1,
  62         const char *__restrict s2,
  63         size_t n)
  64 {
  65 #if defined(PREFER_SIZE_OVER_SPEED) || defined(__OPTIMIZE_SIZE__)
  66   char *s = s1;
  67
  68   while (*s1)
  69     s1++;
  70   while (n-- != 0 && (*s1++ = *s2++))
  71     {
  72       if (n == 0)
  73         *s1 = '\0';
  74     }
  75
  76   return s;
  77 #else
  78   char *s = s1;
  79
  80   /* Skip over the data in s1 as quickly as possible.  */
  81   if (ALIGNED (s1))
  82     {
  83       unsigned long *aligned_s1 = (unsigned long *)s1;
  84       while (!DETECTNULL (*aligned_s1))
  85         aligned_s1++;
  86
  87       s1 = (char *)aligned_s1;
  88     }
  89
  90   while (*s1)
  91     s1++;
  92
  93   /* s1 now points to the its trailing null character, now copy
  94      up to N bytes from S2 into S1 stopping if a NULL is encountered
  95      in S2.
  96
  97      It is not safe to use strncpy here since it copies EXACTLY N
  98      characters, NULL padding if necessary.  */
  99   while (n-- != 0 && (*s1++ = *s2++))
 100     {
 101       if (n == 0)
 102         *s1 = '\0';
 103     }
 104
 105   return s;
 106 #endif /* not PREFER_SIZE_OVER_SPEED */
 107 }