1  /* memrchr -- find the last occurrence of a byte in a memory block
       2     Copyright (C) 1991-2023 Free Software Foundation, Inc.
       3     This file is part of the GNU C Library.
       4  
       5     The GNU C Library is free software; you can redistribute it and/or
       6     modify it under the terms of the GNU Lesser General Public
       7     License as published by the Free Software Foundation; either
       8     version 2.1 of the License, or (at your option) any later version.
       9  
      10     The GNU C Library is distributed in the hope that it will be useful,
      11     but WITHOUT ANY WARRANTY; without even the implied warranty of
      12     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
      13     Lesser General Public License for more details.
      14  
      15     You should have received a copy of the GNU Lesser General Public
      16     License along with the GNU C Library; if not, see
      17     <https://www.gnu.org/licenses/>.  */
      18  
      19  #include <string-fzb.h>
      20  #include <string-fzc.h>
      21  #include <string-fzi.h>
      22  #include <string-shift.h>
      23  #include <string.h>
      24  #include <libc-pointer-arith.h>
      25  
      26  #undef __memrchr
      27  #undef memrchr
      28  
      29  #ifdef MEMRCHR
      30  # define __memrchr MEMRCHR
      31  #endif
      32  
      33  void *
      34  __memrchr (const void *s, int c_in, size_t n)
      35  {
      36    if (__glibc_unlikely (n == 0))
      37      return NULL;
      38  
      39    const op_t *word_ptr = (const op_t *) PTR_ALIGN_UP (s + n, sizeof (op_t));
      40    uintptr_t s_int = (uintptr_t) s + n;
      41  
      42    op_t word = *--word_ptr;
      43    op_t repeated_c = repeat_bytes (c_in);
      44  
      45    /* Compute the address of the word containing the initial byte. */
      46    const op_t *sword = (const op_t *) PTR_ALIGN_DOWN (s, sizeof (op_t));
      47  
      48    /* If the end of buffer is not op_t aligned, mask off the undesirable bits
      49       before find the last byte position.  */
      50    find_t mask = shift_find_last (find_eq_all (word, repeated_c), s_int);
      51    if (mask != 0)
      52      {
      53        char *ret = (char *) word_ptr + index_last (mask);
      54        return ret >= (char *) s ? ret : NULL;
      55      }
      56    if (word_ptr == sword)
      57      return NULL;
      58    word = *--word_ptr;
      59  
      60    while (word_ptr != sword)
      61      {
      62        if (has_eq (word, repeated_c))
      63  	return (char *) word_ptr + index_last_eq (word, repeated_c);
      64        word = *--word_ptr;
      65      }
      66  
      67    if (has_eq (word, repeated_c))
      68      {
      69        /* We found a match, but it might be in a byte past the end of the
      70  	 array.  */
      71        char *ret = (char *) word_ptr + index_last_eq (word, repeated_c);
      72        if (ret >= (char *) s)
      73  	return ret;
      74      }
      75    return NULL;
      76  }
      77  #ifndef MEMRCHR
      78  libc_hidden_def (__memrchr)
      79  weak_alias (__memrchr, memrchr)
      80  #endif