1/* memrchr -- find the last occurrence of a byte in a memory block
2 Copyright (C) 1991-2024 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
33void *
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 (word: find_eq_all (x1: word, x2: repeated_c), s: s_int);
51 if (mask != 0)
52 {
53 char *ret = (char *) word_ptr + index_last (c: 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 (x1: word, x2: repeated_c))
63 return (char *) word_ptr + index_last_eq (x1: word, x2: repeated_c);
64 word = *--word_ptr;
65 }
66
67 if (has_eq (x1: word, x2: 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 (x1: word, x2: repeated_c);
72 if (ret >= (char *) s)
73 return ret;
74 }
75 return NULL;
76}
77#ifndef MEMRCHR
78libc_hidden_def (__memrchr)
79weak_alias (__memrchr, memrchr)
80#endif
81

source code of glibc/string/memrchr.c