1 /* Copyright (C) 1991-2021 Free Software Foundation, Inc.
2 This file is part of the GNU C Library.
3
4 The GNU C Library is free software; you can redistribute it and/or
5 modify it under the terms of the GNU Lesser General Public
6 License as published by the Free Software Foundation; either
7 version 2.1 of the License, or (at your option) any later version.
8
9 The GNU C Library is distributed in the hope that it will be useful,
10 but WITHOUT ANY WARRANTY; without even the implied warranty of
11 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
12 Lesser General Public License for more details.
13
14 You should have received a copy of the GNU Lesser General Public
15 License along with the GNU C Library; if not, see
16 <https://www.gnu.org/licenses/>. */
17
18 #include <string.h>
19 #include <stdlib.h>
20
21 #undef strlen
22
23 #ifndef STRLEN
24 # define STRLEN strlen
25 #endif
26
27 /* Return the length of the null-terminated string STR. Scan for
28 the null terminator quickly by testing four bytes at a time. */
29 size_t
STRLEN(const char * str)30 STRLEN (const char *str)
31 {
32 const char *char_ptr;
33 const unsigned long int *longword_ptr;
34 unsigned long int longword, himagic, lomagic;
35
36 /* Handle the first few characters by reading one character at a time.
37 Do this until CHAR_PTR is aligned on a longword boundary. */
38 for (char_ptr = str; ((unsigned long int) char_ptr
39 & (sizeof (longword) - 1)) != 0;
40 ++char_ptr)
41 if (*char_ptr == '\0')
42 return char_ptr - str;
43
44 /* All these elucidatory comments refer to 4-byte longwords,
45 but the theory applies equally well to 8-byte longwords. */
46
47 longword_ptr = (unsigned long int *) char_ptr;
48
49 /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits
50 the "holes." Note that there is a hole just to the left of
51 each byte, with an extra at the end:
52
53 bits: 01111110 11111110 11111110 11111111
54 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
55
56 The 1-bits make sure that carries propagate to the next 0-bit.
57 The 0-bits provide holes for carries to fall into. */
58 himagic = 0x80808080L;
59 lomagic = 0x01010101L;
60 if (sizeof (longword) > 4)
61 {
62 /* 64-bit version of the magic. */
63 /* Do the shift in two steps to avoid a warning if long has 32 bits. */
64 himagic = ((himagic << 16) << 16) | himagic;
65 lomagic = ((lomagic << 16) << 16) | lomagic;
66 }
67 if (sizeof (longword) > 8)
68 abort ();
69
70 /* Instead of the traditional loop which tests each character,
71 we will test a longword at a time. The tricky part is testing
72 if *any of the four* bytes in the longword in question are zero. */
73 for (;;)
74 {
75 longword = *longword_ptr++;
76
77 if (((longword - lomagic) & ~longword & himagic) != 0)
78 {
79 /* Which of the bytes was the zero? If none of them were, it was
80 a misfire; continue the search. */
81
82 const char *cp = (const char *) (longword_ptr - 1);
83
84 if (cp[0] == 0)
85 return cp - str;
86 if (cp[1] == 0)
87 return cp - str + 1;
88 if (cp[2] == 0)
89 return cp - str + 2;
90 if (cp[3] == 0)
91 return cp - str + 3;
92 if (sizeof (longword) > 4)
93 {
94 if (cp[4] == 0)
95 return cp - str + 4;
96 if (cp[5] == 0)
97 return cp - str + 5;
98 if (cp[6] == 0)
99 return cp - str + 6;
100 if (cp[7] == 0)
101 return cp - str + 7;
102 }
103 }
104 }
105 }
106 libc_hidden_builtin_def (strlen)
107