diff options
Diffstat (limited to 'string')
| -rw-r--r-- | string/strnlen.c | 137 |
1 files changed, 5 insertions, 132 deletions
diff --git a/string/strnlen.c b/string/strnlen.c index 6ff294e..dc23354 100644 --- a/string/strnlen.c +++ b/string/strnlen.c @@ -1,10 +1,6 @@ /* Find the length of STRING, but scan at most MAXLEN characters. Copyright (C) 1991-2023 Free Software Foundation, Inc. - Based on strlen written by Torbjorn Granlund (tege@sics.se), - with help from Dan Sahlin (dan@sics.se); - commentary by Jim Blandy (jimb@ai.mit.edu). - The GNU C Library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the @@ -20,7 +16,6 @@ not, see <https://www.gnu.org/licenses/>. */ #include <string.h> -#include <stdlib.h> /* Find the length of S, but scan at most MAXLEN characters. If no '\0' terminator is found in that many characters, return MAXLEN. */ @@ -32,134 +27,12 @@ size_t __strnlen (const char *str, size_t maxlen) { - const char *char_ptr, *end_ptr = str + maxlen; - const unsigned long int *longword_ptr; - unsigned long int longword, himagic, lomagic; - - if (maxlen == 0) - return 0; - - if (__glibc_unlikely (end_ptr < str)) - end_ptr = (const char *) ~0UL; - - /* Handle the first few characters by reading one character at a time. - Do this until CHAR_PTR is aligned on a longword boundary. */ - for (char_ptr = str; ((unsigned long int) char_ptr - & (sizeof (longword) - 1)) != 0; - ++char_ptr) - if (*char_ptr == '\0') - { - if (char_ptr > end_ptr) - char_ptr = end_ptr; - return char_ptr - str; - } - - /* All these elucidatory comments refer to 4-byte longwords, - but the theory applies equally well to 8-byte longwords. */ - - longword_ptr = (unsigned long int *) char_ptr; - - /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits - the "holes." Note that there is a hole just to the left of - each byte, with an extra at the end: - - bits: 01111110 11111110 11111110 11111111 - bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD - - The 1-bits make sure that carries propagate to the next 0-bit. - The 0-bits provide holes for carries to fall into. */ - himagic = 0x80808080L; - lomagic = 0x01010101L; - if (sizeof (longword) > 4) - { - /* 64-bit version of the magic. */ - /* Do the shift in two steps to avoid a warning if long has 32 bits. */ - himagic = ((himagic << 16) << 16) | himagic; - lomagic = ((lomagic << 16) << 16) | lomagic; - } - if (sizeof (longword) > 8) - abort (); - - /* Instead of the traditional loop which tests each character, - we will test a longword at a time. The tricky part is testing - if *any of the four* bytes in the longword in question are zero. */ - while (longword_ptr < (unsigned long int *) end_ptr) - { - /* We tentatively exit the loop if adding MAGIC_BITS to - LONGWORD fails to change any of the hole bits of LONGWORD. - - 1) Is this safe? Will it catch all the zero bytes? - Suppose there is a byte with all zeros. Any carry bits - propagating from its left will fall into the hole at its - least significant bit and stop. Since there will be no - carry from its most significant bit, the LSB of the - byte to the left will be unchanged, and the zero will be - detected. - - 2) Is this worthwhile? Will it ignore everything except - zero bytes? Suppose every byte of LONGWORD has a bit set - somewhere. There will be a carry into bit 8. If bit 8 - is set, this will carry into bit 16. If bit 8 is clear, - one of bits 9-15 must be set, so there will be a carry - into bit 16. Similarly, there will be a carry into bit - 24. If one of bits 24-30 is set, there will be a carry - into bit 31, so all of the hole bits will be changed. - - The one misfire occurs when bits 24-30 are clear and bit - 31 is set; in this case, the hole at bit 31 is not - changed. If we had access to the processor carry flag, - we could close this loophole by putting the fourth hole - at bit 32! - - So it ignores everything except 128's, when they're aligned - properly. */ - - longword = *longword_ptr++; - - if ((longword - lomagic) & himagic) - { - /* Which of the bytes was the zero? If none of them were, it was - a misfire; continue the search. */ - - const char *cp = (const char *) (longword_ptr - 1); - - char_ptr = cp; - if (cp[0] == 0) - break; - char_ptr = cp + 1; - if (cp[1] == 0) - break; - char_ptr = cp + 2; - if (cp[2] == 0) - break; - char_ptr = cp + 3; - if (cp[3] == 0) - break; - if (sizeof (longword) > 4) - { - char_ptr = cp + 4; - if (cp[4] == 0) - break; - char_ptr = cp + 5; - if (cp[5] == 0) - break; - char_ptr = cp + 6; - if (cp[6] == 0) - break; - char_ptr = cp + 7; - if (cp[7] == 0) - break; - } - } - char_ptr = end_ptr; - } - - if (char_ptr > end_ptr) - char_ptr = end_ptr; - return char_ptr - str; + const char *found = memchr (str, '\0', maxlen); + return found ? found - str : maxlen; } + #ifndef STRNLEN -libc_hidden_def (__strnlen) weak_alias (__strnlen, strnlen) -#endif +libc_hidden_def (__strnlen) libc_hidden_def (strnlen) +#endif |