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glibc/string/strlen.c
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/* Copyright (C) 1991-2016 Free Software Foundation, Inc. | |
This file is part of the GNU C Library. | |
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 License, or (at your option) any later version. | |
The GNU C Library is distributed in the hope that it will be useful, | |
but WITHOUT ANY WARRANTY; without even the implied warranty of | |
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
Lesser General Public License for more details. | |
You should have received a copy of the GNU Lesser General Public | |
License along with the GNU C Library; if not, see | |
<http://www.gnu.org/licenses/>. */ | |
#include <string.h> | |
#include <stdlib.h> | |
#undef strlen | |
#ifndef STRLEN | |
# define STRLEN strlen | |
#endif | |
/* Return the length of the null-terminated string STR. Scan for | |
the null terminator quickly by testing four bytes at a time. */ | |
size_t | |
STRLEN (const char *str) | |
{ | |
const char *char_ptr; | |
const unsigned long int *longword_ptr; | |
unsigned long int longword, himagic, lomagic; | |
/* 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') | |
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. */ | |
for (;;) | |
{ | |
longword = *longword_ptr++; | |
if (((longword - lomagic) & ~longword & himagic) != 0) | |
{ | |
/* 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); | |
if (cp[0] == 0) | |
return cp - str; | |
if (cp[1] == 0) | |
return cp - str + 1; | |
if (cp[2] == 0) | |
return cp - str + 2; | |
if (cp[3] == 0) | |
return cp - str + 3; | |
if (sizeof (longword) > 4) | |
{ | |
if (cp[4] == 0) | |
return cp - str + 4; | |
if (cp[5] == 0) | |
return cp - str + 5; | |
if (cp[6] == 0) | |
return cp - str + 6; | |
if (cp[7] == 0) | |
return cp - str + 7; | |
} | |
} | |
} | |
} | |
libc_hidden_builtin_def (strlen) |