106 lines
3.4 KiB
C
106 lines
3.4 KiB
C
/* Copyright (C) 1991-2015 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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Written by Torbjorn Granlund (tege@sics.se),
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with help from Dan Sahlin (dan@sics.se);
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commentary by Jim Blandy (jimb@ai.mit.edu).
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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#include <string.h>
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#include <stdlib.h>
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#undef strlen
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/* Return the length of the null-terminated string STR. Scan for
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the null terminator quickly by testing four bytes at a time. */
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size_t
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strlen (const char *str)
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{
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const char *char_ptr;
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const unsigned long int *longword_ptr;
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unsigned long int longword, himagic, lomagic;
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/* Handle the first few characters by reading one character at a time.
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Do this until CHAR_PTR is aligned on a longword boundary. */
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for (char_ptr = str; ((unsigned long int) char_ptr
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& (sizeof (longword) - 1)) != 0;
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++char_ptr)
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if (*char_ptr == '\0')
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return char_ptr - str;
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/* All these elucidatory comments refer to 4-byte longwords,
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but the theory applies equally well to 8-byte longwords. */
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longword_ptr = (unsigned long int *) char_ptr;
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/* Bits 31, 24, 16, and 8 of this number are zero. Call these bits
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the "holes." Note that there is a hole just to the left of
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each byte, with an extra at the end:
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bits: 01111110 11111110 11111110 11111111
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bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
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The 1-bits make sure that carries propagate to the next 0-bit.
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The 0-bits provide holes for carries to fall into. */
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himagic = 0x80808080L;
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lomagic = 0x01010101L;
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if (sizeof (longword) > 4)
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{
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/* 64-bit version of the magic. */
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/* Do the shift in two steps to avoid a warning if long has 32 bits. */
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himagic = ((himagic << 16) << 16) | himagic;
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lomagic = ((lomagic << 16) << 16) | lomagic;
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}
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if (sizeof (longword) > 8)
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abort ();
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/* Instead of the traditional loop which tests each character,
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we will test a longword at a time. The tricky part is testing
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if *any of the four* bytes in the longword in question are zero. */
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for (;;)
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{
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longword = *longword_ptr++;
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if (((longword - lomagic) & ~longword & himagic) != 0)
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{
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/* Which of the bytes was the zero? If none of them were, it was
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a misfire; continue the search. */
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const char *cp = (const char *) (longword_ptr - 1);
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if (cp[0] == 0)
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return cp - str;
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if (cp[1] == 0)
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return cp - str + 1;
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if (cp[2] == 0)
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return cp - str + 2;
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if (cp[3] == 0)
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return cp - str + 3;
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if (sizeof (longword) > 4)
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{
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if (cp[4] == 0)
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return cp - str + 4;
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if (cp[5] == 0)
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return cp - str + 5;
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if (cp[6] == 0)
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return cp - str + 6;
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if (cp[7] == 0)
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return cp - str + 7;
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}
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}
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}
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}
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libc_hidden_builtin_def (strlen)
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