glibc
2.9

#include <sys/types.h>
Go to the source code of this file.
Defines  
#define  __ptr_t char * 
#define  reg_char char 
#define  LONG_MAX_32_BITS 2147483647 
#define  LONG_MAX LONG_MAX_32_BITS 
#define  BP_SYM(sym) sym 
Functions  
__ptr_t  __memchr (__ptr_t s, int c_in, size_t n) const 
#define LONG_MAX LONG_MAX_32_BITS 
#define LONG_MAX_32_BITS 2147483647 
Definition at line 68 of file memchr.c.
{ const unsigned char *char_ptr; const unsigned long int *longword_ptr; unsigned long int longword, magic_bits, charmask; unsigned reg_char c; c = (unsigned char) c_in; /* 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 = (const unsigned char *) s; n > 0 && ((unsigned long int) char_ptr & (sizeof (longword)  1)) != 0; n, ++char_ptr) if (*char_ptr == c) return (__ptr_t) char_ptr; /* All these elucidatory comments refer to 4byte longwords, but the theory applies equally well to 8byte 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 1bits make sure that carries propagate to the next 0bit. The 0bits provide holes for carries to fall into. */ if (sizeof (longword) != 4 && sizeof (longword) != 8) abort (); #if LONG_MAX <= LONG_MAX_32_BITS magic_bits = 0x7efefeff; #else magic_bits = ((unsigned long int) 0x7efefefe << 32)  0xfefefeff; #endif /* Set up a longword, each of whose bytes is C. */ charmask = c  (c << 8); charmask = charmask << 16; #if LONG_MAX > LONG_MAX_32_BITS charmask = charmask << 32; #endif /* 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 (n >= sizeof (longword)) { /* 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 915 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 2430 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 2430 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. 3) But wait! Aren't we looking for C, not zero? Good point. So what we do is XOR LONGWORD with a longword, each of whose bytes is C. This turns each byte that is C into a zero. */ longword = *longword_ptr++ ^ charmask; /* Add MAGIC_BITS to LONGWORD. */ if ((((longword + magic_bits) /* Set those bits that were unchanged by the addition. */ ^ ~longword) /* Look at only the hole bits. If any of the hole bits are unchanged, most likely one of the bytes was a zero. */ & ~magic_bits) != 0) { /* Which of the bytes was C? If none of them were, it was a misfire; continue the search. */ const unsigned char *cp = (const unsigned char *) (longword_ptr  1); if (cp[0] == c) return (__ptr_t) cp; if (cp[1] == c) return (__ptr_t) &cp[1]; if (cp[2] == c) return (__ptr_t) &cp[2]; if (cp[3] == c) return (__ptr_t) &cp[3]; #if LONG_MAX > 2147483647 if (cp[4] == c) return (__ptr_t) &cp[4]; if (cp[5] == c) return (__ptr_t) &cp[5]; if (cp[6] == c) return (__ptr_t) &cp[6]; if (cp[7] == c) return (__ptr_t) &cp[7]; #endif } n = sizeof (longword); } char_ptr = (const unsigned char *) longword_ptr; while (n > 0) { if (*char_ptr == c) return (__ptr_t) char_ptr; else ++char_ptr; } return 0; }