對原始crc32校驗和的請求

有人可以向我解釋爲什麼crc被倒置兩次？對原始crc32校驗和的請求

/* ======================================================================== 
* Table of CRC-32's of all single-byte values (made by make_crc_table) 
*/// definicao de uint e uchar 

#include <stdio.h> 

//#define TEST 1 

#ifndef uchar 
#define uchar unsigned char 
#endif 

#ifndef uint 
#define uint unsigned int 
#endif 

#ifndef ushort 
#define ushort unsigned short 
#endif 

static const uint crc_table[256] = { 
    0x00000000L, 0x77073096L, 0xee0e612cL, 0x990951baL, 0x076dc419L, 
    0x706af48fL, 0xe963a535L, 0x9e6495a3L, 0x0edb8832L, 0x79dcb8a4L, 
    0xe0d5e91eL, 0x97d2d988L, 0x09b64c2bL, 0x7eb17cbdL, 0xe7b82d07L, 
    0x90bf1d91L, 0x1db71064L, 0x6ab020f2L, 0xf3b97148L, 0x84be41deL, 
    0x1adad47dL, 0x6ddde4ebL, 0xf4d4b551L, 0x83d385c7L, 0x136c9856L, 
    0x646ba8c0L, 0xfd62f97aL, 0x8a65c9ecL, 0x14015c4fL, 0x63066cd9L, 
    0xfa0f3d63L, 0x8d080df5L, 0x3b6e20c8L, 0x4c69105eL, 0xd56041e4L, 
    0xa2677172L, 0x3c03e4d1L, 0x4b04d447L, 0xd20d85fdL, 0xa50ab56bL, 
    0x35b5a8faL, 0x42b2986cL, 0xdbbbc9d6L, 0xacbcf940L, 0x32d86ce3L, 
    0x45df5c75L, 0xdcd60dcfL, 0xabd13d59L, 0x26d930acL, 0x51de003aL, 
    0xc8d75180L, 0xbfd06116L, 0x21b4f4b5L, 0x56b3c423L, 0xcfba9599L, 
    0xb8bda50fL, 0x2802b89eL, 0x5f058808L, 0xc60cd9b2L, 0xb10be924L, 
    0x2f6f7c87L, 0x58684c11L, 0xc1611dabL, 0xb6662d3dL, 0x76dc4190L, 
    0x01db7106L, 0x98d220bcL, 0xefd5102aL, 0x71b18589L, 0x06b6b51fL, 
    0x9fbfe4a5L, 0xe8b8d433L, 0x7807c9a2L, 0x0f00f934L, 0x9609a88eL, 
    0xe10e9818L, 0x7f6a0dbbL, 0x086d3d2dL, 0x91646c97L, 0xe6635c01L, 
    0x6b6b51f4L, 0x1c6c6162L, 0x856530d8L, 0xf262004eL, 0x6c0695edL, 
    0x1b01a57bL, 0x8208f4c1L, 0xf50fc457L, 0x65b0d9c6L, 0x12b7e950L, 
    0x8bbeb8eaL, 0xfcb9887cL, 0x62dd1ddfL, 0x15da2d49L, 0x8cd37cf3L, 
    0xfbd44c65L, 0x4db26158L, 0x3ab551ceL, 0xa3bc0074L, 0xd4bb30e2L, 
    0x4adfa541L, 0x3dd895d7L, 0xa4d1c46dL, 0xd3d6f4fbL, 0x4369e96aL, 
    0x346ed9fcL, 0xad678846L, 0xda60b8d0L, 0x44042d73L, 0x33031de5L, 
    0xaa0a4c5fL, 0xdd0d7cc9L, 0x5005713cL, 0x270241aaL, 0xbe0b1010L, 
    0xc90c2086L, 0x5768b525L, 0x206f85b3L, 0xb966d409L, 0xce61e49fL, 
    0x5edef90eL, 0x29d9c998L, 0xb0d09822L, 0xc7d7a8b4L, 0x59b33d17L, 
    0x2eb40d81L, 0xb7bd5c3bL, 0xc0ba6cadL, 0xedb88320L, 0x9abfb3b6L, 
    0x03b6e20cL, 0x74b1d29aL, 0xead54739L, 0x9dd277afL, 0x04db2615L, 
    0x73dc1683L, 0xe3630b12L, 0x94643b84L, 0x0d6d6a3eL, 0x7a6a5aa8L, 
    0xe40ecf0bL, 0x9309ff9dL, 0x0a00ae27L, 0x7d079eb1L, 0xf00f9344L, 
    0x8708a3d2L, 0x1e01f268L, 0x6906c2feL, 0xf762575dL, 0x806567cbL, 
    0x196c3671L, 0x6e6b06e7L, 0xfed41b76L, 0x89d32be0L, 0x10da7a5aL, 
    0x67dd4accL, 0xf9b9df6fL, 0x8ebeeff9L, 0x17b7be43L, 0x60b08ed5L, 
    0xd6d6a3e8L, 0xa1d1937eL, 0x38d8c2c4L, 0x4fdff252L, 0xd1bb67f1L, 
    0xa6bc5767L, 0x3fb506ddL, 0x48b2364bL, 0xd80d2bdaL, 0xaf0a1b4cL, 
    0x36034af6L, 0x41047a60L, 0xdf60efc3L, 0xa867df55L, 0x316e8eefL, 
    0x4669be79L, 0xcb61b38cL, 0xbc66831aL, 0x256fd2a0L, 0x5268e236L, 
    0xcc0c7795L, 0xbb0b4703L, 0x220216b9L, 0x5505262fL, 0xc5ba3bbeL, 
    0xb2bd0b28L, 0x2bb45a92L, 0x5cb36a04L, 0xc2d7ffa7L, 0xb5d0cf31L, 
    0x2cd99e8bL, 0x5bdeae1dL, 0x9b64c2b0L, 0xec63f226L, 0x756aa39cL, 
    0x026d930aL, 0x9c0906a9L, 0xeb0e363fL, 0x72076785L, 0x05005713L, 
    0x95bf4a82L, 0xe2b87a14L, 0x7bb12baeL, 0x0cb61b38L, 0x92d28e9bL, 
    0xe5d5be0dL, 0x7cdcefb7L, 0x0bdbdf21L, 0x86d3d2d4L, 0xf1d4e242L, 
    0x68ddb3f8L, 0x1fda836eL, 0x81be16cdL, 0xf6b9265bL, 0x6fb077e1L, 
    0x18b74777L, 0x88085ae6L, 0xff0f6a70L, 0x66063bcaL, 0x11010b5cL, 
    0x8f659effL, 0xf862ae69L, 0x616bffd3L, 0x166ccf45L, 0xa00ae278L, 
    0xd70dd2eeL, 0x4e048354L, 0x3903b3c2L, 0xa7672661L, 0xd06016f7L, 
    0x4969474dL, 0x3e6e77dbL, 0xaed16a4aL, 0xd9d65adcL, 0x40df0b66L, 
    0x37d83bf0L, 0xa9bcae53L, 0xdebb9ec5L, 0x47b2cf7fL, 0x30b5ffe9L, 
    0xbdbdf21cL, 0xcabac28aL, 0x53b39330L, 0x24b4a3a6L, 0xbad03605L, 
    0xcdd70693L, 0x54de5729L, 0x23d967bfL, 0xb3667a2eL, 0xc4614ab8L, 
    0x5d681b02L, 0x2a6f2b94L, 0xb40bbe37L, 0xc30c8ea1L, 0x5a05df1bL, 
    0x2d02ef8dL 
}; 


/* ========================================================================= 
* This function can be used by asm versions of crc32() 
*/ 
const uint * get_crc_table() 
{ 
#ifdef DYNAMIC_CRC_TABLE 
    if (crc_table_empty) make_crc_table(); 
#endif 
    return (const uint *)crc_table; 
} 

/* ========================================================================= */ 
#define DO1(buf) crc = crc_table[((int)crc^(*buf++)) & 0xff]^(crc >> 8); 
#define DO2(buf) DO1(buf); DO1(buf); 
#define DO4(buf) DO2(buf); DO2(buf); 
#define DO8(buf) DO4(buf); DO4(buf); 

/* ========================================================================= */ 
uint crc32(
    uint crc, 
    const uchar *buf, 
    uint len) 
{ 
    if (buf == NULL) return 0L; 
#ifdef DYNAMIC_CRC_TABLE 
    if (crc_table_empty) 
     make_crc_table(); 
#endif 
    crc = crc^0xffffffffL; 
    while (len >= 8) 
    { 
     DO8(buf); 
     len -= 8; 
    } 
    if (len) do { 
     DO1(buf); 
    } while (--len); 
    return crc^0xffffffffL; 
} 


#ifdef TEST 

int main() 
{ 

    printf("Testando crc \n"); 
     printf("%0.8x \n ", crc32(0,"ala", 1)); 



} 


#endif

來源

2009-11-18 Mandrake

是那個完整的代碼？它似乎不完整（考慮到crc除了反轉之外沒有改變）。 DOx函數是否定義了？ – barkmadley 2009-11-18 13:30:02

常見，這是任何人使用它的原始crc32實現 – Mandrake 2009-11-18 13:32:58

#defines與意想不到的副作用是邪惡的 – pmg 2009-11-18 13:39:01

第一翻轉做是因爲否則，加入前導零個比特的緩衝器不會改變所得的CRC值（假設初始CRC值爲零）。

我相信最後的反轉是這樣完成的，因此可以逐漸調用這個函數：不必一次調用整個數據，而是可以調用它，並且兩個反轉會相互取消在相鄰的街區。

請注意，這些反轉不是在所有的CRC32實現上完成的。

來源

2009-11-18 13:46:04 interjay

這是zip格式crc32 – Mandrake 2009-11-18 13:49:35

如果你有興趣在CRC校驗，那麼這是一個必須閱讀：

http://www.ross.net/crc/download/crc_v3.txt

雖然已經很長一段時間，因爲我讀過它，所以我不知道它會回答你的具體問題。

編輯：至少它包含參數化。

 
Here is the specification for the CRC-32 algorithm which is reportedly 
used in PKZip, AUTODIN II, Ethernet, and FDDI. 

    Name : "CRC-32" 
    Width : 32 
    Poly : 04C11DB7 
    Init : FFFFFFFF 
    RefIn : True 
    RefOut : True 
    XorOut : FFFFFFFF 
    Check : CBF43926

來源

2009-11-18 15:10:39 Secure

對原始crc32校驗和的請求

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