crc循环校验的具体算法.txt

Filename: main.c * Description: A simple test program for the CRC implementations. * Notes: To test

********************************************************************** 
* 
* Filename: main.c 
* 
* Description: A simple test program for the CRC implementations. 
* 
* Notes: To test a different CRC standard, modify crc.h. 
* 
* 
* Copyright (c) 2000 by Michael Barr. This software is placed into 
* the public domain and may be used for any purpose. However, this 
* notice must not be changed or removed and no warranty is either 
* expressed or implied by its publication or distribution. 
**********************************************************************/ 


#include <stdio.h> 
#include <string.h> 

#include "crc.h" 


void 
main(void) 
{ 
unsigned char test[] = "123456789"; 


/* 
* Print the check value for the selected CRC algorithm. 
*/ 
printf("The check value for the %s standard is 0x%X\n", CRC_NAME, 
CHECK_VALUE); 

/* 
* Compute the CRC of the test message, slowly. 
*/ 
*/ 
printf("The crcSlow() of \"123456789\" is 0x%X\n", crcSlow(test, 
strlen(test))); 

/* 
* Compute the CRC of the test message, more efficiently. 
*/ 
crcInit(); 
printf("The crcFast() of \"123456789\" is 0x%X\n", crcFast(test, 
strlen(test))); 

} /* main() */ 

/********************************************************************** 
* 
* Filename: crc.h 
* 
* Description: A header file describing the various CRC standards. 
* 
* Notes: 
* 
* 
* Copyright (c) 2000 by Michael Barr. This software is placed into 

* the public domain and may be used for any purpose. However, this 
* notice must not be changed or removed and no warranty is either 
* expressed or implied by its publication or distribution. 
**********************************************************************/ 

#ifndef _crc_h 
#define _crc_h 


#define FALSE 0 
#define TRUE !FALSE 

/* 
* Select the CRC standard from the list that follows. 
*/ 
#define CRC_CCITT 


#if defined(CRC_CCITT) 

typedef unsigned short crc; 


#define CRC_NAME "CRC-CCITT" 
#define POLYNOMIAL 0x1021 
#define INITIAL_REMAINDER 0xFFFF 
#define FINAL_XOR_VALUE 0x0000 
#define REFLECT_DATA FALSE 
#define REFLECT_REMAINDER FALSE 
#define CHECK_VALUE 0x29B1 

#elif defined(CRC16) 

typedef unsigned short crc; 

#define CRC_NAME "CRC-16" 
#define POLYNOMIAL 0x8005 
#define INITIAL_REMAINDER 0x0000 
#define FINAL_XOR_VALUE 0x0000 
#define REFLECT_DATA TRUE 
#define REFLECT_REMAINDER TRUE 
#define CHECK_VALUE 0xBB3D 

#elif defined(CRC32) 


typedef unsigned long crc; 

#define CRC_NAME "CRC-32" 
#define POLYNOMIAL 0x04C11DB7 
#define INITIAL_REMAINDER 0xFFFFFFFF 
#define FINAL_XOR_VALUE 0xFFFFFFFF 
#define REFLECT_DATA TRUE 
#define REFLECT_REMAINDER TRUE 
#define CHECK_VALUE 0xCBF43926 

#else 

#error "One of CRC_CCITT, CRC16, or CRC32 must be #define'd." 

#endif 


void crcInit(void); 
crc crcSlow(unsigned char const message[], int nBytes); 
crc crcFast(unsigned char const message[], int nBytes); 



#endif /* _crc_h */ 



/********************************************************************** 
* 
* Filename: crc.c 
* 
* Description: Slow and fast implementations of the CRC standards. 
* 
* Notes: The parameters for each supported CRC standard are 
* defined in the header file crc.h. The implementtations 
* here should stand up to further additions to thaat list. 
* 
* 
* Copyright (c) 2000 by Michael Barr. This software is placed into 
* the public domain and may be used for any purpose. However, this 
* notice must not be changed or removed and no warranty is either 
* expressed or implied by its publication or distribution. 
**********************************************************************/ 

#include "crc.h" 
#include "crc.h" 


/* 
* Derive parameters from the standard-specific parameters in crc.h. 
*/ 
#define WIDTH (8 * sizeof(crc)) 
#define TOPBIT (1 << (WIDTH - 1)) 

#if (REFLECT_DATA == TRUE) 
#undef REFLECT_DATA 
#define REFLECT_DATA(X) ((unsigned char) reflect((X), 8)) 
#else 
#undef REFLECT_DATA 
#define REFLECT_DATA(X) (X) 
#endif 

#if (REFLECT_REMAINDER == TRUE) 
#undef REFLECT_REMAINDER 
#define REFLECT_REMAINDER(X) ((crc) reflect((X), WIDTH)) 
#else 
#undef REFLECT_REMAINDER 
#define REFLECT_REMAINDER(X) (X) 

#endif 


/********************************************************************* 
* 
* Function: reflect() 
* 
* Description: Reorder the bits of a binary sequence, by reflecting 
* them about the middle position. 
* 
* Notes: No checking is done that nBits <= 32. 
* 
* Returns: The reflection of the original data. 
* 
*********************************************************************/ 
static unsigned long 
reflect(unsigned long data, unsigned char nBits) 
{ 
unsigned long reflection = 0x00000000; 
unsigned char bit; 

/* 
/* 
* Reflect the data about the center bit. 
*/ 
for (bit = 0; bit < nBits; ++bit) 
{ 
/* 
* If the LSB bit is set, set the reflection of it. 
*/ 
if (data & 0x01) 
{ 
reflection |= (1 << ((nBits - 1) - bit)); 
} 

data = (data >> 1); 
} 

return (reflection); 

} /* reflect() */ 


/********************************************************************* 
* 
* 
* Function: crcSlow() 
* 
* Description: Compute the CRC of a given message. 
* 
* Notes: 
* 
* Returns: The CRC of the message. 
* 
*********************************************************************/ 
crc 
crcSlow(unsigned char const message[], int nBytes) 
{ 
crc remainder = INITIAL_REMAINDER; 
int byte; 
unsigned char bit; 


/* 
* Perform modulo-2 division, a byte at a time. 
*/ 
for (byte = 0; byte < nBytes; ++byte) 
{ 
{ 
/* 
* Bring the next byte into the remainder. 
*/ 
remainder ^= (REFLECT_DATA(message[byte]) << (WIDTH - 8)); 

/* 
* Perform modulo-2 division, a bit at a time. 
*/ 
for (bit = 8; bit > 0; --bit) 
{ 
/* 
* Try to divide the current data bit. 
*/ 
if (remainder & TOPBIT) 
{ 
remainder = (remainder << 1) ^ POLYNOMIAL; 
} 
else 
{ 
remainder = (remainder << 1); 
} 
} 
} 
} 

/* 
* The final remainder is the CRC result. 
*/ 
return (REFLECT_REMAINDER(remainder) ^ FINAL_XOR_VALUE); 

} /* crcSlow() */ 


crc crcTable[256]; 


/********************************************************************* 
* 
* Function: crcInit() 
* 
* Description: Populate the partial CRC lookup table. 
* 
* Notes: This function must be rerun any time the CRC standard 
* is changed. If desired, it can be run "offline"" and 
* the table results stored in an embedded system'ss ROM. 

* 
* Returns: None defined. 
* 
*********************************************************************/ 
void 
crcInit(void) 
{ 
crc remainder; 
int dividend; 
unsigned char bit; 


/* 
* Compute the remainder of each possible dividend. 
*/ 
for (dividend = 0; dividend < 256; ++dividend) 
{ 
/* 
* Start with the dividend followed by zeros. 
*/ 
remainder = dividend << (WIDTH - 8); 


/* 
* Perform modulo-2 division, a bit at a time. 
*/ 
for (bit = 8; bit > 0; --bit) 
{ 
/* 
* Try to divide the current data bit. 
*/ 
if (remainder & TOPBIT) 
{ 
remainder = (remainder << 1) ^ POLYNOMIAL; 
} 
else 
{ 
remainder = (remainder << 1); 
} 
} 

/* 
* Store the result into the table. 
*/ 
crcTable[dividend] = remainder; 

} 

} /* crcInit() */ 


/********************************************************************* 
* 
* Function: crcFast() 
* 
* Description: Compute the CRC of a given message. 
* 
* Notes: crcInit() must be called first. 
* 
* Returns: The CRC of the message. 
* 
*********************************************************************/ 
crc 
crcFast(unsigned char const message[], int nBytes) 
{ 
crc remainder = INITIAL_REMAINDER; 
unsigned char data; 
int byte; 
int byte; 


/* 
* Divide the message by the polynomial, a byte at a time. 
*/ 
for (byte = 0; byte < nBytes; ++byte) 
{ 
data = REFLECT_DATA(message[byte]) ^ (remainder >> (WIDTH - 8)); 
remainder = crcTable[data] ^ (remainder << 8); 
} 

/* 
* The final remainder is the CRC. 
*/ 
return (REFLECT_REMAINDER(remainder) ^ FINAL_XOR_VALUE); 

} /* crcFast() */