C语言中CRC循环校验的一个程序

while(len--!=0)

这句的len的值循环一次就减少1，先执行len!=0，再执行腔衫len--。

当len为0时退出循环前圆锋。

for(i=0x80

i!=0

i/=2)

0x80是十慧晌六进制数，也即128

当i!=0时，执行循环体，

然后i=i/2,即i值减半。

下面我们以CRC-16为例来说明任意长度数据流的CRC校验码生成过程。我们采用将数据流分成若干个8bit字符，并由低字节到高字绝塌节传送的并行方法来求CRC校验码。具体计算过程为：用一个16bit的寄存器来存放CRC校验值，且设定其初值为0x0000；将数据流的第一个8bit与16bit的CRC寄存器的高字节相异或，并将结果存入CRC寄存器高字节；CRC寄存器左移一位，最低1bit补零，同时检查移出的最高1bit，若移出的最高1bit为0，则继续按上述过程左移，若最高1bit为1，则将CRC寄存器中的值与生成多项式码相异或，结果存入CRC寄存器值；继续左移并重复上述处理方法，直到将8bit数据处理完为止，则此时CRC寄存器中的值就是第一个8bit数据对应的CRC校验码；然后将此时CRC寄存器的值作为初值，用同样的处理方法重复上述步骤来处理下一个8bit数据流，直到将所有的8bit字符都处理完后，此刻CRC寄存器中的值即为整个数据流对应的CRC校验码。

下面示出了其计算过程的流程图：

在用C语言编写CRC校验码的实现程序时我们应该注意，生成多项式 对应的十六进制数为0x18005，由于CRC寄存器左移过程中，移出的最高位为1时与 相异或，所以与16bit的CRC寄存器对应并塌圆的生成多项式的十六进制数可用0x8005表示。下面给出并行处理8bit数据流的C源程序：

unsigned short crc_dsp(unsigned short reg, unsigned char data_crc)

//reg为crc寄存器， data_crc为将要处理的8bit数据流

{

unsigned short msb//crc寄存器将移出的最高1bit

unsigned short data

unsigned short gx = 0x8005, i = 0//i为左移次数， gx为生成多项式

data = (unsigned short)data_crc

data = data <<8

reg = reg ^ data

do

{

msb = reg &0x8000

reg = reg <<1

if(msb == 0x8000)

{

reg = reg ^ gx

}

i++

}

while(i <8)

return (reg)

}

以上为处理每一个8bit数据流的子程序，在计衫扮算整个数据流的CRC校验码时，我们只需将CRC_reg的初值置为0x0000，求第一个8bit的CRC值，之后，即可将上次求得的CRC值和本次将要处理的8bit数据作为函数实参传递给上述子程序的形参进行处理即可，最终返回的reg值便是我们所想得到的整个数据流的CRC校验值。

/////crc.c

//*****************************************************************************

//

// The CRC table for the polynomial C(x) = x^8 + x^2 + x + 1 (CRC-8-CCITT).

//

//*****************************************************************************

static const unsigned char g_pucCrc8CCITT[256] =

{

0x00, 0x07, 0x0E, 0x09, 0x1C, 0x1B, 0x12, 0x15,

0x38, 0x3F, 0x36, 0x31, 0x24, 0x23, 0x2A, 0x2D,

0x70, 0x77, 0x7E, 0x79, 0x6C, 0x6B, 0x62, 0x65,

0x48, 0x4F, 0x46, 0x41, 0x54, 0x53, 0x5A, 0x5D,

0xE0, 0xE7, 0xEE, 0xE9, 0xFC, 0xFB, 0xF2, 0xF5,

0xD8, 0xDF, 0xD6, 0xD1, 0xC4, 0xC3, 0xCA, 0xCD,

0x90, 0x97, 0x9E, 0x99, 0x8C, 0x8B, 0x82, 0x85,

0xA8, 0xAF, 0xA6, 0xA1, 0xB4, 0xB3, 0xBA, 0xBD,

0xC7, 0xC0, 0xC9, 0xCE, 0xDB, 0xDC, 0xD5, 0xD2,

0xFF, 0xF8, 0xF1, 0xF6, 0xE3, 0xE4, 0xED, 0xEA,

0xB7, 0xB0, 0xB9, 0xBE, 0xAB, 0xAC, 0xA5, 0xA2,

0x8F, 0x88, 0x81, 0x86, 0x93, 0x94, 0x9D, 0x9A,

0x27, 0x20, 0x29, 0x2E, 0x3B, 0x3C, 0x35, 0x32,

0x1F, 0x18, 0x11, 0x16, 0x03, 0x04, 0x0D, 0x0A,

0x57, 0x50, 0x59, 0x5E, 0x4B, 0x4C, 0x45, 0x42,

0x6F, 0x68, 0x61, 0x66, 0x73, 0x74, 0x7D, 0x7A,

0x89, 0x8E, 0x87, 0x80, 0x95, 0x92, 0x9B, 0x9C,

0xB1, 0xB6, 0xBF, 0xB8, 0xAD, 0xAA, 0xA3, 0xA4,

0xF9, 0xFE, 0xF7, 0xF0, 0xE5, 0xE2, 0xEB, 0xEC,

0xC1, 0xC6, 0xCF, 0xC8, 0xDD, 0xDA, 0xD3, 0xD4,

0x69, 0x6E, 0x67, 0x60, 0x75, 0x72, 0x7B, 0x7C,

0x51, 0x56, 0x5F, 0x58, 0x4D, 0x4A, 0x43, 0x44,

0x19, 0x1E, 0x17, 0x10, 0x05, 0x02, 0x0B, 0x0C,

0x21, 0x26, 0x2F, 0x28, 0x3D, 0x3A, 0x33, 0x34,

0x4E, 0x49, 0x40, 0x47, 0x52, 0x55, 0x5C, 0x5B,

0x76, 0x71, 0x78, 0x7F, 0x6A, 0x6D, 0x64, 0x63,

0x3E, 0x39, 0x30, 0x37, 0x22, 0x25, 0x2C, 0x2B,

0x06, 0x01, 0x08, 0x0F, 0x1A, 0x1D, 0x14, 0x13,

0xAE, 0xA9, 0xA0, 0xA7, 0xB2, 0xB5, 0xBC, 0xBB,

0x96, 0x91, 0x98, 0x9F, 0x8A, 0x8D, 0x84, 0x83,

0xDE, 0xD9, 0xD0, 0xD7, 0xC2, 0xC5, 0xCC, 0xCB,

0xE6, 0xE1, 0xE8, 0xEF, 0xFA, 0xFD, 0xF4, 0xF3

}

//*****************************************************************************

//

// This macro executes one iteration of the CRC-8-CCITT.

//

//*****************************************************************************

#define CRC8_ITER(crc, data)g_pucCrc8CCITT[(unsigned char)((crc) ^ (data))]

//*****************************************************************************

//

//! Calculates the CRC-8-CCITT of an array of bytes.

//!

//! \param ucCrc is the starting CRC-8-CCITT value.

//! \param pucData is a pointer to the data buffer.

//! \param ulCount is the number of bytes in the data buffer.

//!

//! This function is used to calculate the CRC-8-CCITT of the input buffer.

//! The CRC-8-CCITT is computed in a running fashion, meaning that the entire

//! data block that is to have its CRC-8-CCITT computed does not need to be

//! supplied all at once. If the input buffer contains the entire block of

//! data, then \b ucCrc should be set to 0. If, however, the entire block of

//! data is not available, then \b ucCrc should be set to 0 for the first

//! portion of the data, and then the returned value should be passed back in

//! as \b ucCrc for the next portion of the data.

//!

//! For example, to compute the CRC-8-CCITT of a block that has been split into

//! three pieces, use the following:

//!

//! \verbatim

//! ucCrc = Crc8CCITT(0, pucData1, ulLen1)

//! ucCrc = Crc8CCITT(ucCrc, pucData2, ulLen2)

//! ucCrc = Crc8CCITT(ucCrc, pucData3, ulLen3)

//! \endverbatim

//!

//! Computing a CRC-8-CCITT in a running fashion is useful in cases where the

//! data is arriving via a serial link (for example) and is therefore not all

//! available at one time.

//!

//! \return The CRC-8-CCITT of the input data.

//

//*****************************************************************************

unsigned char

Crc8CCITT(unsigned char ucCrc, const unsigned char *pucData,

unsigned long ulCount)

{

unsigned long ulTemp

//

// If the data buffer is not short-aligned, then perform a single step of

// the CRC to make it short-aligned.

//

if((unsigned long)pucData &1)

{

//

// Perform the CRC on this input byte.

//

ucCrc = CRC8_ITER(ucCrc, *pucData)

//

// Skip this input byte.

//

pucData++

ulCount--

}

//

// If the data buffer is not word-aligned and there are at least two bytes

// of data left, then perform two steps of the CRC to make it word-aligned.

//

if(((unsigned long)pucData &2) &&(ulCount >1))

{

//

// Read the next short.

//

ulTemp = *(unsigned short *)pucData

//

// Perform the CRC on these two bytes.

//

ucCrc = CRC8_ITER(ucCrc, ulTemp)

ucCrc = CRC8_ITER(ucCrc, ulTemp >>8)

//

// Skip these input bytes.

//

pucData += 2

ulCount -= 2

}

//

// While there is at least a word remaining in the data buffer, perform

// four steps of the CRC to consume a word.

//

while(ulCount >3)

{

//

// Read the next word.

//

ulTemp = *(unsigned long *)pucData

//

// Perform the CRC on these four bytes.

//

ucCrc = CRC8_ITER(ucCrc, ulTemp)

ucCrc = CRC8_ITER(ucCrc, ulTemp >>8)

ucCrc = CRC8_ITER(ucCrc, ulTemp >>16)

ucCrc = CRC8_ITER(ucCrc, ulTemp >>24)

//

// Skip these input bytes.

//

pucData += 4

ulCount -= 4

}

//

// If there is a short left in the input buffer, then perform two steps of

// the CRC.

//

if(ulCount >1)

{

//

// Read the short.

//

ulTemp = *(unsigned short *)pucData

//

// Perform the CRC on these two bytes.

//

ucCrc = CRC8_ITER(ucCrc, ulTemp)

ucCrc = CRC8_ITER(ucCrc, ulTemp >>8)

//

// Skip these input bytes.

//

pucData += 2

ulCount -= 2

}

//

// If there is a final byte remaining in the input buffer, then perform a

// single step of the CRC.

//

if(ulCount != 0)

{

ucCrc = CRC8_ITER(ucCrc, *pucData)

}

//

// Return the resulting CRC-8-CCITT value.

//

return(ucCrc)

}

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