C语言中的CRC4实现 [英] CRC4 Implementation in C

问题描述

我修改了此处找到的实现，以构建一个CRC4表生成函数如下：

I have modified the implementation found here, to build a table generating function for a CRC4 as follows:

#define bufferSize 16
crc crcTable[bufferSize];
#define POLYNOMIAL 0x13

void Init()
{
    crc remainder;

     for(int dividend = 0; dividend < bufferSize; ++dividend)
     {
          remainder = dividend;
          for(uint8_t bit = 8; bit > 0; --bit)
          {
              if(remainder & 1)
                  remainder = (remainder >> 1) ^ POLYNOMIAL;
              else
                  remainder = (remainder >> 1);
          }

          crcTable[dividend] = remainder;
          printf("%hu\n", remainder);
    }
}

然后像这样计算CRC：

And then calculate the CRC like this:

uint8_t calc_crc4(uint8_t start_crc, uint8_t byte)
{
    byte ^= start_crc;
    start_crc = crcTable[byte] ^ (start_crc >> 8);

    return start_crc;
}

生成了crc表格：

/*
* Table based on Polynomial 0x13
*/
uint8_t crcTable[tableSize] = {
    0x00, 0x0E, 0x1C, 0x12,
    0x1F, 0x11, 0x03, 0x0D,
    0x19, 0x17, 0x05, 0x0B,
    0x06, 0x08, 0x1A, 0x14
};

问题是，当我对ERF文件运行它时，我生成的CRC值均不相等附加到ERF帧末尾的那些。当我打印值时，似乎在 calc_crc4 函数中对 crcTable [byte] 的调用几乎总是返回值0x00，但是我不太了解这个概念，无法知道这是否是正确的值。我唯一能想到的是，它在字节的索引位置找不到任何内容，因此返回0x00。我当时的假设是，CRC4只能有16个值，因此该位置必须要有一些值。

The issue is that when I run it against an ERF file, none of my generated CRC values are equal to the ones attached to the end of an ERF frame. When I print values, it looks like the call to crcTable[byte] in the calc_crc4 function is almost always returning the value 0x00, but I don't grasp the concept well enough to know if this is the correct value or not. The only thing I can think of is that it's not finding anything at the index location of byte, so it returns 0x00. I was under the assumption that there can only be 16 values of a CRC4 though, so there would have to be something in that location.

推荐答案

您没有所需CRC的完整定义，并且尝试将实现外推到4位会出现很多错误。

You don't have the full definition of the desired CRC, and your attempt at extrapolating the implementation to four bits has many errors.

首先，您需要知道的不只是多项式。您需要知道是否反映了CRC，是否也反映了输出，初始寄存器值是什么以及输出是否被某些值互斥或。

First off, you need to know more than the polynomial. You need to know if the CRC is reflected, if the output is also reflected, what the initial register value is, and whether the output is exclusive-or'ed with some value or not.

第二，如果一次要处理一个字节，则该表需要具有256个条目，而与CRC的长度无关。此外，每个条目必须是CRC的长度，在这种情况下为4位，其中条目的长度为5。另外，您还需要将CRC放置在初始排他的字节的适当末尾，或者在表查找之前，或者将表移位。如前所述，向下移位八位的八位值是零，因此与它的异或运算不起作用。

Second, if you are processing a byte at a time, the table needs to have 256 entries, regardless of the length of the CRC. Furthermore, each entry must be the length of the CRC, in this case four bits, where you have entries with five. Also you need to put the CRC at the proper end of the byte for the initial exclusive-or before the table lookup, or shift the table. As already noted, an eight-bit value shifted down by eight bits is zero, so exclusive-or'ing with that does nothing.

表驱动的a实现

static unsigned char const table_byte[256] = {
    0x90, 0xa0, 0xf0, 0xc0, 0x50, 0x60, 0x30, 0x00, 0x20, 0x10, 0x40, 0x70, 0xe0,
    0xd0, 0x80, 0xb0, 0xc0, 0xf0, 0xa0, 0x90, 0x00, 0x30, 0x60, 0x50, 0x70, 0x40,
    0x10, 0x20, 0xb0, 0x80, 0xd0, 0xe0, 0x30, 0x00, 0x50, 0x60, 0xf0, 0xc0, 0x90,
    0xa0, 0x80, 0xb0, 0xe0, 0xd0, 0x40, 0x70, 0x20, 0x10, 0x60, 0x50, 0x00, 0x30,
    0xa0, 0x90, 0xc0, 0xf0, 0xd0, 0xe0, 0xb0, 0x80, 0x10, 0x20, 0x70, 0x40, 0xe0,
    0xd0, 0x80, 0xb0, 0x20, 0x10, 0x40, 0x70, 0x50, 0x60, 0x30, 0x00, 0x90, 0xa0,
    0xf0, 0xc0, 0xb0, 0x80, 0xd0, 0xe0, 0x70, 0x40, 0x10, 0x20, 0x00, 0x30, 0x60,
    0x50, 0xc0, 0xf0, 0xa0, 0x90, 0x40, 0x70, 0x20, 0x10, 0x80, 0xb0, 0xe0, 0xd0,
    0xf0, 0xc0, 0x90, 0xa0, 0x30, 0x00, 0x50, 0x60, 0x10, 0x20, 0x70, 0x40, 0xd0,
    0xe0, 0xb0, 0x80, 0xa0, 0x90, 0xc0, 0xf0, 0x60, 0x50, 0x00, 0x30, 0x70, 0x40,
    0x10, 0x20, 0xb0, 0x80, 0xd0, 0xe0, 0xc0, 0xf0, 0xa0, 0x90, 0x00, 0x30, 0x60,
    0x50, 0x20, 0x10, 0x40, 0x70, 0xe0, 0xd0, 0x80, 0xb0, 0x90, 0xa0, 0xf0, 0xc0,
    0x50, 0x60, 0x30, 0x00, 0xd0, 0xe0, 0xb0, 0x80, 0x10, 0x20, 0x70, 0x40, 0x60,
    0x50, 0x00, 0x30, 0xa0, 0x90, 0xc0, 0xf0, 0x80, 0xb0, 0xe0, 0xd0, 0x40, 0x70,
    0x20, 0x10, 0x30, 0x00, 0x50, 0x60, 0xf0, 0xc0, 0x90, 0xa0, 0x00, 0x30, 0x60,
    0x50, 0xc0, 0xf0, 0xa0, 0x90, 0xb0, 0x80, 0xd0, 0xe0, 0x70, 0x40, 0x10, 0x20,
    0x50, 0x60, 0x30, 0x00, 0x90, 0xa0, 0xf0, 0xc0, 0xe0, 0xd0, 0x80, 0xb0, 0x20,
    0x10, 0x40, 0x70, 0xa0, 0x90, 0xc0, 0xf0, 0x60, 0x50, 0x00, 0x30, 0x10, 0x20,
    0x70, 0x40, 0xd0, 0xe0, 0xb0, 0x80, 0xf0, 0xc0, 0x90, 0xa0, 0x30, 0x00, 0x50,
    0x60, 0x40, 0x70, 0x20, 0x10, 0x80, 0xb0, 0xe0, 0xd0};

unsigned crc4interlaken_byte(unsigned crc, void const *mem, size_t len) {
    unsigned char const *data = mem;
    if (data == NULL)
        return 0;
    crc &= 0xf;
    crc <<= 4;
    while (len--)
        crc = table_byte[crc ^ *data++];
    crc >>= 4;
    return crc;
}

或

static unsigned char const table_byte[256] = {
    0x0, 0x7, 0xe, 0x9, 0x5, 0x2, 0xb, 0xc, 0xa, 0xd, 0x4, 0x3, 0xf, 0x8, 0x1, 0x6,
    0xd, 0xa, 0x3, 0x4, 0x8, 0xf, 0x6, 0x1, 0x7, 0x0, 0x9, 0xe, 0x2, 0x5, 0xc, 0xb,
    0x3, 0x4, 0xd, 0xa, 0x6, 0x1, 0x8, 0xf, 0x9, 0xe, 0x7, 0x0, 0xc, 0xb, 0x2, 0x5,
    0xe, 0x9, 0x0, 0x7, 0xb, 0xc, 0x5, 0x2, 0x4, 0x3, 0xa, 0xd, 0x1, 0x6, 0xf, 0x8,
    0x6, 0x1, 0x8, 0xf, 0x3, 0x4, 0xd, 0xa, 0xc, 0xb, 0x2, 0x5, 0x9, 0xe, 0x7, 0x0,
    0xb, 0xc, 0x5, 0x2, 0xe, 0x9, 0x0, 0x7, 0x1, 0x6, 0xf, 0x8, 0x4, 0x3, 0xa, 0xd,
    0x5, 0x2, 0xb, 0xc, 0x0, 0x7, 0xe, 0x9, 0xf, 0x8, 0x1, 0x6, 0xa, 0xd, 0x4, 0x3,
    0x8, 0xf, 0x6, 0x1, 0xd, 0xa, 0x3, 0x4, 0x2, 0x5, 0xc, 0xb, 0x7, 0x0, 0x9, 0xe,
    0xc, 0xb, 0x2, 0x5, 0x9, 0xe, 0x7, 0x0, 0x6, 0x1, 0x8, 0xf, 0x3, 0x4, 0xd, 0xa,
    0x1, 0x6, 0xf, 0x8, 0x4, 0x3, 0xa, 0xd, 0xb, 0xc, 0x5, 0x2, 0xe, 0x9, 0x0, 0x7,
    0xf, 0x8, 0x1, 0x6, 0xa, 0xd, 0x4, 0x3, 0x5, 0x2, 0xb, 0xc, 0x0, 0x7, 0xe, 0x9,
    0x2, 0x5, 0xc, 0xb, 0x7, 0x0, 0x9, 0xe, 0x8, 0xf, 0x6, 0x1, 0xd, 0xa, 0x3, 0x4,
    0xa, 0xd, 0x4, 0x3, 0xf, 0x8, 0x1, 0x6, 0x0, 0x7, 0xe, 0x9, 0x5, 0x2, 0xb, 0xc,
    0x7, 0x0, 0x9, 0xe, 0x2, 0x5, 0xc, 0xb, 0xd, 0xa, 0x3, 0x4, 0x8, 0xf, 0x6, 0x1,
    0x9, 0xe, 0x7, 0x0, 0xc, 0xb, 0x2, 0x5, 0x3, 0x4, 0xd, 0xa, 0x6, 0x1, 0x8, 0xf,
    0x4, 0x3, 0xa, 0xd, 0x1, 0x6, 0xf, 0x8, 0xe, 0x9, 0x0, 0x7, 0xb, 0xc, 0x5, 0x2};

unsigned crc4g_704_byte(unsigned crc, void const *mem, size_t len) {
    unsigned char const *data = mem;
    if (data == NULL)
        return 0;
    crc &= 0xf;
    while (len--)
        crc = table_byte[crc ^ *data++];
    return crc;
}

此代码和表格由我的crcany代码。函数使用 data 处的 len 个字节推进CRC。当调用 data 等于 NULL 时，将返回初始CRC（即零字节CRC）。 CRC位于返回值的最低有效位。

This code and tables were generated by my crcany code. The functions advance a CRC using the len bytes at data. The initial CRC (i.e. a CRC of zero bytes) is returned when called with data equal to NULL. The CRCs are in the least-significant bits of the return value.

这两个CRC在格雷格·库克（Greg Cook）的目录，其中两个4位CRC定义为：

Those two CRCs are defined in Greg Cook's catalog, where the two 4-bit CRC definitions are:

width=4 poly=0x3 init=0xf refin=false refout=false xorout=0xf check=0xb residue=0x2 name="CRC-4/INTERLAKEN"
width=4 poly=0x3 init=0x0 refin=true refout=true xorout=0x0 check=0x7 residue=0x0 name="CRC-4/G-704"

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