CODEWITHSUNDEEP
c++crc32
tulipe
tulipe

Reputation: 746

CRC32 not calculated right

I use a very simple algorithm to calculate the CRC32 but it gives wrong values.

I compare my output values with calculator ones but it always looks different

unsigned int crc32_tab[256] = {
        0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419, 0x706af48f,
        0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988,
        0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91, 0x1db71064, 0x6ab020f2,
        0xf3b97148, 0x84be41de, 0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
      .........,..............,.............,.........,...........
    };

Function that use the lookup table is 

 unsigned int MyClass::crc32(unsigned int crc, const void *buf, unsigned int   size)
{
const unsigned int *p;

p = (const quint8 *)buf;
crc = crc ^~ 0xFFFFFFFF;

while(size--)
{
    crc = this->crc32_tab[(crc ^ *p++) & 0xFF] ^ (crc >> 8);
}

return crc ^~ 0xFFFFFFFF;
}

I call it by this way 

 QString test= QString::number(mclass.crc32(0, crcval, 6))

Upvotes: 3

Views: 1362

Answers (1)

Iwillnotexist Idonotexist
Iwillnotexist Idonotexist

Reputation: 13457

Solution drawn from chat dicussion

The CRC-32 algorithm being implemented is CRC-32 Ethernet (generator polynomial 0x04C11DB7).

This CRC-32 requires:

  • Being initialized with 0xFFFFFFFF.
  • And being finalized by XORing with 0xFFFFFFFF.

Therefore, you should remove the crc ^~ 0xFFFFFFFF statements within your function, pass 0xFFFFFFFF on your call to the function, and once you're done CRCing the data, you should XOR the return value with 0xFFFFFFFF.

Upvotes: 2

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