# 32-Bit CRC Functions

Topics
Overview | Functions

Overview

The CRC32 functions (Cyclic Redundancy Check) are used to calculate a sophisticated checksum based on the algebra of polynomials. The Cyclic Redundancy Check, is a way to detect bit errors that occur during data storage or transmission. The CRC-32 algorithm operates on a block of data as a single large numerical value. The algorithm divides this large value by the CRC-32 polynomial or generator polynomial, leaving the remainder 32-bit, which is the checksum.

The CRC32 functions use a CRC table to calculate a checksum value for a single byte, a raw pattern of bytes, or a file. The CRC table is an array of all possible single-byte values needed to generate a 32-bit CRC for data one a byte at a time for all combinations of incoming bytes. It is based on the Autodin/Ethernet/ADCCP polynomial of 0x4C11DB7:

0000 0100 1100 0001 0001 1101 1011 0111 (binary) or a poly of x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7 +x^5+x^4+x^2+x^1+x^0

In this implementation the coefficient of x^0 is stored in the MSB of the 32-bit word and the coefficient of x^31 is stored in the LSB. Thus 0x4C11DB7 becomes 0xEDB88320:

1110 1101 1011 1000 1000 0011 0010 0000 (binary)

Functions

unsigned long calcCRC32(char *buf, unsigned len)

- Calculates a 32-bit CRC for a raw pattern of bytes. Returns a 32-bit checksum.

unsigned long calcCRC32(const char *buf, unsigned len)

- Calculates a 32-bit CRC for a raw pattern of bytes. Returns a 32-bit checksum.

unsigned long calcCRC32(unsigned char c, unsigned long CRC)

- Calculates a 32-bit CRC table value for a single byte. NOTE: The initial CRC value must be set to 0xffffffffL and the final 32-bit value that must be XOR'ed with 0xffffffffL to obtain the checksum.

unsigned long calcCRC32(fstream &infile)

- Calculates a 32-bit CRC for a file. Assumes the stream is already open. Returns a 32-bit checksum.

int makeCRC32(ostream &stream)

- Writes a CRC 32 table for a byte-wise 32-bit CRC calculation based on the Autodin/Ethernet/ADCCP polynomial of 0x4C11DB7. This function creates a table of CRC values for every possible combination of bytes. It was used to generate the table included in the "crc32tab.h" file.

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