cksum

NAME

cksum, sum - display file checksums and sizes

SYNOPSIS 

cksum [-o 1 | 2] [file ...]


sum [-r] [file ...]

DESCRIPTION

Both the cksum(1) and sum(1) utilities calculate the checksum and size of a file. If cksum(1) is invoked as sum(1), it behaves as the sum(1) utility. The sum(1) command is obsolete and should not be used.

The cksum(1) utility writes to the standard output three, white-space separated fields for each input file:

crc  #_octets file_name
These fields contain a checksum (crc), the total number of octets in the file (#_octets) and the file name. If no file name is specified, the standard input is used and no file name is written.

The sum(1) utility writes to the standard output three, white-space separated fields for each input file:

crc #_blocks file_name

These fields contain a checksum (-crc), the total number of 512-byte blocks in the file (#_blocks), and the file name.

OPTIONS

-o n
Use historic algorithms instead of the default one. n can be either 1 or 2:
1
Algorithm 1 is the algorithm used by historic Berkeley Software Distribution (BSD) systems as the sum(1) algorithm, and by historic AT&T System V systems as the sum(1) algorithm when using the -r option. This is a 16-bit checksum, with a right rotation before each addition; overflow is discarded.
2
Algorithm 2 is the algorithm used by historic AT&T System V systems as the default sum(1) algorithm. This is a 32-bit checksum, and is defined as follows:

s = sum of all bytes;
r = s % 2^16 + (s % 2^32) / 2^16;
cksum = (r % 2^16) + r / 2^16;

Both algorithm 1 and 2 write to the standard output the same fields as the default algorithm, except that the size of the file in bytes is replaced with the size of the file in blocks. For historic reasons, the block size is 1024 for algorithm 1 and 512 for algorithm 2. Partial blocks are rounded up.
-r
For the sum(1) command, this is equivalent to: 
cksum -o 1
Without the -r option, the sum(1) command is equivalent to: 
cskum -o 2

The default Cyclical Redundancy Checking (CRC) used is based on the polynomial used for CRC error checking in the networking standard ISO 8802-3:1989. The CRC checksum encoding is defined by the generating polynomial:

G(x) = 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
Mathematically, the CRC value corresponding to a given file is defined by the following procedure: The n bits to be evaluated are considered to be the coefficients of a mod 2 polynomial M(x) of degree n-1. These n bits are the bits from the file. The most significant bit being the most significant bit of the first octet of the file, and the last bit being the least significant bit of the last octet, padded with zero bits (if necessary) to achieve an integral number of octets, followed by one or more octets representing the length of the file as a binary value, least significant octet first. The smallest number of octets capable of representing this integer is used.

M(x) is multiplied by x^32 (that is, shifted left 32 bits) and divided by G(x) using mod 2 division, producing a remainder R(x) of degree <= 31.

The coefficients of R(x) are considered to be a 32-bit sequence.

The bit sequence is complemented and the result is the CRC.

DIAGNOSTICS

The cksum(1) utility exits 0 on success, and >0 if an error occurs.