CKSUM(1) MidnightBSD General Commands Manual CKSUM(1)

**NAME**

**cksum**, **sum** — display
file checksums and block counts

**SYNOPSIS**

**cksum**
[**−o ***1 *| *2 *| *3*]
[*file ...*] **
sum** [

**DESCRIPTION**

The **cksum** utility writes to the
standard output three whitespace separated fields for each input
file. These fields are a checksum CRC, the total number of octets
in the file and the file name. If no file name is specified, the
standard input is used and no file name is written.

The **sum** utility
is identical to the **cksum** utility, except that it defaults
to using historic algorithm 1, as described below. It is provided
for compatibility only.

The options are as follows:

**−o**

Use historic algorithms instead of the (superior) default one.

Algorithm 1 is the
algorithm used by historic BSD systems as the sum(1) algorithm and
by historic AT&T System V UNIX 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.

Algorithm 2 is the algorithm used by historic AT&T System V UNIX 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;

Algorithm 3 is what is commonly called the ‘32bit CRC’ algorithm. This is a 32-bit checksum.

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.

The default CRC used is based on the polynomial used for CRC error checking in the networking standard ISO/IEC 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, with 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 are used.

M(x) is multiplied by x^32 (i.e., 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.

**EXIT STATUS**

The **cksum** and **sum**
utilities exit 0 on success, and >0 if an error
occurs.

**SEE ALSO**

md5(1)

The default calculation is identical to that given in pseudo-code in the following ACM article.

Dilip V. Sarwate

, "

Computation of Cyclic Redundancy Checks Via Table
Lookup ", *
Communications of the ACM* ,

August 1988 .

**STANDARDS**

The **cksum** utility is expected to
conform to IEEE Std 1003.2-1992
(‘‘POSIX.2’’).

**HISTORY**

The **cksum** utility appeared in
4.4BSD.

MidnightBSD 0.3 April 28, 1995 MidnightBSD 0.3