# cksum

```
CKSUM(1) General Commands Manual CKSUM(1)
NAME
cksum, sum – display file checksums and block counts
SYNOPSIS
cksum [-o 1 | 2 | 3] [file ...]
sum [file ...]
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 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 Tn 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.
macOS 12.1 April 28, 1995 macOS 12.1
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