cksum - display file checksums and block counts
cksum [-o 1 | 2] [file ...]
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
The options are as follows:
-o Use historic algorithms instead of the (superior) default one.
Algorithm 1 is the algorithm used by historic Unix systems as the sum 1
algorithm. This is a 16-bit checksum, with a right rotation before each
addition; overflow is discarded.
Algorithm 2 is the algorithm used by historic System V Unix systems as
the default sum algorithm. This is a 32-bit checksum, and is defined as
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.
The default CRC used is based on the polynomial used for CRC error
checking in the networking standard -iso8802-3. 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:
1. The n bits to be evaluated are considered to be the coefficients of
a mod 2 polynomial M(x) of degree n-1.
2. 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.
3. The smallest number of octets capable of representing this integer
4. 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
5. The coefficients of R(x) are considered to be a 32-bit sequence.
6. The bit sequence is complemented and the result is the CRC.
The cksum utility exits 0 on success, and >0 if an error occurs.
The default calculation is identical to that given in pseudo-code in the
following ACM article.
Title: Computation of Cyclic Redundancy Checks Via Table Lookup
Author: Dilip V. Sarwate
Publication: Communications of the ACM; August 1988
The cksum utility is expected to be POSIX 1003.2 compatible.
The cksum utility appears in BSD 4.4 .
PPPPaaaaggggeeee 2222 [ Back ]