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CKSUM(1)		  BSD 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 sepa‐
     rated 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 pro‐
     vided 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 check‐
	     sum, 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 check‐
     ing 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 signif‐
	   icant 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.

BSD				April 28, 1995				   BSD
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