manpagez: man pages & more
man cksum(1)
Home | html | info | man
cksum(1)                  BSD General Commands Manual                 cksum(1)


     cksum, sum -- display file checksums and block counts


     cksum [-o 1 | 2 | 3] [file ...]
     sum [file ...]


     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

             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.


     The cksum and sum utilities exit 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.

     Dilip V. Sarwate, "Computation of Cyclic Redundancy Checks Via Table
     Lookup", Communications of the ACM, August 1988.


     The cksum utility is expected to conform to IEEE Std 1003.2-1992


     The cksum utility appeared in 4.4BSD.

BSD                             April 28, 1995                             BSD

Mac OS X 10.8 - Generated Thu Aug 23 07:43:56 CDT 2012
© 2000-2021
Individual documents may contain additional copyright information.