projects / unix-history / blame
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1 | .\" Copyright (c) 1991 The Regents of the University of California. |
| 2 | .\" All rights reserved. | |
| 3 | .\" | |
| 4 | .\" %sccs.include.redist.man% | |
| 5 | .\" | |
| 6 | .\" @(#)cksum.1 5.1 (Berkeley) %G% | |
| 7 | .\" | |
| 8 | .TH CKSUM 1 "" | |
| 9 | .UC 7 | |
| 10 | .SH NAME | |
| 11 | cksum \- display file checksums and block counts | |
| 12 | .SH SYNOPSIS | |
| 13 | .ft B | |
| 14 | .nf | |
| 15 | cksum [file ...] | |
| 16 | .fi | |
| 17 | .ft R | |
| 18 | .SH DESCRIPTION | |
| 19 | The | |
| 20 | .I cksum | |
| 21 | utility calculates and writes to standard output a total number of bytes, | |
| 22 | a checksum CRC and the file name for each input file (or the standard input | |
| 23 | by default). | |
| 24 | .PP | |
| 25 | The CRC used is based on the polynomial used for CRC error checking in | |
| 26 | the networking standard ISO 8802-3: 1989. | |
| 27 | The CRC checksum encoding is defined by the generating polynomial: | |
| 28 | .sp | |
| 29 | .RS | |
| 30 | G(x) = | |
| 31 | .RS | |
| 32 | x^32 + x^26 + x^23 + x^22 + x^16 + x^12 + | |
| 33 | x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1 | |
| 34 | .RE | |
| 35 | .RE | |
| 36 | .PP | |
| 37 | Mathematically, the CRC value corresponding to a given file is defined by | |
| 38 | the following procedure: | |
| 39 | .RS | |
| 40 | .PP | |
| 41 | The | |
| 42 | .I n | |
| 43 | bits of the file are considered to be the coefficients of a mod 2 | |
| 44 | polynomial M(x) of degree | |
| 45 | .IR n -1. | |
| 46 | .PP | |
| 47 | M(x) is multiplied by x^32 (i.e., shifted left 32 bits) and divided by | |
| 48 | G(x) using mod 2 division, producing a remainder R(x) of degree <= 31. | |
| 49 | During the division, each time the intermediate remainder is zero, it | |
| 50 | is changed to the next value from a predefined sequence of 32-bit | |
| 51 | integers before completing the division. | |
| 52 | This sequence is long and complex -- see the source code for more | |
| 53 | information. | |
| 54 | .PP | |
| 55 | The coefficients of R(x) are considered to be a 32-bit sequence. | |
| 56 | .RE | |
| 57 | .PP | |
| 58 | The calculation used is identical to that given in pseudo-code in | |
| 59 | the ACM article listed below. | |
| 60 | .PP | |
| 61 | The | |
| 62 | .I cksum | |
| 63 | utility exits 0 on success, and >0 if an error occurs. | |
| 64 | .SH SEE ALSO | |
| 65 | .IR "Computation of Cyclic Redundancy Checks Via Table Lookup" , | |
| 66 | Dilip V. Sarwate, Communications of the ACM, August 1988. | |
| 67 | .SH STANDARDS | |
| 68 | .I Cksum | |
| 69 | is expected to conform to IEEE Std 1003.2 (``POSIX''). | |
| 70 | .SH HISTORY | |
| 71 | .I Cksum | |
| 72 | first appeared in 4.4BSD. |