391 lines
16 KiB
HTML
391 lines
16 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<meta name="generator" content="HTML Tidy, see www.w3.org">
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<link type="text/css" rel="stylesheet" href="style.css"><!-- Generated by The Open Group's rhtm tool v1.2.1 -->
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<!-- Copyright (c) 2001 The Open Group, All Rights Reserved -->
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<title>cksum</title>
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</head>
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<body bgcolor="white">
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<script type="text/javascript" language="JavaScript" src="../jscript/codes.js">
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</script>
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<basefont size="3"> <a name="cksum"></a> <a name="tag_04_20"></a><!-- cksum -->
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<!--header start-->
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<center><font size="2">The Open Group Base Specifications Issue 6<br>
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IEEE Std 1003.1-2001<br>
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Copyright © 2001 The IEEE and The Open Group, All Rights reserved.</font></center>
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<!--header end-->
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<hr size="2" noshade>
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<h4><a name="tag_04_20_01"></a>NAME</h4>
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<blockquote>cksum - write file checksums and sizes</blockquote>
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<h4><a name="tag_04_20_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<p><code><tt>cksum</tt> <b>[</b><i>file</i> <tt>...</tt><b>]</b></code></p>
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</blockquote>
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<h4><a name="tag_04_20_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The <i>cksum</i> utility shall calculate and write to standard output a cyclic redundancy check (CRC) for each input file, and
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also write to standard output the number of octets in each file. The CRC used is based on the polynomial used for CRC error
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checking in the ISO/IEC 8802-3:1996 standard (Ethernet).</p>
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<p>The encoding for the CRC checksum is defined by the generating polynomial:</p>
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<pre>
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<i>G</i><tt>(</tt><i>x</i><tt>)=</tt><i>x</i><tt><sup><small>32</small></sup>+</tt><i>x</i><tt><sup><small>26</small></sup>+</tt><i>x</i><tt><sup><small>23</small></sup>+</tt><i>x</i><tt><sup><small>22</small></sup>+</tt><i>x</i><tt><sup><small>16</small></sup>+</tt><i>x</i><tt><sup><small>12</small></sup>+</tt><i>x</i><tt><sup><small>11</small></sup>+</tt><i>x</i><tt><sup><small>10</small></sup>+</tt><i>x</i><tt><sup><small>8</small></sup>+</tt><i>x</i><tt><sup><small>7</small></sup>+</tt><i>x</i><tt><sup><small>5</small></sup>+</tt><i>x</i><tt><sup><small>4</small></sup>+</tt><i>x</i><tt><sup><small>2</small></sup>+</tt><i>x</i><tt>+1
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</tt>
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</pre>
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<p>Mathematically, the CRC value corresponding to a given file shall be defined by the following procedure:</p>
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<ol>
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<li>
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<p>The <i>n</i> bits to be evaluated are considered to be the coefficients of a mod 2 polynomial <i>M</i>( <i>x</i>) of degree
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<i>n</i>-1. These <i>n</i> bits are the bits from the file, with the most significant bit being the most significant bit of the
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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
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achieve an integral number of octets, followed by one or more octets representing the length of the file as a binary value, least
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significant octet first. The smallest number of octets capable of representing this integer shall be used.</p>
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</li>
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<li>
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<p><i>M</i>( <i>x</i>) is multiplied by <i>x</i><sup><small>32</small></sup> (that is, shifted left 32 bits) and divided by
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<i>G</i>( <i>x</i>) using mod 2 division, producing a remainder <i>R</i>( <i>x</i>) of degree <= 31.</p>
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</li>
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<li>
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<p>The coefficients of <i>R</i>( <i>x</i>) are considered to be a 32-bit sequence.</p>
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</li>
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<li>
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<p>The bit sequence is complemented and the result is the CRC.</p>
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</li>
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</ol>
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</blockquote>
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<h4><a name="tag_04_20_04"></a>OPTIONS</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_04_20_05"></a>OPERANDS</h4>
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<blockquote>
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<p>The following operand shall be supported:</p>
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<dl compact>
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<dt><i>file</i></dt>
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<dd>A pathname of a file to be checked. If no <i>file</i> operands are specified, the standard input shall be used.</dd>
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</dl>
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</blockquote>
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<h4><a name="tag_04_20_06"></a>STDIN</h4>
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<blockquote>
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<p>The standard input shall be used only if no <i>file</i> operands are specified. See the INPUT FILES section.</p>
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</blockquote>
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<h4><a name="tag_04_20_07"></a>INPUT FILES</h4>
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<blockquote>
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<p>The input files can be any file type.</p>
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</blockquote>
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<h4><a name="tag_04_20_08"></a>ENVIRONMENT VARIABLES</h4>
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<blockquote>
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<p>The following environment variables shall affect the execution of <i>cksum</i>:</p>
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<dl compact>
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<dt><i>LANG</i></dt>
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<dd>Provide a default value for the internationalization variables that are unset or null. (See the Base Definitions volume of
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IEEE Std 1003.1-2001, <a href="../basedefs/xbd_chap08.html#tag_08_02">Section 8.2, Internationalization Variables</a> for
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the precedence of internationalization variables used to determine the values of locale categories.)</dd>
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<dt><i>LC_ALL</i></dt>
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<dd>If set to a non-empty string value, override the values of all the other internationalization variables.</dd>
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<dt><i>LC_CTYPE</i></dt>
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<dd>Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as
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opposed to multi-byte characters in arguments).</dd>
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<dt><i>LC_MESSAGES</i></dt>
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<dd>Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard
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error.</dd>
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<dt><i>NLSPATH</i></dt>
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<dd><sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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Determine the location of message catalogs for the processing of <i>LC_MESSAGES .</i> <img src="../images/opt-end.gif" alt=
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"[Option End]" border="0"></dd>
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</dl>
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</blockquote>
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<h4><a name="tag_04_20_09"></a>ASYNCHRONOUS EVENTS</h4>
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<blockquote>
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<p>Default.</p>
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</blockquote>
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<h4><a name="tag_04_20_10"></a>STDOUT</h4>
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<blockquote>
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<p>For each file processed successfully, the <i>cksum</i> utility shall write in the following format:</p>
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<pre>
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<tt>"%u %d %s\n", <</tt><i>checksum</i><tt>>, <</tt><i># of octets</i><tt>>, <</tt><i>pathname</i><tt>>
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</tt>
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</pre>
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<p>If no <i>file</i> operand was specified, the pathname and its leading <space> shall be omitted.</p>
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</blockquote>
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<h4><a name="tag_04_20_11"></a>STDERR</h4>
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<blockquote>
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<p>The standard error shall be used only for diagnostic messages.</p>
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</blockquote>
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<h4><a name="tag_04_20_12"></a>OUTPUT FILES</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_04_20_13"></a>EXTENDED DESCRIPTION</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_04_20_14"></a>EXIT STATUS</h4>
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<blockquote>
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<p>The following exit values shall be returned:</p>
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<dl compact>
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<dt> 0</dt>
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<dd>All files were processed successfully.</dd>
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<dt>>0</dt>
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<dd>An error occurred.</dd>
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</dl>
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</blockquote>
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<h4><a name="tag_04_20_15"></a>CONSEQUENCES OF ERRORS</h4>
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<blockquote>
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<p>Default.</p>
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</blockquote>
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<hr>
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<div class="box"><em>The following sections are informative.</em></div>
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<h4><a name="tag_04_20_16"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>The <i>cksum</i> utility is typically used to quickly compare a suspect file against a trusted version of the same, such as to
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ensure that files transmitted over noisy media arrive intact. However, this comparison cannot be considered cryptographically
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secure. The chances of a damaged file producing the same CRC as the original are small; deliberate deception is difficult, but
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probably not impossible.</p>
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<p>Although input files to <i>cksum</i> can be any type, the results need not be what would be expected on character special device
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files or on file types not described by the System Interfaces volume of IEEE Std 1003.1-2001. Since this volume of
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IEEE Std 1003.1-2001 does not specify the block size used when doing input, checksums of character special files need not
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process all of the data in those files.</p>
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<p>The algorithm is expressed in terms of a bitstream divided into octets. If a file is transmitted between two systems and
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undergoes any data transformation (such as changing little-endian byte ordering to big-endian), identical CRC values cannot be
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expected. Implementations performing such transformations may extend <i>cksum</i> to handle such situations.</p>
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</blockquote>
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<h4><a name="tag_04_20_17"></a>EXAMPLES</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_04_20_18"></a>RATIONALE</h4>
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<blockquote>
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<p>The following C-language program can be used as a model to describe the algorithm. It assumes that a <b>char</b> is one octet.
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It also assumes that the entire file is available for one pass through the function. This was done for simplicity in demonstrating
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the algorithm, rather than as an implementation model.</p>
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<pre>
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<tt>static unsigned long crctab[] = {
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0x00000000,
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0x04c11db7, 0x09823b6e, 0x0d4326d9, 0x130476dc, 0x17c56b6b,
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0x1a864db2, 0x1e475005, 0x2608edb8, 0x22c9f00f, 0x2f8ad6d6,
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0x2b4bcb61, 0x350c9b64, 0x31cd86d3, 0x3c8ea00a, 0x384fbdbd,
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0x4c11db70, 0x48d0c6c7, 0x4593e01e, 0x4152fda9, 0x5f15adac,
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0x5bd4b01b, 0x569796c2, 0x52568b75, 0x6a1936c8, 0x6ed82b7f,
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0x639b0da6, 0x675a1011, 0x791d4014, 0x7ddc5da3, 0x709f7b7a,
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0x745e66cd, 0x9823b6e0, 0x9ce2ab57, 0x91a18d8e, 0x95609039,
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0x8b27c03c, 0x8fe6dd8b, 0x82a5fb52, 0x8664e6e5, 0xbe2b5b58,
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0xbaea46ef, 0xb7a96036, 0xb3687d81, 0xad2f2d84, 0xa9ee3033,
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0xa4ad16ea, 0xa06c0b5d, 0xd4326d90, 0xd0f37027, 0xddb056fe,
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0xd9714b49, 0xc7361b4c, 0xc3f706fb, 0xceb42022, 0xca753d95,
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0xf23a8028, 0xf6fb9d9f, 0xfbb8bb46, 0xff79a6f1, 0xe13ef6f4,
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0xe5ffeb43, 0xe8bccd9a, 0xec7dd02d, 0x34867077, 0x30476dc0,
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0x3d044b19, 0x39c556ae, 0x278206ab, 0x23431b1c, 0x2e003dc5,
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0x2ac12072, 0x128e9dcf, 0x164f8078, 0x1b0ca6a1, 0x1fcdbb16,
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0x018aeb13, 0x054bf6a4, 0x0808d07d, 0x0cc9cdca, 0x7897ab07,
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0x7c56b6b0, 0x71159069, 0x75d48dde, 0x6b93dddb, 0x6f52c06c,
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0x6211e6b5, 0x66d0fb02, 0x5e9f46bf, 0x5a5e5b08, 0x571d7dd1,
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0x53dc6066, 0x4d9b3063, 0x495a2dd4, 0x44190b0d, 0x40d816ba,
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0xaca5c697, 0xa864db20, 0xa527fdf9, 0xa1e6e04e, 0xbfa1b04b,
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0xbb60adfc, 0xb6238b25, 0xb2e29692, 0x8aad2b2f, 0x8e6c3698,
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0x832f1041, 0x87ee0df6, 0x99a95df3, 0x9d684044, 0x902b669d,
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0x94ea7b2a, 0xe0b41de7, 0xe4750050, 0xe9362689, 0xedf73b3e,
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0xf3b06b3b, 0xf771768c, 0xfa325055, 0xfef34de2, 0xc6bcf05f,
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0xc27dede8, 0xcf3ecb31, 0xcbffd686, 0xd5b88683, 0xd1799b34,
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0xdc3abded, 0xd8fba05a, 0x690ce0ee, 0x6dcdfd59, 0x608edb80,
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0x644fc637, 0x7a089632, 0x7ec98b85, 0x738aad5c, 0x774bb0eb,
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0x4f040d56, 0x4bc510e1, 0x46863638, 0x42472b8f, 0x5c007b8a,
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0x58c1663d, 0x558240e4, 0x51435d53, 0x251d3b9e, 0x21dc2629,
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0x2c9f00f0, 0x285e1d47, 0x36194d42, 0x32d850f5, 0x3f9b762c,
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0x3b5a6b9b, 0x0315d626, 0x07d4cb91, 0x0a97ed48, 0x0e56f0ff,
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0x1011a0fa, 0x14d0bd4d, 0x19939b94, 0x1d528623, 0xf12f560e,
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0xf5ee4bb9, 0xf8ad6d60, 0xfc6c70d7, 0xe22b20d2, 0xe6ea3d65,
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0xeba91bbc, 0xef68060b, 0xd727bbb6, 0xd3e6a601, 0xdea580d8,
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0xda649d6f, 0xc423cd6a, 0xc0e2d0dd, 0xcda1f604, 0xc960ebb3,
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0xbd3e8d7e, 0xb9ff90c9, 0xb4bcb610, 0xb07daba7, 0xae3afba2,
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0xaafbe615, 0xa7b8c0cc, 0xa379dd7b, 0x9b3660c6, 0x9ff77d71,
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0x92b45ba8, 0x9675461f, 0x8832161a, 0x8cf30bad, 0x81b02d74,
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0x857130c3, 0x5d8a9099, 0x594b8d2e, 0x5408abf7, 0x50c9b640,
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0x4e8ee645, 0x4a4ffbf2, 0x470cdd2b, 0x43cdc09c, 0x7b827d21,
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0x7f436096, 0x7200464f, 0x76c15bf8, 0x68860bfd, 0x6c47164a,
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0x61043093, 0x65c52d24, 0x119b4be9, 0x155a565e, 0x18197087,
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0x1cd86d30, 0x029f3d35, 0x065e2082, 0x0b1d065b, 0x0fdc1bec,
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0x3793a651, 0x3352bbe6, 0x3e119d3f, 0x3ad08088, 0x2497d08d,
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0x2056cd3a, 0x2d15ebe3, 0x29d4f654, 0xc5a92679, 0xc1683bce,
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0xcc2b1d17, 0xc8ea00a0, 0xd6ad50a5, 0xd26c4d12, 0xdf2f6bcb,
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0xdbee767c, 0xe3a1cbc1, 0xe760d676, 0xea23f0af, 0xeee2ed18,
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0xf0a5bd1d, 0xf464a0aa, 0xf9278673, 0xfde69bc4, 0x89b8fd09,
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0x8d79e0be, 0x803ac667, 0x84fbdbd0, 0x9abc8bd5, 0x9e7d9662,
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0x933eb0bb, 0x97ffad0c, 0xafb010b1, 0xab710d06, 0xa6322bdf,
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0xa2f33668, 0xbcb4666d, 0xb8757bda, 0xb5365d03, 0xb1f740b4
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};
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<br>
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unsigned long memcrc(const unsigned char *b, size_t n)
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{
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/* Input arguments:
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* const char* b == byte sequence to checksum
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* size_t n == length of sequence
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*/
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<br>
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register unsigned i, c, s = 0;
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<br>
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for (i = n; i > 0; --i) {
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c = (unsigned)(*b++);
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s = (s << 8) ^ crctab[(s >> 24) ^ c];
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}
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<br>
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/* Extend with the length of the string. */
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while (n != 0) {
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c = n & 0377;
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n >>= 8;
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s = (s << 8) ^ crctab[(s >> 24) ^ c];
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}
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<br>
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return ˜s;
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}
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</tt>
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</pre>
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<p>The historical practice of writing the number of "blocks" has been changed to writing the number of octets, since the latter
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is not only more useful, but also since historical implementations have not been consistent in defining what a "block" meant.
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Octets are used instead of bytes because bytes can differ in size between systems.</p>
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<p>The algorithm used was selected to increase the operational robustness of <i>cksum</i>. Neither the System V nor BSD <i>sum</i>
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algorithm was selected. Since each of these was different and each was the default behavior on those systems, no realistic
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compromise was available if either were selected-some set of historical applications would break. Therefore, the name was changed
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to <i>cksum</i>. Although the historical <i>sum</i> commands will probably continue to be provided for many years, programs
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designed for portability across systems should use the new name.</p>
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<p>The algorithm selected is based on that used by the ISO/IEC 8802-3:1996 standard (Ethernet) for the frame check sequence
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field. The algorithm used does not match the technical definition of a <i>checksum</i>; the term is used for historical reasons.
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The length of the file is included in the CRC calculation because this parallels inclusion of a length field by Ethernet in its
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CRC, but also because it guards against inadvertent collisions between files that begin with different series of zero octets. The
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chance that two different files produce identical CRCs is much greater when their lengths are not considered. Keeping the length
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and the checksum of the file itself separate would yield a slightly more robust algorithm, but historical usage has always been
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that a single number (the checksum as printed) represents the signature of the file. It was decided that historical usage was the
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more important consideration.</p>
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<p>Early proposals contained modifications to the Ethernet algorithm that involved extracting table values whenever an intermediate
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result became zero. This was demonstrated to be less robust than the current method and mathematically difficult to describe or
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justify.</p>
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<p>The calculation used is identical to that given in pseudo-code in the referenced Sarwate article. The pseudo-code rendition
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is:</p>
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<pre>
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<tt>X <- 0; Y <- 0;
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for i <- m -1 step -1 until 0 do
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begin
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T <- X(1) ^ A[i];
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X(1) <- X(0); X(0) <- Y(1); Y(1) <- Y(0); Y(0) <- 0;
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comment: f[T] and f'[T] denote the T-th words in the
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table f and f' ;
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X <- X ^ f[T]; Y <- Y ^ f'[T];
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end
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</tt>
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</pre>
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<p>The pseudo-code is reproduced exactly as given; however, note that in the case of <i>cksum</i>, <b>A[i]</b> represents a byte of
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the file, the words <b>X</b> and <b>Y</b> are treated as a single 32-bit value, and the tables <b>f</b> and <b>f'</b> are a single
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table containing 32-bit values.</p>
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<p>The referenced Sarwate article also discusses generating the table.</p>
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</blockquote>
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<h4><a name="tag_04_20_19"></a>FUTURE DIRECTIONS</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_04_20_20"></a>SEE ALSO</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_04_20_21"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 4.</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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