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<?xml version='1.0'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
<!ENTITY project_name "pycrc">
<!ENTITY project_version "0.8">
<!ENTITY project_homepage "http://www.tty1.net/pycrc/">
<!ENTITY project_models "http://www.tty1.net/pycrc/crc-models_en.html">
<!ENTITY date "2013-07-25">
<!ENTITY author_firstname "Thomas">
<!ENTITY author_surname "Pircher">
<!ENTITY author_email "tehpeh@gmx.net">
<!ENTITY bit-by-bit "bit-by-bit">
<!ENTITY bbb "bbb">
<!ENTITY bit-by-bit-fast "bit-by-bit-fast">
<!ENTITY bbf "bbf">
<!ENTITY bitwise-expression "bitwise-expression">
<!ENTITY bwe "bwe">
<!ENTITY table-driven "table-driven">
<!ENTITY tbl "tbl">
<!ENTITY width "Width">
<!ENTITY poly "Polynomial">
<!ENTITY reflect_in "ReflectIn">
<!ENTITY xor_in "XorIn">
<!ENTITY reflect_out "ReflectOut">
<!ENTITY xor_out "XorOut">
<!ENTITY check "Check">
]>
<refentry id="&project_name;">
<refentryinfo>
<title>&project_name;</title>
<productname>&project_name;</productname>
<productnumber>&project_version;</productnumber>
<author>
<firstname>&author_firstname;</firstname>
<surname>&author_surname;</surname>
<contrib>Author of &project_name; and this manual page.</contrib>
<email>&author_email;</email>
</author>
<date>&date;</date>
</refentryinfo>
<refmeta>
<refentrytitle>&project_name;</refentrytitle>
<manvolnum>1</manvolnum>
</refmeta>
<refnamediv>
<refname>&project_name;</refname>
<refpurpose>a free, easy to use Cyclic Redundancy Check (CRC) calculator and C source code generator.</refpurpose>
</refnamediv>
<refsynopsisdiv>
<cmdsynopsis>
<command>python pycrc.py</command>
<arg>OPTIONS</arg>
</cmdsynopsis>
</refsynopsisdiv>
<refsect1><title>Description</title>
<para>
<ulink url="&project_homepage;">&project_name;</ulink>
is a CRC reference implementation in Python and a C source code generator for parametrised CRC models.
The generated C source code can be optimised for simplicity,
speed or small memory footprint, as required on small embedded systems.
The following operations are implemented:
<itemizedlist>
<listitem>
<para>calculate the checksum of a string (ASCII or hex)</para>
</listitem>
<listitem>
<para>calculate the checksum of a file</para>
</listitem>
<listitem>
<para>generate the header and source files for a C implementation.</para>
</listitem>
</itemizedlist>
</para>
<para>
&project_name; supports the following variants of the CRC algorithm:
<itemizedlist>
<listitem>
<para><replaceable>&bit-by-bit;</replaceable> or <replaceable>&bbb;</replaceable>:
the basic algorithm which operates individually on every bit of the augmented message
(i.e. the input data with <replaceable>&width;</replaceable> zero bits added at the end).
This algorithm is a straightforward implementation of the basic polynomial division and
is the easiest one to understand, but it is also the slowest one among all possible
variants.
</para>
</listitem>
<listitem>
<para><replaceable>&bit-by-bit-fast;</replaceable> or <replaceable>&bbf;</replaceable>:
a variation of the simple <replaceable>&bit-by-bit;</replaceable> algorithm.
This algorithm still iterates over every bit of the message, but does not augment
it (does not add <replaceable>&width;</replaceable> zero bits at the end).
It gives the same result as the <replaceable>&bit-by-bit;</replaceable> method by
carefully choosing the initial value of the algorithm.
This method might be a good choice for embedded platforms, where code space is more
important than execution speed.
</para>
</listitem>
<listitem>
<para><replaceable>&bitwise-expression;</replaceable> or <replaceable>&bwe;</replaceable>:
calculates the CRC byte-wise using bit-wise expressions (such as binary and, not, xor,
bit-shifts etc).
This algorithm uses the same approach as the <replaceable>&table-driven;</replaceable>
variant, but uses a logical operations instead of a look-up table. It is slightly
slower than the <replaceable>&table-driven;</replaceable> algorithm, but without the
overhead of a look-up table.
</para>
<para>
This is an experimental feature and will in most cases result in a slower and bigger
binary than any other algorithm.
This option is only valid for code generation, not for checking strings or files.
It can take a long time to generate the output for models with a big &width; parameter.
Use with care.
</para>
</listitem>
<listitem>
<para><replaceable>&table-driven;</replaceable> or <replaceable>&tbl;</replaceable>:
the standard table driven algorithm.
This is the fastest variant because it operates on one byte at a time, as opposed to one
bit at the time.
This method uses a look-up table (usually of 256 elements), which might not be acceptable
for small embedded systems. The number of elements in the look-up table can be reduced
with the <option>--table-idx-width</option> command line switch.
The value of 4 bits for the table index (16 elements in the look-up table) can be a good
compromise between execution speed and code size.
</para>
</listitem>
</itemizedlist>
</para>
</refsect1>
<refsect1><title>Options</title>
<variablelist>
<varlistentry>
<term>
<option>--version</option>
</term>
<listitem>
<para>show the program version number and exit.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-h</option>
</term>
<term>
<option>--help</option>
</term>
<listitem>
<para>show this help message and exit.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--verbose</option>
</term>
<listitem>
<para>be more verbose; in particular, print the value of the parameters and the chosen model to <filename>stdout</filename>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--check-string=</option><replaceable>STRING</replaceable>
</term>
<listitem>
<para>calculate the checksum of a string (default: <quote><replaceable>123456789</replaceable></quote>).</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--check-hexstring=</option><replaceable>STRING</replaceable>
</term>
<listitem>
<para>calculate the checksum of a hexadecimal number string.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--check-file=</option><replaceable>FILE</replaceable>
</term>
<listitem>
<para>calculate the checksum of a file.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--generate=</option><replaceable>CODE</replaceable>
</term>
<listitem>
<para>generate C source code; choose the type from {<replaceable>h</replaceable>,
<replaceable>c</replaceable>, <replaceable>c-main</replaceable>, <replaceable>table</replaceable>}.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--std=</option><replaceable>STD</replaceable>
</term>
<listitem>
<para>specify the C dialect of the generated code from {C89, ANSI, C99}.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--algorithm=</option><replaceable>ALGO</replaceable>
</term>
<listitem>
<para>choose an algorithm from {<replaceable>bit-by-bit</replaceable>, <replaceable>bbb</replaceable>,
<replaceable>bit-by-bit-fast</replaceable>, <replaceable>bbf</replaceable>,
<replaceable>bitwise-expression</replaceable>, <replaceable>bwe</replaceable>,
<replaceable>table-driven</replaceable>, <replaceable>tbl</replaceable>,
<replaceable>all</replaceable>}.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--model=</option><replaceable>MODEL</replaceable>
</term>
<listitem>
<para>choose a parameter set from
{<replaceable>crc-5</replaceable>,
<replaceable>crc-8</replaceable>,
<replaceable>dallas-1-wire</replaceable>,
<replaceable>crc-12-3gpp</replaceable>,
<replaceable>crc-15</replaceable>,
<replaceable>crc-16</replaceable>,
<replaceable>crc-16-usb</replaceable>,
<replaceable>crc-16-modbus</replaceable>,
<replaceable>crc-16-genibus</replaceable>,
<replaceable>ccitt</replaceable>,
<replaceable>r-crc-16</replaceable>,
<replaceable>kermit</replaceable>,
<replaceable>x-25</replaceable>,
<replaceable>xmodem</replaceable>,
<replaceable>zmodem</replaceable>,
<replaceable>crc-24</replaceable>,
<replaceable>crc-32</replaceable>,
<replaceable>crc-32c</replaceable>,
<replaceable>crc-32-mpeg</replaceable>,
<replaceable>crc-32-bzip2</replaceable>,
<replaceable>posix</replaceable>,
<replaceable>jam</replaceable>,
<replaceable>xfer</replaceable>,
<replaceable>crc-64</replaceable>,
<replaceable>crc-64-jones</replaceable>,
<replaceable>crc-64-xz</replaceable>}.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--width=</option><replaceable>NUM</replaceable>
</term>
<listitem>
<para>use <replaceable>NUM</replaceable> bits in the <replaceable>&poly;</replaceable>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--poly=</option><replaceable>HEX</replaceable>
</term>
<listitem>
<para>use <replaceable>HEX</replaceable> as <replaceable>&poly;</replaceable>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--reflect-in=</option><replaceable>BOOL</replaceable>
</term>
<listitem>
<para>reflect the octets in the input message.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--xor-in=</option><replaceable>HEX</replaceable>
</term>
<listitem>
<para>use <replaceable>HEX</replaceable> as initial value.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--reflect-out=</option><replaceable>BOOL</replaceable>
</term>
<listitem>
<para>reflect the resulting checksum before applying the &xor_out; value.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--xor-out=</option><replaceable>HEX</replaceable>
</term>
<listitem>
<para>xor the final CRC value with <replaceable>HEX</replaceable>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--table-idx-width=</option><replaceable>NUM</replaceable>
</term>
<listitem>
<para>use <replaceable>NUM</replaceable> bits to index the CRC table;
<replaceable>NUM</replaceable> must be one of the values
{<replaceable>1</replaceable>, <replaceable>2</replaceable>,
<replaceable>4</replaceable>, <replaceable>8</replaceable>}.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--symbol-prefix=</option><replaceable>STRING</replaceable>
</term>
<listitem>
<para>when generating source code, use <replaceable>STRING</replaceable>
as prefix to the exported C symbols.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--crc-type=</option><replaceable>STRING</replaceable>
</term>
<listitem>
<para>when generating source code, use <replaceable>STRING</replaceable> as crc_t type.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--include-file=</option><replaceable>FILE</replaceable>
</term>
<listitem>
<para>when generating source code, include also <replaceable>FILE</replaceable> as header file.
This option can be specified multiple times.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-o</option><replaceable>FILE</replaceable>
</term>
<term>
<option>--output=</option><replaceable>FILE</replaceable>
</term>
<listitem>
<para>write the generated code to <replaceable>FILE</replaceable> instead of <filename>stdout</filename>.</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1><title>The CRC Parametric Model</title>
<para>
The parametric model follows Ross N. Williams' convention described in
<ulink url="http://www.ross.net/crc/crcpaper.html">A Painless Guide to CRC Error Detection Algorithms</ulink>,
often called the Rocksoft Model.
Since most people are familiar with this kind of parameters, &project_name; follows this convention, described as follows:
<glosslist>
<glossentry>
<glossterm><replaceable>&width;</replaceable></glossterm>
<glossdef>
<para>
The number of significant bits in the CRC <replaceable>&poly;</replaceable>,
excluding the most significant 1.
This will also be the number of bits in the final CRC result.
In previous versions of &project_name; only multiples of 8 could be used as
<replaceable>&width;</replaceable> for the <replaceable>&table-driven;</replaceable> algorithm.
As of version 0.7.5 any value is accepted for <replaceable>&width;</replaceable> for all algorithms.
</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm><replaceable>&poly;</replaceable></glossterm>
<glossdef>
<para>
The unreflected polynomial of the CRC algorithm.
</para>
<para>
The <replaceable>&poly;</replaceable> may be specified in its standard form,
i.e. with bit <replaceable>&width;</replaceable>+1 set to 1, but the most significant
bit may also be omitted.
For example, both numbers 0x18005 and 0x8005 are accepted for a 16-bit
<replaceable>&poly;</replaceable>.
</para>
<para>
Most <replaceable>&poly;</replaceable>s used in real world applications are odd (the least significant
bit is 1), but there are some good even ones.
&project_name; allows the use of even <replaceable>&poly;</replaceable>s but some of them may yield
incorrect checksums depending on the used algorithm.
Use at your own risk and if in doubt pick a well-known <replaceable>MODEL</replaceable> above.
</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm><replaceable>&reflect_in;</replaceable></glossterm>
<glossdef>
<para>
Reflect the octets of the message before processing them.
</para>
<para>
A word is reflected or reversed by <quote>flipping</quote> its bits around the
mid-point of the word.
The most significant bit of the word is moved to the least significant position,
the second-most significant bit is moved to the second-least significant position
and so on.
The reflected value of 0xa2 (10100010b) is 0x45 (01000101b), for example.
</para>
<para>
Some CRC algorithms can be implemented more efficiently in a bit reversed version,
that's why many of the standard CRC models use reflected input octets.
</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm><replaceable>&xor_in;</replaceable></glossterm>
<glossdef>
<para>
The initial value (usually all 0 or all 1) for algorithms which operate on the
non-augmented message, that is, any algorithm other than the
<replaceable>&bit-by-bit;</replaceable> one.
This value can be interpreted as a value which will be XOR-ed into the CRC register
after <replaceable>&width;</replaceable> iterations of the
<replaceable>&bit-by-bit;</replaceable> algorithm.
This implies that the simple <replaceable>&bit-by-bit;</replaceable> algorithm must
calculate the initial value using some sort of reverse CRC algorithm on the
<replaceable>&xor_in;</replaceable> value.
</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm><replaceable>&reflect_out;</replaceable></glossterm>
<glossdef>
<para>
Reflect the final CRC result. This operation takes place before XOR-ing the final CRC
value with the <replaceable>&xor_out;</replaceable> parameter.
</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm><replaceable>&xor_out;</replaceable></glossterm>
<glossdef>
<para>
A value (usually all bits 0 or all 1) which will be XOR-ed to the final CRC value.
</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm><replaceable>✓</replaceable></glossterm>
<glossdef>
<para>
This value is not exactly a parameter of a model but it is sometimes given together
with the Rocksoft Model parameters.
It is the CRC value of the parametrised model over the string
<quote><replaceable>123456789</replaceable></quote> and
can be used as a sanity check for a particular CRC implementation.
</para>
</glossdef>
</glossentry>
</glosslist>
</para>
</refsect1>
<refsect1><title>Code generation</title>
<para>
In the default configuration, the generated code is strict ISO C99.
A minimal set of three functions are defined for each algorithm:
<function>crc_init()</function>, <function>crc_update()</function> and <function>crc_finalize()</function>.
Depending on the number of parameters given to &project_name;, a different interface will be defined.
A fully parametrised model has a simpler API, while the generated code for a runtime-specified
implementation requires a pointer to a configuration structure as first parameter to all functions.
</para>
<para>
The generated source code uses the type <type>crc_t</type>, which is used throughout the code
to hold intermediate results and also the final CRC value.
It is defined in the generated header file and its type may be overridden with the
<option>--crc-type</option> option.
</para>
<refsect2><title>Fully parametrised models</title>
<para>
The prototypes of the CRC functions are normally generated by &project_name; using the
<replaceable>--generate h</replaceable> option.
The CRC functions for a fully parametrised model will look like:
</para>
<funcsynopsis>
<funcsynopsisinfo>
#include <stdlib.h>
typedef uint16_t crc_t; /* &project_name; will use an appropriate size here */
</funcsynopsisinfo>
<funcprototype>
<?dbhtml funcsynopsis-style='ansi'?>
<funcdef>crc_t <function>crc_init</function></funcdef>
<void/>
</funcprototype>
<funcprototype>
<?dbhtml funcsynopsis-style='ansi'?>
<funcdef>crc_t <function>crc_update</function></funcdef>
<paramdef>crc_t <parameter>crc</parameter></paramdef>
<paramdef>const unsigned char *<parameter>data</parameter></paramdef>
<paramdef>size_t <parameter>data_len</parameter></paramdef>
</funcprototype>
<funcprototype>
<?dbhtml funcsynopsis-style='ansi'?>
<funcdef>crc_t <function>crc_finalize</function></funcdef>
<paramdef>crc_t <parameter>crc</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<para>
The code snippet below shows how to use the generated functions.
<programlisting>
#include "&project_name;_generated_crc.h"
#include <stdio.h>
int main(void)
{
static const unsigned char str1[] = "1234";
static const unsigned char str2[] = "56789";
crc_t crc;
crc = crc_init();
crc = crc_update(crc, str1, sizeof(str1) - 1);
crc = crc_update(crc, str2, sizeof(str2) - 1);
/* more calls to crc_update... */
crc = crc_finalize(crc);
printf("0x%lx\n", (long)crc);
return 0;
}
</programlisting>
</para>
</refsect2>
<refsect2><title>Models with runtime-configurable parameters</title>
<para>
When the model is not fully defined then the missing parameters are stored in a structure of
type <type>crc_cfg_t</type>.
If a CRC function requires a value from the <type>crc_cfg_t</type> structure, then the first
function argument is always a pointer to that structure.
All fields of the configuration structure must be properly initialised before the first call
to any CRC function.
</para>
<para>
If the <replaceable>&width;</replaceable> was not specified when the code was generated, then
the <type>crc_cfg_t</type> structure will contain three more fields:
<parameter>msb_mask</parameter>, <parameter>crc_mask</parameter> and <parameter>crc_shift</parameter>.
They are defined for performance reasons and must be initialised to the value given next to the
field definition.
</para>
<para>
For example, a completely undefined CRC implementation will generate a <type>crc_cfg_t</type>
structure as below:
<programlisting>
typedef struct {
unsigned int width;
crc_t poly;
bool reflect_in;
crc_t xor_in;
bool reflect_out;
crc_t xor_out;
// internal parameters
crc_t msb_mask; // initialise as (crc_t)1u << (cfg->width - 1)
crc_t crc_mask; // initialise as (cfg->msb_mask - 1) | cfg->msb_mask
unsigned int crc_shift; // initialise as cfg->width < 8 ? 8 - cfg->width : 0
} crc_cfg_t;
</programlisting>
</para>
<para>
<parameter>msb_mask</parameter> is a bitmask with the most significant bit of a
<replaceable>&width;</replaceable> bits wide data type set to 1.
<parameter>crc_mask</parameter> is a bitmask with all bits of a
<replaceable>&width;</replaceable> bits wide data type set to 1.
<parameter>crc_shift</parameter> is a shift counter that is used when
<replaceable>&width;</replaceable> is less than 8.
It is the number of bits to shift the CRC register to align its top bit to a byte boundary.
</para>
<para>
The file <filename>test/main.c</filename> in the source package of &project_name;
contains a fully featured example of how to use the generated source code.
A shorter, more compact <code>main()</code> function can be generated with the
<replaceable>--generate c-main</replaceable> option.
This second variant is the better option as it will always output valid code when
some of the CRC parameters are known and some are unknown during code generation.
</para>
</refsect2>
</refsect1>
<refsect1><title>Examples</title>
<para>
<glosslist>
<glossentry>
<glossterm>Calculate the CRC-32 checksum of the string <quote>123456789</quote>:</glossterm>
<glossdef>
<para>
<userinput>python pycrc.py --model crc-32 --check-string 123456789</userinput>
</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>Generate the source code of the table-driven algorithm for an embedded application.</glossterm>
<glossdef>
<para>
The table index width of 4 bits ensures a moderate memory usage.
To be precise, the size of the resulting table will be <code>16 * sizeof(crc_t)</code>.
</para>
<para>
<userinput>python pycrc.py --model crc-16 --algorithm table-driven --table-idx-width 4 --generate h -o crc.h</userinput>
</para>
<para>
<userinput>python pycrc.py --model crc-16 --algorithm table-driven --table-idx-width 4 --generate c -o crc.c</userinput>
</para>
<para>
A variant of the <replaceable>c</replaceable> target is <replaceable>c-main</replaceable>:
this target will generate a simple <replaceable>main()</replaceable> function in addition to
the CRC functions:
</para>
<para>
<userinput>python pycrc.py --model crc-16 --algorithm table-driven --table-idx-width 4 --generate c-main -o crc.c</userinput>
</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>Generate the CRC table only:</glossterm>
<glossdef>
<para>
<userinput>python pycrc.py --model kermit --generate table -o crc-table.txt</userinput>
</para>
</glossdef>
</glossentry>
</glosslist>
</para>
</refsect1>
<refsect1><title>See Also</title>
<para>
The homepage of &project_name; is <ulink url="&project_homepage;">&project_homepage;</ulink>.
</para>
<para>
A list of common CRC models is at <ulink url="&project_models;">&project_models;</ulink>.
For a long list of known CRC models, see Greg Cook's
<ulink url="http://reveng.sourceforge.net/crc-catalogue/">Catalogue of Parameterised CRC Algorithms</ulink>.
</para>
</refsect1>
<refsect1><title>Copyright</title>
<para>
This work is licensed under a
<ulink url="http://creativecommons.org/licenses/by-sa/3.0/">Creative Commons Attribution-Share Alike 3.0 Unported License</ulink>.
</para>
</refsect1>
</refentry>
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