<PART ID="services-crc">
<TITLE>CRC Algorithms</TITLE>
<PARTINTRO>
<PARA>
The CRC package provides implementation of CRC algorithms. This
includes the POSIX CRC calculation which produces the same result as
the cksum command on Linux, another 32 bit CRC by Gary S. Brown and a
16bit CRC. The CRC used for Ethernet FCS is also implemented.
</PARA>
</PARTINTRO>
<CHAPTER id="crc-functions">
<TITLE>CRC Functions</TITLE>
<SECT1 id="services-crc-api">
<TITLE>CRC API</TITLE>
<para>
The package implements a number of CRC functions as described below.
The API to these functions is in the include file
<filename>cyg/crc/crc.h</filename>.
</para>
<sect2 id="services-crc-api-cyg-posix-crc32">
<title>cyg_posix_crc32</title>
<para>
This function implements a 32 bit CRC which is compliant to the POSIX
1008.2 Standard. This is the same as the Linux cksum program.
</para>
<programlisting>
unsigned long cyg_posix_crc32(unsigned char &ast; s, int len);
</programlisting>
<para>
The CRC calculation is run over the data pointed to by
<parameter>s</parameter>, of length <parameter>len</parameter>. The
CRC is returned as an unsigned long.</para>
</sect2>

<sect2 id="services-crc-api-cyg-crc32">
<title>cyg_crc32</title>
<para>
These functions implement a 32 bit CRC by Gary S. Brown. They use the
polynomial
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+X^0.
</para>
<programlisting>
unsigned long cyg_crc32(unsigned char &ast; s, int len);
unsigned long cyg_crc32_accumulate(unsigned long crc, unsigned char &ast; s, int len);
</programlisting>
<para>
The CRC calculation is run over the data pointed to by
<parameter>s</parameter>, of length <parameter>len</parameter>. The
CRC is returned as an unsigned long.</para> 

<para> The CRC can be calculated over data separated into multiple
buffers by using the function <parameter> cyg_crc32_accumulate()
</parameter>. The parameter <parameter>crc</parameter> should be the
result from the previous CRC calculation.
</para>
</sect2>

<sect2 id="services-crc-api-cyg-ether-crc32">
<title>cyg_ether_crc32</title>
<para>
These functions implement the 32 bit CRC used by the Ethernet FCS word.
</para>
<programlisting>
unsigned long cyg_ether_crc32(unsigned char &ast; s, int len);
unsigned long cyg_ether_crc32_accumulate(unsigned long crc, unsigned char &ast; s, int len);
</programlisting>
<para>
The CRC calculation is run over the data pointed to by
<parameter>s</parameter>, of length <parameter>len</parameter>. The
CRC is returned as an unsigned long.</para> 

<para> The CRC can be calculated over data separated into multiple
buffers by using the function <parameter> cyg_ether_crc32_accumulate()
</parameter>. The parameter <parameter>crc</parameter> should be the
result from the previous CRC calculation.
</para>
</sect2>

<sect2 id="services-crc-api-cyg-crc16">
<title>cyg_crc16</title>
<para>
This function implements a 16 bit CRC. It uses the polynomial
x^16+x^12+x^5+1.
</para>
<programlisting>
unsigned short cyg_crc16(unsigned char &ast; s, int len);
</programlisting>
<para>
The CRC calculation is run over the data pointed to by
<parameter>s</parameter>, of length <parameter>len</parameter>. The
CRC is returned as an unsigned short.</para>
</sect2>

</SECT1>
</CHAPTER>
</PART>

