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.TH crc-mktab 1 "9 November 2003" "Straylight/Edgeware" "mLib utilities library"
.SH NAME
crc-mktab \- construct CRC tables for efficient computation
.SH SYNOPSIS
.B crc-mktab
.RB [ \-cr ]
.RB [ \-s
.IR symbol ]
.RB [ \-t
.IR type ]
.RB [ \-i
.IR header ]
.br
	\c
.RB [ \-g
.IR macro ]
.RB [ \-b
.IR bits ]
.RB [ \-B
.IR chunk ]
.RB [ \-p
.IR poly ]
.br
	\c
.RB [ \-o
.IR filename ]
.SH DESCRIPTION
The
.B crc-mktab
program constructs tables for efficient computation of CRC (cyclic
redundancy check) values.  It will produce the table as either an array
defined in a C source file or as an initializer macro defined in a C
header file.
.SS "Options"
The program accepts no non-option arguments.  At least one of
.B \-b
or
.B \-p
must be given.  The options are as follows.
.TP
.B "\-h, \-\-help"
Print a help message to standard output and exit successfully.
.TP
.B "\-v, \-\-version"
Print the program's version number to standard output and exit
successfully.
.TP
.B "\-u, \-\-usage"
Print a one-line usage summary to standard output and exit successfully.
.TP
.B "\-c, \-\-c-source"
Produce a C source file which exports a symbol naming the array, instead
of a C header file.
.TP
.BI "\-s, \-\-symbol=" symbol
Name the table
.IR symbol .
This is the name of the macro defined by a header file, or the array
exported by a C source.  The default macro name is
.BR CRC_TAB ;
the default array name is
.BR crctab .
.TP
.BI "\-t, \-\-type=" type
Specify the element type of the array defined by a C source output.  The
default is
.B "unsigned short"
if the polynomial has degree 16 or less, or
.B "unsigned long"
otherwise.  This option does nothing without the
.B \-c
option.
.TP
.BI "\-i, \-\-include=" header
Request that generated C source include the named
.I header
file.  Inserts a
line of the form
.PP
.nf
.BI "          #include """ header """"
.fi
.IP
at the top of the generated C source.  The default is not to include a
header file.  This option does nothing without the
.B \-c
option.
.TP
.BI "\-g, \-\-guard=" macro
Use the named
.I macro
as a guard against multiple inclusion of the generated header file.
Inserts a pair of lines of the form
.PP
.nf
.BI "          #ifndef " macro
.BI "          #define " macro
.fi
.IP
at the top of the generated header, and a line
.PP
.nf
.BI "          #endif"
.fi
.IP
at the end.  The default guard macro name is built from the output file
name specified with
.B \-o
by uppercasing all alphabetic characters in the name and replacing
nonalphanumeric characters by underscores
.RB ` _ '.
This option does nothing with the
.B \-c
option.
.TP
.BI "\-b, \-\-bits=" bits
Specifies the degree of the CRC polynomial or, equivalently, the length
of the generated CRC.  This must be an integer between 1 and 32
inclusive.  If it is not specified, the polynomial given by
.B \-p
is examined and an educated guess made.  (Currently we choose the
smallest multiple of 8 which is large enough.)
.TP
.BI "\-B, \-\-bit-chunk=" chunk
Chunk size in which bits are taken from the input.  The number of
entries in the table is precisely
.RI 2\*(ss chunk \*(se.
The default chunk size is 8.
.TP
.BI "\-p, \-\-polynomial=" poly
Specifies the CRC polynomial as an integer.  The polynomial has a
nonzero coefficient in its
.IR x \*(ss i \*(se
term if and only if bit
.I i
of
.I poly
is nonzero.  Note that if you want to specify the polynomial in
hexadecimal, you must prefix it with
.BR 0x .
Default polynomials of degree 16 and 32 are known.
.TP
.B "\-r, \-\-reverse"
Construct the table to compensate for unusual bit-ordering.  Without
this option, you'd have to reverse the order of all input chunks and the
output CRC.
.SS "The table and how to use it"
Describing in detail the contents of the table would take too long.  For
an example of use, see the header file
.BR crc32.h .
.SH "SEE ALSO"
.BR crc32 (3).
.PP
.I A painless guide to CRC error detection algorithms
by Ross N. Williams.
.SH "AUTHOR"
Mark Wooding, <mdw@distorted.org.uk>