NAME¶

pycrc - a free, easy to use Cyclic Redundancy Check (CRC) calculator and C source code generator.

SYNOPSIS¶

python pycrc.py [OPTIONS]

DESCRIPTION¶

pycrc[1] 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:

pycrc supports the following variants of the CRC algorithm:

OPTIONS¶

--version

-h, --help

--verbose

--check-string=STRING

--check-hexstring=STRING

--check-file=FILE

--generate=CODE

--std=STD

--algorithm=ALGO

--model=MODEL

--width=NUM

--poly=HEX

--reflect-in=BOOL

--xor-in=HEX

--reflect-out=BOOL

--xor-out=HEX

--slice-by=NUM

--table-idx-width=NUM

--force-poly

--symbol-prefix=STRING

--crc-type=STRING

--include-file=FILE

-oFILE, --output=FILE

THE CRC PARAMETRIC MODEL¶

The parametric model follows Ross N. Williams' convention described in A Painless Guide to CRC Error Detection Algorithms[2], often called the Rocksoft Model. Since most people are familiar with this kind of parameters, pycrc follows this convention, described as follows:

Width

Polynomial

ReflectIn

XorIn

ReflectOut

XorOut

Check

CODE GENERATION¶

In the default configuration, the generated code is strict ISO C99. A minimal set of three functions are defined for each algorithm: crc_init(), crc_update() and crc_finalize(). Depending on the number of parameters given to pycrc, 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.

The generated source code uses the type crc_t, 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 --crc-type option.

#include <stdlib.h>
typedef uint16_t crc_t;         /* pycrc will use an appropriate size here */


                

#include "pycrc_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;
}


            

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;


            

EXAMPLES¶

Calculate the CRC-32 checksum of the string “123456789”:

Generate the source code of the table-driven algorithm for an embedded application.

Generate the CRC table only:

SEE ALSO¶

The homepage of pycrc is https://pycrc.org.

A list of common CRC models is at https://pycrc.org/models.html. For a long list of known CRC models, see Greg Cook's Catalogue of Parameterised CRC Algorithms[3].

COPYRIGHT¶

This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International[4].

AUTHOR¶

Thomas Pircher <tehpeh-web@tty1.net>

NOTES¶

1.

pycrc

2.

A Painless Guide to CRC Error Detection Algorithms

3.

Catalogue of Parameterised CRC Algorithms

4.

Creative Commons Attribution-ShareAlike 4.0 International