pycrc — a free, easy to use Cyclic Redundancy Check (CRC) calculator and C source code generator.
python pycrc.py
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pycrc 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:
calculate the checksum of a string (ASCII or hex)
calculate the checksum of a file
generate the header and source files for a C implementation.
pycrc supports the following variants of the CRC algorithm:
bit-by-bit
or bbb
:
the basic algorithm which operates individually on every bit of the augmented message
(i.e. the input data with Width
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.
bit-by-bit-fast
or bbf
:
a variation of the simple bit-by-bit
algorithm.
This algorithm still iterates over every bit of the message, but does not augment
it (does not add Width
zero bits at the end).
It gives the same result as the bit-by-bit
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.
table-driven
or tbl
:
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 --table-idx-width
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.
The --slice-by
option enables a variant of the table-driven
algorithm that operates on 32 bits of data or more at a time rather than 8 bits.
This can dramatically speed-up the calculation of the CRC, at the cost of
increased code and data size.
Note: this option is experimental and not well-tested.
Check your results and please raise bugs if you find problems.
--version
show the program version number and exit.
-h
,
--help
show this help message and exit.
--verbose
be more verbose; in particular, print the value of the parameters and the chosen model to stdout
.
--check-string=
STRING
calculate the checksum of a string (default: “123456789
”). If the string contains non-ASCII characters then it will be UTF-8 decoded.
--check-hexstring=
STRING
calculate the checksum of a hexadecimal number string.
--check-file=
FILE
calculate the checksum of a file. If the file contains non-ASCII characters then it will be UTF-8 decoded.
--generate=
CODE
generate C source code; choose the type from {h
,
c
, c-main
, table
}.
--std=
STD
specify the C dialect of the generated code from {C89, ANSI, C99}.
--algorithm=
ALGO
choose an algorithm from {bit-by-bit
, bbb
,
bit-by-bit-fast
, bbf
,
table-driven
, tbl
,
all
}.
--model=
MODEL
choose a parameter set from
{crc-5
,
crc-8
,
dallas-1-wire
,
crc-12-3gpp
,
crc-15
,
crc-16
,
crc-16-usb
,
crc-16-modbus
,
crc-16-genibus
,
crc-16-ccitt
,
r-crc-16
,
kermit
,
x-25
,
xmodem
,
zmodem
,
crc-24
,
crc-32
,
crc-32c
,
crc-32-mpeg
,
crc-32-bzip2
,
posix
,
jam
,
xfer
,
crc-64
,
crc-64-jones
,
crc-64-xz
}.
--width=
NUM
use NUM
bits in the Polynomial
.
--poly=
HEX
use HEX
as Polynomial
.
--reflect-in=
BOOL
reflect the octets in the input message.
--xor-in=
HEX
use HEX
as initial value.
--reflect-out=
BOOL
reflect the resulting checksum before applying the XorOut value.
--xor-out=
HEX
xor the final CRC value with HEX
.
--slice-by=
NUM
speed-up the table-driven calculation by operating on
NUM
octets of data rather than a
single octet at a time.
NUM
must be one of the values
{4
, 8
,
16
}.
--table-idx-width=
NUM
use NUM
bits to index the CRC table;
NUM
must be one of the values
{1
, 2
,
4
, 8
}.
--force-poly
override any errors about possibly unsuitable polynoms. pycrc does not allow even polynoms or polynoms that are wider than Width. Use this option to override the error, if you know what you are doing.
--symbol-prefix=
STRING
when generating source code, use STRING
as prefix to the exported C symbols.
--crc-type=
STRING
when generating source code, use STRING
as crc_t type.
--include-file=
FILE
when generating source code, include also FILE
as header file.
This option can be specified multiple times.
-o
FILE
,
--output=
FILE
write the generated code to FILE
instead of stdout
.
The parametric model follows Ross N. Williams' convention described in A Painless Guide to CRC Error Detection Algorithms, often called the Rocksoft Model. Since most people are familiar with this kind of parameters, pycrc follows this convention, described as follows:
Width
The number of significant bits in the CRC Polynomial
,
excluding the most significant 1.
This will also be the number of bits in the final CRC result.
In previous versions of pycrc only multiples of 8 could be used as
Width
for the table-driven
algorithm.
As of version 0.7.5 any value is accepted for Width
for all algorithms.
Polynomial
The unreflected polynomial of the CRC algorithm.
The Polynomial
may be specified in its standard form,
i.e. with bit Width
+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
Polynomial
.
Most polynomials used in real world applications are odd (the least significant
bit is 1), but there are some good even ones.
pycrc allows the use of even polynomials with the --force-poly
option.
Some even polynomials may yield incorrect checksums depending on the used algorithm.
Use at your own risk and if at all possible use a well-known MODEL
above.
ReflectIn
Reflect the octets of the message before processing them.
A word is reflected or reversed by “flipping” 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.
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.
XorIn
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
bit-by-bit
one.
This value can be interpreted as a value which will be XOR-ed into the CRC register
after Width
iterations of the
bit-by-bit
algorithm.
This implies that the simple bit-by-bit
algorithm must
calculate the initial value using some sort of reverse CRC algorithm on the
XorIn
value.
ReflectOut
Reflect the final CRC result. This operation takes place before XOR-ing the final CRC
value with the XorOut
parameter.
XorOut
A value (usually all bits 0 or all 1) which will be XOR-ed to the final CRC value.
Check
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
“123456789
” and
can be used as a sanity check for a particular CRC implementation.
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.
The prototypes of the CRC functions are normally generated by pycrc using the
--generate h
option.
The CRC functions for a fully parametrised model will look like:
#include <stdlib.h> typedef uint16_t crc_t; /* pycrc will use an appropriate size here */
crc_t crc_init( | void) ; |
crc_t crc_update( | crc_t crc, |
const unsigned char *data, | |
size_t data_len) ; |
crc_t crc_finalize( | crc_t crc) ; |
The code snippet below shows how to use the generated functions.
#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; }
When the model is not fully defined then the missing parameters are stored in a structure of type crc_cfg_t. If a CRC function requires a value from the crc_cfg_t 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.
If the Width
was not specified when the code was generated, then
the crc_cfg_t structure will contain three more fields:
msb_mask
, crc_mask
and crc_shift
.
They are defined for performance reasons and must be initialised to the value given next to the
field definition.
For example, a completely undefined CRC implementation will generate a crc_cfg_t structure as below:
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;
msb_mask
is a bitmask with the most significant bit of a
Width
bits wide data type set to 1.
crc_mask
is a bitmask with all bits of a
Width
bits wide data type set to 1.
crc_shift
is a shift counter that is used when
Width
is less than 8.
It is the number of bits to shift the CRC register to align its top bit to a byte boundary.
The file test/main.c
in the source package of pycrc
contains a fully featured example of how to use the generated source code.
A shorter, more compact main()
function can be generated with the
--generate c-main
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.
python pycrc.py --model crc-32 --check-string 123456789
The table index width of 4 bits ensures a moderate memory usage.
To be precise, the size of the resulting table will be 16 * sizeof(crc_t)
.
python pycrc.py --model crc-16 --algorithm table-driven --table-idx-width 4 --generate h -o crc.h
python pycrc.py --model crc-16 --algorithm table-driven --table-idx-width 4 --generate c -o crc.c
A variant of the c
target is c-main
:
this target will generate a simple main()
function in addition to
the CRC functions:
python pycrc.py --model crc-16 --algorithm table-driven --table-idx-width 4 --generate c-main -o crc.c
python pycrc.py --model kermit --generate table -o crc-table.txt
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.
This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International.