dbcc

DBC converter/compiler

This program turns a DBC file into a number of different formats.

Introduction

dbcc is a program for converting a DBC file primarily into into C code that can serialize and deserialize CAN messages into structures that represent those messages and signals. It is also possible to print out the information contained in a structure.

Building, Licenses and Dependencies

See the license file for details of the license for this program, it is released under the MIT license. Dependencies, if linked against, may have their own license and their own set of restrictions if built against.

The sources file mpc.c and mpc.h originate from a parser combinator written in C called MPC and are licensed under the 3 Clause BSD license.

To build, you only need a C (C99) compiler and Make (probably GNU make, I make no effort to support other Make implementations). The dbcc program itself it written in what should be portable C with the only external dependency being your platforms C library.

You should be able to type:

make

To build, an executable called 'dbcc' is produced. To test run the tests, xmllint is required.

C Coding Standards

• When in doubt, format with indent with the "-linux" option.
• Use tabs, not spaces for formatting
• Use assertions where possible (not for error checking, for checking pre/post conditions and invariants).
• The tool should run on Windows and Linux with no modification. The project is written in pure C.
• No external dependencies should brought into the project.

Notes on generated code

• If you want a specific format in the generated code, integrate indent into you toolchain instead of trying to change the code generator.
• The output of the generated code is not stable, it may change from commit to commit, download and maintain a specific version if you want stability.

How to use the generated code

The code generator can make code to unpack a message (turn some bytes into a data structure), decode a message (apply a scaling/offset minimum and maximum values to the values in a data structure), and the inverse can be done (pack/encode).

You can look at the code generated from the DBC files within the project to get an understanding of how it should work.

If you want to process a CAN message that you have received you will need to call the 'unpack_message'. The code generate is agnostic to the CPUs byte order, it takes a 'uint64_t' value containing a single CAN packet (along with the CAN ID and the DLC for the that packet) and unpacks that into a structure it generates. The first byte of the CAN packet should be put in the least significant byte of the 'uint64_t'.

You can use the following functions to convert to/from a CAN message:

static uint64_t u64_from_can_msg(const uint8_t m[8]) {
	return ((uint64_t)m[7] << 56) | ((uint64_t)m[6] << 48) | ((uint64_t)m[5] << 40) | ((uint64_t)m[4] << 32) 
		| ((uint64_t)m[3] << 24) | ((uint64_t)m[2] << 16) | ((uint64_t)m[1] << 8) | ((uint64_t)m[0] << 0);
}

static void u64_to_can_msg(const uint64_t u, uint8_t m[8]) {
	m[7] = u >> 56;
	m[6] = u >> 48;
	m[5] = u >> 40;
	m[4] = u >> 32;
	m[3] = u >> 24;
	m[2] = u >> 16;
	m[1] = u >>  8;
	m[0] = u >>  0;
}

The code generator will make a structure based on the file name of the DBC file, so for the example DBC file 'ex1.dbc' a data structure called 'can_obj_ex1_h_t' is made. This structure contains all of the CAN message structures, which in turn contain all of the signals. Having all of the messages/signals in one structure has advantages and disadvantages, one of the things it makes easier is defining the data structures needed.

/* reminder of the 'unpack_message' prototype */
int unpack_message(can_obj_ex1_h_t *o, const unsigned long id, uint64_t data, uint8_t dlc);

static can_obj_ex1_h_t ex1;

uint8_t can_message_raw[8];
unsigned long id = 0;
uint8_t dlc = 0;
your_function_to_receive_a_can_message(can_message_raw, &id, &dlc);
if (unpack_message(&ex1, id, can_message_u64, dlc) < 0) {
	// Error Condition; something went wrong
	return -1;
}

'unpack_message' calls the correct unpack function for that ID, as an example for ID '0x020':

case 0x020: return unpack_can_0x020_MagicCanNode1RBootloaderAddress(&o->can_0x020_MagicCanNode1RBootloaderAddress, data, dlc);

The unpack function populates the message object in the 'can_obj_ex1_h_t' structure for that ID. The individual signals can then be decoded with the appropriate functions for that signal. For example:

uint16_t b = 0;
if (decode_can_0x020_MagicNode1R_BLAddy(o, &b)) {
	/* error */
}

To transmit a message, each signal has to be encoded, then the pack function will return a packed message.

Some other notes:

• Asserts can be disabled with a command line option
• An option to force the encode/decode function to only use the double width floating point type has been added, so different function types do not have to be dealt with by the programmer.

DBC file specification

For a specification, as I understand it, of the DBC file format, see dbc.md. This is a work in progress.

DBC VIM syntax file

There is a Vim syntax file for DBC files in the project, called dbc.vim

XML Generation

As well as C, XML can be generated, the project contains an XSD and XSLT file for the generated XML.

BSM (beSTORM Module) Generation

An XML based file that can be imported into Beyond Security's beSTORM and used to test CAN BUS infrastructure.

• Note: May be replaced with an XSLT in the future, deriving from the XML output. The BSM output is itself and XML file.

CSV Generation

A flat CSV file can be generated, which is easier to import into Excel.

JSON Generation

A JSON file can be generated, which is what all the cool kids use nowadays.

Operation

Consult the manual page for more information about the precise operation of the program.

Bugs / To Do

• The floating point conversion routines assume your platform is using IEEE-754 floats. If it does not, then tough.
• A lot of the DBC file format is not dealt with:
  • Special values
  • Timeouts
  • Error frames
  • ...
• The generated code is not MISRA C compliant.
• Integers that cannot be represented in a double width floating point number should be packed/unpacked correctly, however the encode/decode and printing functions will not as they use doubles for calculations (pack/unpack do not). This affects numbers larger than 2^53.
• FYI AUTOSAR sucks.
• There are two pieces of information that are useful to any CAN stack for received messages; the time stamp of the received message, and the status (error CRC/timeout, message okay, or message never set). This information could be included in the generated C code.
• The code generator makes code for packing/encoding and unpacking/decoding, this could be done in one step to simplify the code and data structures, it means decoded/encoded values do not need to recalculated.

It would be possible to generate nice (ASCII ART) images that show how a message is structured, which helps in understanding the message in question, and is useful for documentation purposes, for example, something like:

Message Name: Example-1
Message ID: 0x10, 16
DLC: 1 (8-bits)
+-----+-----.-----.-----.-----+-----.-----+-----+
|     |                       |           |     |
|     |                       |           |     |
+-----+-----.-----.-----.-----+-----.-----+-----+
   0     1     2     3     4     5     6     7
Bit     0: Signal-Name-1, 1 bit signal, scalar 1.0, offset 0
Bits  1-2: Signal-Name-2, 4 bit signal, signed, Motorola, ...
... etcetera ...

Or something similar. This would be another output module.