DBC Parser and Runtime Decode Module Documentation

Overview

This module provides a minimal but extensible DBC parser and runtime decode foundation for the FrameTap project.

Its purpose is to:

• parse DBC files
• store parsed frame and signal metadata
• build a tree for future UI integration
• build a runtime-ready decode database
• decode live CAN frames
• decode CAN trace records using the same decoder

The implementation follows a simple and practical design:

• parser logic is separated from UI logic
• runtime decode structures are separated from tree structures
• Qt is not required at the parser or decoder level
• the same decode engine can be reused for live traffic and trace replay

The core library uses C++17 and has no Qt dependency. Qt Core and Qt Test are required only for the regression tests. Live CAN acquisition, trace-file readers, and a Qt model adapter are left to the consuming application.

See changelog.md for the release and development history, tests/README.md for test instructions, and generated API documentation for the API reference.

---

High-Level Architecture

The module is divided into two main paths.

1. Parse and UI path

Used for:

• browsing frames and signals
• later integration with Qt Model/View
• displaying metadata

Pipeline:

DBC file
   ↓
DbcParser
   ↓
DbcDatabase
   ↓
DbcTreeBuilder
   ↓
TreeNode hierarchy
   ↓
future Qt UI

2. Runtime decode path

Used for:

• live CAN decoding
• CAN trace decoding
• fast lookup by CAN ID

Pipeline:

DBC file
   ↓
DbcParser
   ↓
DbcDatabase
   ↓
DbcDecodeBuilder
   ↓
DecodeDatabase
   ↓
DbcDecoder
   ↓
Decoded values

This separation is intentional.

The tree is useful for UI, but it is not the main data structure for runtime decoding.

---

Why the Runtime Decode Layer Exists

A tree structure is convenient for browsing, but a runtime decoder needs something different:

• fast lookup by CAN ID
• direct access to signal decode definitions
• minimal overhead during repeated decoding
• the same logic for live frames and trace frames

Because of that, the design uses a dedicated runtime-ready structure:

• DecodeDatabase
• DecodeFrame
• DecodeSignal

This avoids forcing UI-oriented structures into a decode role they were not meant for.

---

Module Layout

core/       C++17 parser, statement handlers, metadata, tree, and decoder
examples/   Command-line demo (built separately)
tests/      QtTest parser and decoder regression suites
docs/       Generated Doxygen documentation
dbc.pro     Root qmake project: core + tests
Doxyfile    API documentation configuration

Component	Source files
Parsed metadata	core/dbc_database.h, core/frame_info.h, core/signal_info.h
Parser and dispatch	core/dbc_parser.*, core/dbc_statement_handler.*, core/typefactory.h
UI tree	core/tree_node.*, core/dbc_tree_builder.*
Runtime database	core/decode_database.h, core/dbc_decode_builder.*
Decoder	core/dbc_decoder.*
Demo	examples/main.cpp, examples/demo.pro
Build integration	core/core.pro, core/core_files.pri, core/core.pri

---

Parsed Data Structures

SignalInfo

Represents one signal as parsed from the DBC file.

Fields:

• name
• startBit
• length
• isLittleEndian
• isSigned
• factor
• offset
• minimum
• maximum
• unit
• receivers
• comment

Notes:

• receivers is a list because a signal may have more than one receiver ECU
• factor and offset define physical conversion
• this structure is close to DBC content and easy to inspect

Physical value rule:

physical = raw * factor + offset

---

FrameInfo

Represents one frame as parsed from the DBC file.

Fields:

• name
• canId
• isExtended
• pgn
• hasPgn
• dlc
• transmitter
• comment
• signals

Notes:

• signals is a list of SignalInfo
• isExtended is determined during CAN ID normalization
• pgn is derived using simplified J1939 logic when applicable

---

DbcDatabase

Top-level parsed DBC container.

Conceptually:

DbcDatabase
 └── vector<FrameInfo>

This is the central structure produced by DbcParser.

---

UI Tree Layer

TreeNode

The UI tree contains three node types:

• Root
• Frame
• Signal

Example hierarchy:

dbc
 ├── EngineData
 │    ├── EngineSpeed
 │    ├── OilTemp
 │    └── CoolantTemp
 └── VehicleData
      ├── VehicleSpeed
      └── Odometer

Frame and signal nodes store copies of the corresponding metadata; the root has no metadata payload. The payload types are:

• FrameInfo
• SignalInfo

The tree is intended for browsing and later Qt model integration.

It is not the primary runtime decode structure.

---

Runtime Decode Layer

Purpose

The decode layer exists so that decoding can be fast and independent from UI concerns.

Instead of searching a tree, the decoder uses a prepared database with direct lookup.

---

ByteOrder

Runtime byte order enum:

• Intel
• Motorola

This is better for decode code than passing around raw DBC characters.

---

ValueType

Numeric type enum:

• Unsigned
• Signed

This is clearer than combining multiple boolean flags during runtime logic.

---

DecodeSignal

Represents one runtime-ready signal definition.

Fields:

• name
• startBit
• length
• byteOrder
• valueType
• factor
• offset
• minimum
• maximum
• unit
• receivers
• comment

This structure contains all information required for extracting and converting a signal value from raw frame data.

---

DecodeFrame

Represents one runtime-ready frame definition.

Fields:

• name
• canId
• isExtended
• dlc
• pgn
• hasPgn
• transmitter
• comment
• signals

This structure is used directly by the decoder.

---

FrameKey

Fast lookup key for runtime frame matching.

Fields:

• canId
• isExtended

This matters because the same numeric identifier must not be confused between standard and extended frames.

---

DecodeDatabase

Top-level runtime decode container.

Fields:

• frames
• frameIndexByKey

Conceptually:

DecodeDatabase
 ├── vector<DecodeFrame>
 └── unordered_map<FrameKey, index>

This gives the decoder fast access to a frame definition using CAN ID and frame type.

---

Decoder Layer

RawCanFrame

Represents a raw CAN frame to decode.

Fields:

• canId
• isExtended
• data

This same structure can be used for:

• live CAN bus input
• replayed trace records
• unit tests

---

DecodedSignalValue

Represents one decoded signal result.

Fields:

• definition
• rawValue
• physicalValue
• valid

---

Raw value representation and precision

DecodedSignalValue::rawValue is a C++17 std::variant<std::int64_t, std::uint64_t>. For a valid signal, signed definitions produce the int64_t alternative and unsigned definitions produce the uint64_t alternative. Raw values remain exact across their full 64-bit ranges. Check valid before using the value; an invalid result's default value is not decoded data.

Use std::get when the numeric type is known, or std::get_if to handle both:

if (const auto *value = std::get_if<std::int64_t>(&decodedSignal.rawValue)) {
    std::cout << *value;
} else {
    std::cout << std::get<std::uint64_t>(decodedSignal.rawValue);
}

This replaces the previous int64_t field: consumers that read or print rawValue directly must handle the variant.

Physical conversion uses the active numeric type: physicalValue = static_cast<double>(raw) * factor + offset. physicalValue remains a double and is approximate for large integers. On usual IEEE 754 binary64 implementations, integers are exactly representable through 2^53; above that, some integers round to the same floating-point value. For example, unsigned UINT64_MAX remains exact in rawValue, but converts to approximately 18446744073709551616 in binary64 before scaling. Use rawValue whenever exact integer values are required.

---

DecodedFrameValue

Represents one decoded frame result.

Fields:

• definition
• signals
• valid

valid means a matching frame definition was found. Check each signal's valid separately: insufficient payload data can leave individual signals invalid even when the frame is valid. An unknown frame returns valid == false.

The frame and signal definition pointers refer into DecodeDatabase. Keep that database alive and avoid changes that invalidate its frame or signal storage while using decoded results. The decoder applies factor and offset; it does not clamp physical values to the declared minimum and maximum.

---

DbcDecoder

Main runtime decoder class.

Responsibilities:

• find a frame definition by CAN ID
• decode all signals in a frame
• extract raw values
• sign-extend signed values
• convert raw values into physical values

Main methods:

• FindFrame(...)
• Decode(...)

---

Parser Support

The current parser supports the following DBC elements:

• BO_
• SG_
• CM_ BO_
• CM_ SG_

---

Supported DBC Syntax

Frame definition

Example:

BO_ 256 EngineData: 8 EEC1

Parsed fields:

• frame CAN ID
• frame name
• DLC
• transmitter ECU

---

Frame validation and parse errors

BO_ definitions require a decimal CAN ID, a nonempty single-token frame name, a colon, a decimal payload length, and a transmitter token. Spaces or tabs may separate fields, and whitespace before the colon is accepted. Additional trailing fields are rejected.

Numeric fields must contain decimal digits only; signs, hexadecimal notation, fractional values, suffixes, and overflow are rejected. CAN IDs may contain the 29-bit identifier plus the optional DBC extended flag (bit 31). Bits 29 and 30 are rejected rather than silently discarded. For compatibility, unflagged IDs above 0x7FF are still treated as extended.

The payload length must fit uint8_t (0..255). This is a storage-range check, not CAN or CAN FD protocol validation; no new 8-byte or 64-byte limit is imposed.

Parsing stops at the first detected error. Exceptions raised while parsing a line are reported as std::runtime_error with the supplied file path and the one-based physical line number, including blank lines:

example.dbc:3: Invalid CAN ID: invalid

Frame and signal definitions are validated as described here; comprehensive DBC grammar coverage is still planned. File-open errors contain the path but have no line number.

---

Signal definition

Example:

SG_ EngineSpeed : 0|16@1+ (0.125,0) [0|8000] "rpm" ECU1,ECU2

Parsed fields:

• signal name
• start bit
• signal length
• byte order
• signedness
• factor
• offset
• minimum
• maximum
• unit
• receivers

---

Signal validation and parse errors

SG_ statements are matched completely. Signal names must be nonempty and contain no whitespace. Spaces and tabs are accepted between syntax elements; byte order and signedness remain adjacent (@0+, @0-, @1+, or @1-). A quoted unit (which may be empty) and a nonempty comma-separated receiver list are required. Unexpected trailing fields are rejected.

Start bits must be unsigned decimal integers fitting uint32_t; lengths must be within 1..64 bits. Signs, fractional values, numeric suffixes, and overflow are rejected for these fields. Every signal bit must fit within the frame's declared payload length, using Intel increasing bit positions or Motorola sawtooth traversal. This uses the stored payload length without introducing an additional CAN or CAN FD payload limit.

Factor, offset, minimum, and maximum accept signed decimal numbers, fractions, and scientific notation. The entire numeric token must be valid; hexadecimal numbers, NaN, infinity, conversion overflow, and conversion underflow are rejected. Minimum must not exceed maximum. Zero and negative factors are allowed.

Multiplexing markers (M, mN, and combinations) are currently rejected rather than interpreted as part of a signal name. Multiplexing support and duplicate frame/signal detection remain separate work.

Invalid signal definitions stop parsing with path:line: description, including when the signal exceeds the declared payload length.

---

Comments

Frame comment example:

CM_ BO_ 256 "Engine data frame";

Signal comment example:

CM_ SG_ 256 EngineSpeed "Actual engine speed";

Stored in:

• FrameInfo::comment
• SignalInfo::comment

---

CAN ID Normalization

The parser normalizes frame identifiers.

Common DBC behavior:

• extended identifiers may be stored with bit 31 set
• the actual 29-bit identifier must be extracted from that value

The parser therefore stores:

• normalized canId
• separate isExtended flag

This is important both for correct lookup and for future interoperability with live CAN APIs.

---

PGN Extraction

PGN is derived only when the frame is treated as extended.

The current logic is simplified J1939 extraction:

• pf
• ps
• dp

This is enough for a practical start but should not be treated as full J1939 validation.

---

Decode Flow

Typical runtime decode flow:

RawCanFrame
   ↓
Find frame in DecodeDatabase
   ↓
For each signal:
    extract raw bits
    apply sign extension if needed
    convert to physical value
   ↓
DecodedFrameValue

---

Intel and Motorola Extraction

The decoder currently has separate extraction paths:

• ExtractIntel(...)
• ExtractMotorola(...)

This is important because byte order is not just metadata once decoding starts.

Intel and Motorola require different bit extraction logic.

This is one of the main reasons why the runtime decode layer should be explicit and prepared in advance.

---

Example Usage

Include the public headers from the core/ include directory supplied by core/core.pri:

#include "dbc_parser.h"
#include "dbc_tree_builder.h"
#include "dbc_decode_builder.h"
#include "dbc_decoder.h"

Parse DBC

DbcParser parser;
DbcDatabase database = parser.ParseFile("example.dbc");

Build UI tree

DbcTreeBuilder treeBuilder;
std::unique_ptr<TreeNode> root = treeBuilder.Build(database);

Build runtime decode database

DbcDecodeBuilder decodeBuilder;
DecodeDatabase decodeDatabase = decodeBuilder.Build(database);

Decode a raw frame

RawCanFrame rawFrame;
rawFrame.canId = 256; // EngineData from the DBC example above
rawFrame.isExtended = false;
rawFrame.data = {0x01, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};

DbcDecoder decoder;
DecodedFrameValue decoded = decoder.Decode(decodeDatabase, rawFrame);

---

Unified Decode Strategy

A key design goal is that the same decoder should work for both:

• live CAN frames
• trace replay frames

That means this architecture supports:

live path

live CAN input
   ↓
RawCanFrame
   ↓
DbcDecoder
   ↓
decoded signal values

trace path

trace reader
   ↓
RawCanFrame
   ↓
DbcDecoder
   ↓
decoded signal values

This avoids duplicating decode logic in two separate parts of the application.

---

Intended Use in FrameTap

This module is meant to support at least the following FrameTap workflows:

• load a DBC file
• browse frames and signals
• search signals
• drag a signal into a plot later
• decode live CAN traffic
• decode recorded traces
• convert raw values into physical values
• show metadata like units, comments, transmitter, receivers, and PGN

Example combined workflow:

Load DBC
   ↓
Parse into DbcDatabase
   ↓
Build UI tree
   ↓
Build DecodeDatabase
   ↓
Use same decode engine for:
    - live frames
    - trace replay

---

Why the Tree Is Not Enough

The tree exists for browsing.

However, runtime decode should not rely on tree traversal because that would introduce unnecessary coupling and inefficiency.

A runtime decoder needs:

• fast key-based access
• minimal interpretation at decode time
• direct signal definitions already prepared

That is why DecodeDatabase is a separate layer.

---

Statement Handler Factory

DbcParser owns file reading, trimming, line numbers, and error reporting. Each ParseFile() call creates a fresh DbcParseContext and a DbcStatementDispatcher. The context contains the parsed database and current frame; handlers receive it by reference and retain no parsing state.

The dispatcher uses TypeFactory<std::string, DbcStatementHandler> to create registered handlers once per parsing session, then reuses them for all matching lines:

• BO_ → DbcFrameHandler
• SG_ → DbcSignalHandler
• CM_ → DbcCommentHandler

Grammar implementations live in core/dbc_statement_handler.cpp; their interface, context, and dispatcher live in core/dbc_statement_handler.h. core/typefactory.h provides the generic registry and object creation.

To add a construct, implement a default-constructible handler deriving from DbcStatementHandler with:

void Handle(const std::string &line, DbcParseContext &context) const override;

Register it in the dispatcher constructor:

RegisterType<DbcValueHandler>("VAL_"); // after implementing DbcValueHandler

Registration creates one handler; replacing a registration replaces that instance. Register during initialization before dispatching. A line is routed using its full first whitespace-delimited keyword, so BO_TX_BU_ cannot match BO_. Unregistered keywords remain ignored. SG_ accepts spaces and tabs after its keyword; CM_ retains its existing policy requiring a literal space.

This factory handles statements inside DBC files. A separate format-level factory can be introduced when another file format is implemented. The decoder and database builders remain independent of statement dispatch.

---

Current Limitations

This is still a minimal implementation.

Not supported yet:

• multiplexed signals
• VAL_ tables
• BA_ attributes
• BA_DEF_ definitions
• advanced comment handling
• full DBC grammar coverage
• full J1939 validation
• extensive edge-case handling for unusual DBC formatting

Additional behavior to account for:

• unregistered DBC keywords are ignored
• an SG_ statement before the first frame is ignored
• duplicate frame and signal definitions are not rejected
• payload lengths are checked against storage bounds (0..255), rather than CAN or CAN FD protocol limits
• comment handling is limited to the supported forms and already parsed targets

Motorola extraction uses DBC sawtooth numbering: a start bit of 7 traverses 7..0, 15..8, 23..16. Regression tests cover aligned and unaligned signals, byte boundaries, and 64-bit values.

---

Recommended Next Steps

A practical development order would be:

Stage 1 - already implemented

• BO_
• SG_
• CM_
• normalized CAN ID
• isExtended
• transmitter
• receivers
• comments
• tree representation
• runtime decode database
• runtime decoder

Stage 2

Recommended additions:

• parent pointer in TreeNode
• Qt model adapter
• VAL_ support for enum-style signals
• better display strings for UI
• selective decoding of only chosen signals

Stage 3

Recommended additions:

• multiplexing support
• attribute parsing
• richer J1939 support
• CSV export of decoded traces
• optimized filtering and signal selection

Stage 4

Advanced functionality:

• live plot integration
• signal subscriptions
• per-signal trace decode pipelines
• decoder-assisted export formats

---

Build Integration

The repository supplies qmake projects. A C++17 compiler is required; the root build also requires Qt Core and Qt Test development packages. Other build systems can compile the sources listed in core/core_files.pri; no CMake configuration is currently provided.

Open dbc.pro in Qt Creator to see two subprojects: core and tests. core/core.pro builds the C++17 static library dbc_core without a Qt dependency. tests/tests.pro builds the QtTest executable dbc_tests and links that library. The root project declares tests.depends = core for parallel builds.

Build out of source (Qt 6; use qmake for Qt 5):

mkdir build
cd build
qmake6 ../dbc.pro
make -j
make check

core/core_files.pri lists the library sources and headers. Consumers include core/core.pri for include paths and library linkage; the test source lists live in tests/parser_tests.pri and tests/decoder_tests.pri. When building a consumer separately, pass DBC_CORE_BUILD_DIR=/absolute/path/to/build/core to qmake.

Build core without Qt libraries

From the repository root:

mkdir -p build/core
cd build/core
qmake6 ../../core/core.pro
make -j

qmake is still the build tool; this target does not link Qt libraries.

Build and run the demo

After building core, run these commands from the repository root:

mkdir -p build/examples
cd build/examples
qmake6 ../../examples/demo.pro DBC_CORE_BUILD_DIR=../core
make -j
./dbc_demo /absolute/path/to/example.dbc

The demo prints the parsed tree and decodes eight zero bytes using the first frame definition. Replace that sample payload in examples/main.cpp to feed live CAN data or trace records.

Generate API documentation

With Doxygen installed, run from the repository root:

doxygen Doxyfile

Open docs/html/index.html to view the generated reference.

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Regression tests

See tests/README.md for QtTest build and execution commands. Tests cover frame validation and error locations, Intel and Motorola extraction, signal syntax and payload bounds, statement dispatch, signed and unsigned 64-bit boundaries, physical conversion, numeric output, and invalid signal payloads.