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 integration targets the installed Qt 6.8.2, which is the only supported Qt version. Qt Core and Qt Test are used by the regression tests; model tests also link QtGui for graphical metatypes. The optional adapter requires only Qt Core. Live CAN acquisition, trace-file readers, are left to the consuming application. An optional Qt Core model adapter is provided in qt/.

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
   ↓
DbcTreeModel → Qt views

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)
qt/         Optional read-only Qt Core model adapter
tests/      Parser, decoder, tree, and optional model tests
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_diagnostic.h, 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 supports browsing through the optional DbcTreeModel adapter.

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

Stable identifier for runtime frame matching and application state.

Fields:

• canId
• isExtended

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

---

DecodeDatabase

Immutable runtime snapshot with a private frame index. Build it with DbcDecodeBuilder::Build(parsed) or construct it from std::vector<DecodeFrame>. Construction rejects duplicate frame keys and duplicate signal names within a frame with std::invalid_argument, including manually supplied metadata.

• GetFrames() returns a borrowed const std::vector<DecodeFrame>&.
• FindFrame(FrameKey) returns std::shared_ptr<const DecodeFrame>.
• FindSignal(SignalKey) returns std::shared_ptr<const DecodeSignal>.
• Missing definitions return an empty handle.
• Copying a database shares its snapshot; replacing it installs another snapshot.

Frame lookup uses a hash index; signal lookup scans the matched frame's signals. Decoding shares the snapshot without copying definitions or allocating ownership objects per signal.

---

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
• key
• rawValue
• physicalValue
• valid
• status

---

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 std::int64_t *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
• key
• 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 decoder applies factor and offset; it does not clamp physical values to the declared minimum and maximum.

Definition ownership and reload

DecodedFrameValue::definition and DecodedSignalValue::definition are owning std::shared_ptr<const ...> handles. Each handle retains the immutable runtime snapshot. Definitions remain valid until the last database copy, lookup handle, or decoded result owning that snapshot is released. An independently copied signal result retains its definition even after the parent frame result is removed. Copying or moving results preserves ownership.

Reload by assigning a newly built database. Stored results continue to describe the old DBC, including its units, scaling, comments, and VAL_ descriptions; subsequent decoding uses the new DBC. Removed definitions cannot be found in the new database, but old results safely retain their old definitions. Keeping even one signal handle retains the whole old snapshot; release stored results when that metadata is no longer needed.

GetFrames() is a borrowed view. References, iterators, and raw pointers obtained from it or from a handle's .get() do not retain ownership. Keep an owning handle or an unchanged database copy while using them. Do not retain mutable aliases into a vector moved into the database constructor. The raw string pointer from GetValueDescription() follows the same rule: keep the signal definition handle alive while using the string.

Parsed DbcDatabase/FrameInfo/SignalInfo remain ordinary mutable value containers: borrowed pointers follow standard vector invalidation rules. Builders copy their metadata, so parsed input may be changed or destroyed after building. Tree nodes own metadata copies; the Qt model owns its tree and invalidates model indexes on reset. Runtime handles do not depend on the parsed database or tree. Synchronize replacement of a shared database variable with decoding; immutable snapshots do not make concurrent assignment to the same object safe.

Stable frame and signal identifiers

DecodedFrameValue::key is a FrameKey (normalized canId, isExtended), also available through DecodeFrame::GetKey(). DecodedSignalValue::key is a SignalKey (frame key, exact case-sensitive signal name). Both are value types with equality and hashers (FrameKeyHasher, SignalKeyHasher) for application maps and saved selections. Construct the same keys from parsed metadata when needed; do not use addresses or vector positions as identifiers.

Keys stay equal across reloads and reordering when their components stay equal. Frame renaming and changes to units, bit layout, scaling, or descriptions do not change a key. Changing a CAN ID/frame type or renaming a signal produces a new key. The same name in different frames and standard/extended frames with the same numeric CAN ID remain distinct. Keys identify definitions within one logical DBC database; applications combining databases must add their own database ID. A key is not a version identifier: matching keys in different snapshots may have different metadata. Use valid/the definition handle to distinguish a matched result; default keys alone do not indicate successful decoding.

DecodeDatabase runtime = DbcDecodeBuilder().Build(parsed);
DecodedFrameValue stored = DbcDecoder().Decode(runtime, rawFrame);
if (stored.valid && !stored.signals.empty()) {
    DecodedSignalValue saved = stored.signals.front();
    SignalKey selection = saved.key;
    runtime = DbcDecodeBuilder().Build(parser.ParseFile("updated.dbc"));
    std::shared_ptr<const DecodeSignal> current = runtime.FindSignal(selection);
    // saved.definition retains old metadata; current holds new metadata or is empty.
}

API migration: replace reads of runtime.frames with runtime.GetFrames(). To edit runtime definitions, prepare a vector and construct a new database; frameIndexByKey is now private and built automatically. DbcDecoder::FindFrame and result definition fields now use owning handles instead of raw pointers; definition->name, boolean/null checks, and existing decode calls remain usable. Use .get() only for temporary raw-pointer interoperation with an owner alive.

---

DbcDecoder

Main runtime decoder class.

Responsibilities:

• find a frame definition by CAN ID
• decode unconditional signals and the active multiplexed branch
• 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_
• VAL_ signal value descriptions
• basic M / mN multiplexing

Structured diagnostics

ParseFileWithDiagnostics(path) returns DbcParseResult containing database and diagnostics. Each DbcDiagnostic has severity, code, filePath, lineNumber, and message. Paths preserve the caller's supplied spelling. Lines are one-based physical lines, including blank lines; file-level errors use zero.

const DbcParser parser;
const DbcParseResult result = parser.ParseFileWithDiagnostics("example.dbc");
for (const DbcDiagnostic &diagnostic : result.diagnostics) {
    std::cerr << diagnostic.filePath << ":" << diagnostic.lineNumber
              << ": " << diagnostic.message << '\n';
}
if (!result.HasErrors()) {
    const DecodeDatabase runtime = DbcDecodeBuilder().Build(result.database);
    // Inspect warnings before relying on decoding results.
}

Severity / code	Meaning
Error / FileOpenError	Input cannot be opened
Error / FileReadError	Input cannot be read completely
Error / InvalidStatement	Invalid frame/signal definition, duplicate, or payload violation
Error / InvalidValueDescription	Invalid VAL_ syntax, duplicate value, or unresolved reference
Error / InvalidMultiplexing	Selector, branch range, or signal overlap validation failed
Warning / UnsupportedStatement	Unregistered statement skipped; decoding or metadata may be incomplete
Error / UnsupportedStatement	Extended multiplexing (SG_MUL_VAL_) rejected

Parsing stops at the first error and returns an empty database, together with any preceding warnings and the error. Deferred VAL_ and multiplexing errors retain their original source lines. Invalid signal markers use InvalidStatement. HasErrors() checks severity; warnings alone do not make a parse fail.

Unknown statements are reported conservatively, including SIG_VALTYPE_, attributes (BA_, BA_DEF_, defaults and relation forms), signal types/references, environment variables, named VAL_TABLE_ tables, and vendor extensions. For example, ignoring SIG_VALTYPE_ may decode a floating-point signal as an integer. The parser does not implement the warned features; consumers should review warnings.

Common VERSION, NS_, BS_, and BU_ headers produce no diagnostics. Single-token symbols inside the NS_ declaration list are quiet regardless of indentation, even when they name unsupported syntax. Frame, signal, bus, and node definitions or a line with arguments end the list. Actual unsupported statements outside that list are reported. Node and bus metadata are not stored. Other comment forms (CM_, CM_ BU_, CM_ EV_) are ignored by the comment handler; comments only attach to already parsed frames/signals. Supported statements currently require one physical line.

ParseFile(path) remains compatible: it returns the database, discards warnings, and throws std::runtime_error on the first error with the existing path:line: format. Use ParseFileWithDiagnostics to see skipped syntax. Resource failures such as allocation errors may still throw.

---

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 exceptions contain the path but no line number. The structured diagnostic API uses line zero for file-level errors.

---

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.

Basic multiplexing markers M and mN are supported as described below. Overlapping signals are rejected unless their branch values differ. Duplicate frame keys and signal names within a frame are rejected.

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

---

Basic multiplexing

BO_ 256 Status: 3 ECU
 SG_ Mode M : 0|8@1+ (1,0) [0|255] "" ECU
 SG_ Temperature m1 : 8|8@1- (1,0) [-128|127] "C" ECU
 SG_ Pressure m2 : 8|8@1+ (2,0) [0|510] "kPa" ECU
 SG_ Counter : 16|8@1+ (1,0) [0|255] "" ECU

SignalInfo and DecodeSignal contain multiplexing: a MultiplexRole (Unconditional, Selector, or Branch) and an exact uint64_t branch value. Each frame may have one unsigned selector (M); a branch (mN) references that selector implicitly. The selector may appear after its branches. N must contain only decimal digits, fit uint64_t, and fit the selector's bit width. Leading zeros are accepted. A selector without branches is allowed. Signed selectors, missing selectors, multiple selectors, nested/combined markers, and extended SG_MUL_VAL_ statements are rejected with the originating path:line. The dispatcher registers the unsupported statement handler through TypeFactory.

Signal bits may overlap only between branches with different selector values. This validation uses Intel or Motorola bit traversal, including mixed byte orders. It also rejects overlapping unconditional signals. Metadata survives the runtime and tree builders; the optional Qt adapter does not add multiplexing roles yet.

The shared decoder extracts the selector before processing branches, compares its raw unsigned value, and leaves result order equal to definition order. Factor, offset, and VAL_ descriptions do not influence branch selection. Unconditional signals continue to decode. An unmapped selector value leaves all branches inactive.

Each result has SignalDecodeStatus status:

Status	Meaning
Valid	Raw and physical values are available; valid == true
Inactive	Selector is available and chooses another branch
SelectorUnavailable	Branch activation is unknown because selector bytes are missing
InsufficientPayload	Selected or unconditional signal bytes are missing
InvalidDefinition	Invalid bit length or missing/invalid selector in manually built runtime metadata

All statuses except Valid have valid == false; their numeric fields contain no decoded data and GetValueDescription() returns nullptr. Check status to distinguish inactive branches from errors. Consumers constructing runtime metadata manually must obey the same non-overlap rules as the parser. Extended and nested multiplexing are outside this stage and are explicitly rejected, rather than silently decoded as unconditional signals.

---

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
   ↓
Extract multiplexor selector first (when present)
For each unconditional or active 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
• VAL_ → DbcValueDescriptionHandler
• SG_MUL_VAL_ → DbcUnsupportedMultiplexingHandler
• VERSION, NS_, BS_, BU_ → DbcHeaderHandler (also accepts attached colons)

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<MyCustomHandler>("CUSTOM_"); // after implementing MyCustomHandler

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 produce UnsupportedStatement warnings through ParseFileWithDiagnostics. SG_ and CM_ accept spaces and tabs after their keywords.

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.

---

Signal value descriptions (VAL_)

Signal descriptions attach text to exact raw integer values:

BO_ 256 Status: 8 ECU
 SG_ State : 0|8@1+ (1,0) [0|255] "" ECU
VAL_ 256 State 0 "Off" 1 "On" 2 "Error";

Each statement must occupy one physical line, contain at least one integer/text pair, and end with a semicolon. Spaces and tabs are accepted between fields; descriptions may contain whitespace, semicolons, and empty text. Inside quoted text, \" and \\ represent a quote and a backslash; other escapes are rejected. Keys are decimal integers in INT64_MIN..UINT64_MAX, with an optional minus sign; plus signs, fractions, hexadecimal numbers, exponents, and overflow are rejected. Keys never pass through double.

VAL_ may appear before or after its frame and signal declarations. References are resolved after reading the file using the same CAN ID normalization as BO_, including bit 31 for extended identifiers. Standard and extended frames with the same normalized ID remain distinct. Missing or ambiguous references, malformed statements, and duplicate integer keys produce path:line: description errors at the originating VAL_ line. Distinct keys from multiple statements are merged; 0, 00, and -0 are the same key. Keys need not fit the referenced signal's bit width or signedness; unreachable keys simply cannot match a decode.

Mappings are available as SignalInfo::valueDescriptions and DecodeSignal::valueDescriptions, and survive both database builders, including TreeNode::GetSignal(). Consumers can look up a decoded result directly:

const DecodedSignalValue &value = decodedFrame.signals.at(0);
if (const std::string *description = value.GetValueDescription()) {
    std::cout << *description;
}

GetValueDescription() uses the exact raw integer before factor/offset scaling. It returns nullptr for an invalid decode or an unknown value; numeric decoding and physicalValue are unchanged. A mapped empty string returns a non-null pointer. The pointer belongs to the signal definition and stays valid while that definition remains alive. A decoded signal owns its immutable definition across reload.

For metadata or standalone raw values, use FindValueDescription(signal.valueDescriptions, rawValue), where rawValue is RawSignalValue (std::variant<std::int64_t, std::uint64_t>). The helper matches nonnegative signed and unsigned values identically and keeps negative values distinct from large unsigned values. Stored keys use negative int64_t and nonnegative uint64_t; when constructing mappings manually, normalize keys with NormalizeValueKey() before insertion.

Named VAL_TABLE_ tables, environment-variable mappings, multiline statements, are outside this implementation; the optional Qt adapter exposes signal mappings.

---

Current Limitations

This is still a minimal implementation.

Not supported yet:

• extended and nested multiplexing (SG_MUL_VAL_, mNM)
• named VAL_TABLE_ tables and environment-variable value descriptions
• BA_ attributes
• BA_DEF_ definitions and attribute defaults
• SIG_VALTYPE_ floating-point signal types
• signal types/references (SGTYPE_, SIG_TYPE_REF_) and environment variables
• 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 skipped with warnings in the diagnostic API
• an SG_ statement before the first frame is rejected
• duplicate frame keys (normalized CAN ID and frame type) and repeated signal names within a frame are rejected at their definition lines; manually created databases are also checked for ambiguous VAL_ references during resolution
• 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_
• VAL_ signal value descriptions
• normalized CAN ID
• isExtended
• transmitter
• receivers
• comments
• tree representation
• runtime decode database
• runtime decoder

Stage 2

Recommended additions:

• selective decoding of only chosen signals

Stage 3

Recommended additions:

• basic multiplexing (implemented)
• 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.

Qt-dependent qmake targets share qt/qt_version.pri, which rejects Qt versions other than 6.8.2. The core library remains independent of Qt libraries.

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 with Qt 6.8.2:

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.

Optional Qt model adapter

The adapter and its QAbstractItemModelTester suite use Qt 6.8.2. The adapter itself links only Qt Core.

The model test executable additionally links QtGui and uses QGuiApplication so QAbstractItemModelTester can resolve graphical metatypes such as QPixmap. It defaults to the offscreen platform when QT_QPA_PLATFORM is unset or empty; no display server is required. An explicit platform setting is preserved.

Enable its library and focused QtTest suite:

mkdir -p build-qt
cd build-qt
qmake6 ../dbc.pro CONFIG+=dbc_qt
make -j
make check

Consumers include qt/qt.pri instead of core/core.pri, and set DBC_CORE_BUILD_DIR and DBC_QT_BUILD_DIR to the corresponding library build directories when building separately. A widget application also needs QT += widgets. The adapter integration enables no_keywords, because core metadata has a signals member; use Q_SIGNALS, Q_SLOTS, and Q_EMIT in consuming Qt classes.

#include "dbc_parser.h"
#include "dbc_tree_builder.h"
#include "dbc_tree_model.h"
#include <QTreeView>

// Run after constructing QApplication; view owns the model through QObject.
QTreeView view;
DbcTreeModel *model = new DbcTreeModel(&view);
model->SetTree(DbcTreeBuilder().Build(DbcParser().ParseFile("example.dbc")));
view.setModel(model);
view.expandAll();
view.show();
// On reload, pass a newly built tree to SetTree(); to clear:
// model->SetTree(nullptr);

The root is invisible: frames are top-level rows, signals are their children. Only column zero has children. parent() always returns column zero. The model takes exclusive ownership of the tree; do not retain mutable aliases. Metadata is copied by the tree builder, so the parsed database may be destroyed. TreeNode::GetParent() is non-owning and GetRow() gives the zero-based sibling position (zero for a detached node/root). Nodes cannot be copied or moved; transferring their unique_ptr ownership preserves addresses and navigation.

Call SetTree() on the model's thread. It accepts a detached Root or nullptr, uses beginResetModel()/endResetModel(), and destroys the previous tree during reset. All ordinary QModelIndex values expire on reset and must not be reused; QPersistentModelIndex values are invalidated by Qt. Destroying the model also invalidates indexes. Invalid and foreign indexes return no data/flags/children. Valid items are enabled and selectable, with no editing support.

Column (zero-based enum)	Frame display	Signal display
0 Name	Name	Name
1 CanId	Normalized hexadecimal ID	Empty
2 PayloadLength	Bytes	Empty
3 StartBit	Empty	DBC start bit
4 BitLength	Empty	Bit count
5 ByteOrder	Empty	Intel / Motorola
6 Signed	Empty	Signed / Unsigned
7 Unit	Empty	Unit
8 Comment	Comment	Comment

Horizontal DisplayRole headers describe these columns; other header requests return invalid QVariant. ToolTipRole returns the comment on every column. Custom roles also apply on every column; inapplicable roles return invalid QVariant:

Role	Node types	QVariant content
NodeTypeRole	Frame, signal	int corresponding to NodeType
CanIdRole, PayloadLengthRole, ExtendedRole	Frame	uint, int, bool
StartBitRole, BitLengthRole	Signal	uint
LittleEndianRole, SignedRole	Signal	bool
UnitRole	Signal	QString
CommentRole	Frame, signal	QString
ValueDescriptionsRole	Signal	QVariantList of QVariantMap entries

Each value-description entry contains value (qlonglong for negative keys, qulonglong for nonnegative keys) and description (QString). This preserves the full signed/unsigned 64-bit range without converting through double. An unmapped signal returns an empty list. Data comes from SignalInfo::valueDescriptions. Unsupported roles return invalid QVariant.

Standalone tree tests without Qt

From the repository root, use only a C++17 compiler:

c++ -std=c++17 -Icore tests/tree_node_test.cpp core/tree_node.cpp \
  core/dbc_tree_builder.cpp -o /tmp/dbc_tree_node_test
/tmp/dbc_tree_node_test

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.

---

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, VAL_ syntax and reference validation, raw description lookup and preservation through both builders, statement dispatch, signed and unsigned 64-bit boundaries, physical conversion, numeric output, and invalid signal payloads.

Measuring code coverage

Coverage instrumentation is separate from the QtTest suite named CoverageTest: that suite runs extra regression checks, but coverage counters require a specially instrumented build. Use a separate build directory for each coverage tool.

GCC, gcov, and LCOV on Debian/Ubuntu

Install the build and report tools:

sudo apt install build-essential qt6-base-dev qt6-base-dev-tools qmake6 lcov
# Optional IDE:
sudo apt install qtcreator

GCC supplies gcov; the lcov package supplies lcov and genhtml. Use the gcov version matching the GCC used by the selected Qt Creator kit. These instructions use GCC and Qt 6.8.2.

From the repository root, create a fresh coverage build. --coverage must be passed to both compilation and linking; Debug avoids optimization that can make source-level results harder to interpret. -fprofile-abs-path makes source paths in the coverage data independent of the compilation working directory.

DBC_SOURCE_DIR="$(pwd)"
mkdir -p build-coverage
cd build-coverage

qmake6 "$DBC_SOURCE_DIR/dbc.pro" \
  CONFIG+=dbc_qt CONFIG+=debug CONFIG-=release \
  'QMAKE_CXXFLAGS+=--coverage -fprofile-abs-path' \
  'QMAKE_LFLAGS+=--coverage'
make -j4

# The standalone tree test is not a qmake target; instrument it separately.
mkdir -p standalone
c++ -std=c++17 -O0 -g --coverage -fprofile-abs-path \
  -I"$DBC_SOURCE_DIR/core" \
  "$DBC_SOURCE_DIR/tests/tree_node_test.cpp" \
  "$DBC_SOURCE_DIR/core/tree_node.cpp" \
  "$DBC_SOURCE_DIR/core/dbc_tree_builder.cpp" \
  -o standalone/tree_node_tests

# Capture a zero-count baseline so unexecuted compiled code remains in the report.
lcov --capture --initial --directory . --branch-coverage \
  --include "$DBC_SOURCE_DIR/core/*" --include "$DBC_SOURCE_DIR/qt/*" \
  --output-file baseline.info

# Reset counts before each new measurement; otherwise executions accumulate.
lcov --zerocounters --directory .
make check
./standalone/tree_node_tests

lcov --capture --directory . --branch-coverage \
  --include "$DBC_SOURCE_DIR/core/*" --include "$DBC_SOURCE_DIR/qt/*" \
  --output-file executed.info
lcov --add-tracefile baseline.info --add-tracefile executed.info \
  --branch-coverage --output-file coverage.info

genhtml coverage.info --branch-coverage --demangle-cpp --output-directory html
lcov --summary coverage.info --branch-coverage

Open build-coverage/html/index.html in a browser. The report includes the library's core/ and qt/ sources and instantiated header code; test sources, generated moc files, and Qt/STL headers are excluded. Click a filename to inspect execution counts, uncovered lines, and branch outcomes. Branch coverage is a separate metric: executing a line does not necessarily exercise every outcome. C++ branch data can also include compiler-generated exception paths.

For another run with unchanged sources, repeat the reset, test execution, capture, merge, and HTML generation steps. After changing sources or compiler flags, rebuild and regenerate baseline.info before measuring again. A fresh build directory avoids mixing old profiling data with a different build.

An initial measurement with GCC 14.2 and Qt 6.8.2 on 2026-10-07, including both QtTest executables and the standalone tree test, yielded 97.8% line coverage (724/740) and 70.7% branch coverage (833/1178). These are a snapshot, not fixed expected values for later changes or other toolchains.

After adding direct comment-handler and ambiguous VAL_ reference tests, the same configuration measured 98.4% line coverage (728/740) and 71.4% branch coverage (841/1178).

See GCC instrumentation options and the LCOV project documentation.

Configure the GCC coverage build in Qt Creator

1. Open dbc.pro and select a GCC kit with Qt 6.8.2.

2. In Projects > Build Settings, clone Debug as Coverage.

3. Choose a separate build directory, such as build-coverage.

4. Expand the qmake build step and add these Additional arguments:

   CONFIG+=dbc_qt CONFIG+=debug CONFIG-=release "QMAKE_CXXFLAGS+=--coverage -fprofile-abs-path" "QMAKE_LFLAGS+=--coverage"
   

5. Run qmake and build the project. Both dbc_tests and dbc_model_tests must run to include coverage from the core and optional model adapter.

6. In a terminal in the coverage build directory, compile the standalone test and run the LCOV/report commands above. Set DBC_SOURCE_DIR to the absolute repository path if using a new terminal. make check runs both QtTest executables; the standalone tree executable must be run separately.

7. View html/index.html in a browser. This workflow produces HTML reports; it does not require a licensed coverage toolchain.

Configure all build variants in Qt Creator

The local dbc.pro.user stores machine-specific kit IDs and build/run settings. It is ignored by Git (*.user) and should not be committed or copied between machines. Recreate the configurations below through the UI when setting up a new checkout. The Qt4ProjectManager identifiers inside .user are historical Qt Creator setting names; the project still requires Qt 6.8.2.

The local configuration set uses these independent build directories:

Configuration	Build directory example	Purpose
Debug	/tmp/build/dbc/Desktop-Debug	Core library and parser/decoder/coverage regression tests
Release	/tmp/build/dbc/Desktop-Release	The same targets with release settings
Profile	/tmp/build/dbc/Desktop-Profile	Existing Qt Creator profiling configuration; not code coverage
Debug Qt Models	/tmp/build/dbc/Desktop-Debug-Qt-Models	Debug targets plus optional adapter and model tests
Release Qt Models	/tmp/build/dbc/Desktop-Release-Qt-Models	Release targets plus optional adapter and model tests
Debug Qt Models Strict	/tmp/build/dbc/Desktop-Debug-Qt-Models-Strict	Model build with QT_FATAL_WARNINGS=1 for builds and runs
Coverage	/tmp/build/dbc/Desktop-Coverage	Instrumented core/model build, test execution, and LCOV HTML report
Core Only	/tmp/build/dbc/Desktop-Core-Only	The Qt-free core static library
Tree Tests	/tmp/build/dbc/Desktop-Tree-Tests	Standalone C++17 tree test without Qt
Demo	/tmp/build/dbc/Desktop-Demo	Core library and command-line demo

The temporary paths are examples; choose permanent build directories if desired. Do not share a build directory between these configurations.

To add the ordinary qmake configurations:

1. Open dbc.pro with a GCC/Qt 6.8.2 kit.

2. Go to Projects > Build Settings. Keep the existing Debug, Release, and Profile configurations. Remove or disable a Coco build step if one remains.

3. Select Debug, press Clone, and name the clone Debug Qt Models. Set its build directory from the table and add CONFIG+=dbc_qt to the qmake step's Additional arguments.

4. Clone Release as Release Qt Models with the same qmake argument and its own build directory.

5. Clone Debug Qt Models as Debug Qt Models Strict. In Build Environment, add QT_FATAL_WARNINGS=1. Run configurations should use the build environment.

6. Clone Debug Qt Models as Coverage. In qmake Additional arguments, use:

   CONFIG+=dbc_qt CONFIG+=debug CONFIG-=release "QMAKE_CXXFLAGS+=--coverage -fprofile-abs-path" "QMAKE_LFLAGS+=--coverage"
   

7. For Coverage, select Add Build Step > Custom Process Step after qmake and Make, then set:

   Field	Value
   Command	/bin/sh
   Arguments	"%{sourceDir}/tests/coverage_report.sh" "%{sourceDir}" "%{buildDir}"
   Working directory	%{buildDir}

   This script compiles the standalone tree test, resets counters, runs both QtTest executables and the tree test, and generates %{buildDir}/html/index.html. It stops if any test fails. Therefore building Coverage also executes tests. lcov and genhtml must be installed. Use this configuration with the GCC kit; gcov and the compiler used for the standalone test must match that kit.

8. Run qmake and build each new configuration once. The default root builds expose dbc_tests; model configurations also expose dbc_model_tests.

Qt Creator expands %{sourceDir} and %{buildDir} in custom steps to the source and active build directories. Use the field's variable chooser if needed.

For the three independent builds, clone Debug, assign the name and build directory from the table, and replace its qmake/Make build steps with the custom process steps below. These steps are local IDE settings and do not change dbc.pro. Give each step a descriptive name and retain the listed order.

For Core Only:

Command	Arguments	Working directory
/usr/bin/mkdir	-p "%{buildDir}"	%{sourceDir}
/usr/bin/qmake6	"%{sourceDir}/core/core.pro" CONFIG+=debug CONFIG-=release	%{buildDir}
/usr/bin/make	-j4	%{buildDir}

Keep a clean step with make clean in %{buildDir}. Core Only creates a static library, so there is no executable to run.

For Tree Tests:

1. Add a custom step with command /usr/bin/mkdir, arguments -p "%{buildDir}", and working directory %{sourceDir}.

2. Add a custom step with command /usr/bin/c++, working directory %{buildDir}, and these arguments:

   -std=c++17 -O0 -g -I"%{sourceDir}/core" "%{sourceDir}/tests/tree_node_test.cpp" "%{sourceDir}/core/tree_node.cpp" "%{sourceDir}/core/dbc_tree_builder.cpp" -o "%{buildDir}/tree_node_tests"
   

3. Remove the default Make clean step: this direct compiler build creates no Makefile. Run the resulting executable as described below.

For Demo:

Command	Arguments	Working directory
/usr/bin/mkdir	-p "%{buildDir}/core" "%{buildDir}/examples"	%{sourceDir}
/usr/bin/qmake6	"%{sourceDir}/core/core.pro" CONFIG+=debug CONFIG-=release	%{buildDir}/core
/usr/bin/make	-j4	%{buildDir}/core
/usr/bin/qmake6	"%{sourceDir}/examples/demo.pro" "DBC_CORE_BUILD_DIR=%{buildDir}/core" CONFIG+=debug CONFIG-=release	%{buildDir}/examples
/usr/bin/make	-j4	%{buildDir}/examples

Replace the default clean step with two custom steps: /usr/bin/make with arguments clean, first in %{buildDir}/examples, then in %{buildDir}/core.

To configure execution and switch variants:

1. Go to Projects > Run Settings and select the appropriate qmake target: dbc_tests for ordinary builds or dbc_model_tests for model builds.
2. To run all QtTest suites together, add a Custom Executable named All QtTest suites, with executable /usr/bin/make, arguments check, and working directory %{buildDir}. Use it with an ordinary or model qmake configuration. Coverage already runs every test in its final build step.
3. For Tree Tests, add a Custom Executable named Standalone tree test, executable %{buildDir}/tree_node_tests, working directory %{buildDir}, and no arguments.
4. For Demo, add a Custom Executable named Demo, executable %{buildDir}/examples/dbc_demo, working directory %{buildDir}/examples. Set Command line arguments to the quoted absolute path of your DBC file.
5. For Coverage on Linux, optionally add a Custom Executable named Coverage HTML report, executable /usr/bin/xdg-open, arguments "%{buildDir}/html/index.html", working directory %{buildDir}. Running it opens the generated report in the default browser.
6. Use the kit/build/run selector above the Run button to choose the build configuration and its corresponding executable. Build with Ctrl+B, then run with Ctrl+R. The Build Settings configuration list can also switch the active build.
7. If .user was edited externally, close and reopen the project so Qt Creator reads the new settings. Do not edit it while the project is open, because Qt Creator can overwrite external changes when saving its session.

See Qt Creator's build configuration guide and run configuration guide.