Closes #25

README.md

@@ -647,11 +647,52 @@ 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,
+attribute relation forms (`BA_REL_`, `BA_DEF_REL_`), 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.
 
+### Attributes and frame metadata
+
+`BA_DEF_`, `BA_DEF_DEF_`, and `BA_` support database, `BO_`, `SG_`,
+`BU_`, and `EV_` scopes with `INT`, `HEX`, `FLOAT`, `STRING`, and `ENUM`
+definitions. Integer/HEX values use decimal signed 64-bit numbers; floating
+values are finite and locale independent. ENUM assignments use zero-based indices;
+defaults accept an index or a quoted label. Strings support escaped quotes and
+backslashes. Each statement occupies one physical line and ends with `;`.
+
+```dbc
+BA_DEF_ BO_ "VFrameFormat" ENUM "StandardCAN","ExtendedCAN","StandardCAN_FD";
+BA_DEF_DEF_ "VFrameFormat" "StandardCAN";
+BA_DEF_ BO_ "GenMsgCycleTime" INT 0 10000;
+BA_DEF_DEF_ "GenMsgCycleTime" 100;
+BA_ "GenMsgCycleTime" BO_ 256 20;
+```
+
+Definitions, defaults, assignments, and frame/signal declarations may appear in
+any order. Defaults apply to every object of the defined scope; explicit values
+override them, with the last repeated assignment winning. Duplicate definitions
+or defaults, undefined names, wrong types/scopes, out-of-range values, and missing
+frame/signal targets produce `Error / InvalidAttribute` at the source line.
+
+`DbcDatabase::attributeDefinitions` retains types, ranges, enum labels, and defaults;
+`attributeAssignments` retains explicit values and targets. Effective values are
+available in `attributes` on the database, frames and signals. Node/environment
+assignments are retained by name without checking declarations; their defaults
+remain on the definitions. Frame/signal attributes are copied into runtime
+snapshots and tree payloads. Other valid attributes remain available as metadata
+without changing decoding.
+
+`FrameInfo` and `DecodeFrame` expose optional `frameFormat` (the enum label) and
+`cycleTimeMs` (`GenMsgCycleTime`, unsigned 32-bit milliseconds). Missing values
+remain unset; zero cycle time means unspecified. `StandardCAN` and `ExtendedCAN`
+are recognized. Other format labels, including CAN FD and `J1939PG`, are preserved
+with `Warning / UnsupportedAttribute` when effective on a frame. A supported
+format conflicting with the encoded `BO_` ID also warns. Warnings identify the
+assignment or default line. Attributes do not change CAN ID normalization or
+automatically enable CAN FD or J1939 decoding; J1939 still requires parser opt-in.
+Use `ParseFileWithDiagnostics()` to review these 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
@@ -1207,8 +1248,7 @@ 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
+- attribute relation forms (`BA_REL_`, `BA_DEF_REL_`)
 - `SIG_VALTYPE_` floating-point signal types
 - signal types/references (`SGTYPE_`, `SIG_TYPE_REF_`) and environment variables
 - advanced comment handling
@@ -1264,7 +1304,7 @@ Recommended additions:
 Recommended additions:
 
 - basic multiplexing (implemented)
-- attribute parsing
+- attribute parsing (implemented for definitions, defaults and values)
 - richer J1939 support
 - CSV export of decoded traces
 - optimized filtering and signal selection