Closes #3 Validate DBC frame definitions and report error locations

README.md

@@ -505,6 +505,36 @@ Parsed fields:
 
 ---
 
+### 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:
+
+```text
+example.dbc:3: Invalid CAN ID: invalid
+```
+
+This change validates frame definitions. Signal syntax and unsupported DBC
+constructs retain their existing behavior; comprehensive validation is still
+planned. File-open errors contain the path but have no line number.
+
+---
+
 ## Signal definition
 
 Example:
@@ -871,5 +901,6 @@ This gives FrameTap a much better foundation for real use, because both browsing
 ## Regression tests
 
 See [tests/README.md](tests/README.md) for compiler and qmake commands.
-Tests cover Intel and Motorola extraction, signed and unsigned 64-bit boundaries,
+Tests cover frame validation and error locations, Intel and Motorola extraction,
+signed and unsigned 64-bit boundaries,
 physical conversion, numeric output, and invalid signal payloads.