DBC文件结构如下：
version
new_symbols
bit_timing
(*obsolete but required*)
nodes
value_tables
messages
message_transmitters
environment_variables
environment_variables_data
signal_types
comments
attribute_definitions
sigtype_attr_list
attribute_defaults
attribute_values
value_descriptions
category_definitions
(*obsolete*)
categories
(*obsolete*)
filter
(*obsolete*)
signal_type_refs
signal_groups
signal_extended_value_type_list