Method: CTypes::Struct::Builder#attribute

Defined in:
lib/ctypes/struct/builder.rb

#attribute(name, type = nil) ⇒ Object

declare an attribute in the structure This function supports the use of CTypes::Struct and Union types for declaring unnamed fields (ISO C11). See example below for more details.

Examples:

attribute(:name, string)

add an attribute with a struct type

include CTypes::Helpers
attribute(:header, struct(id: uint32, len: uint32))

add an unnamed field (ISO C11)

include CTypes::Helpers

# declare the type to be used in the unnamed field
header = struct(id: uint32, len: uint32)

# create our struct type with an unnamed field
t = struct do
  # add the unnamed field, in this case the header type
  attribute(header)

  # add any other field needed in the struct
  attribute(:body, string)
  size { |struct| struct[:len] }
end

# now unpack an instance of the struct type
packet = t.unpack("\x01\0\0\0\x13\0\0\0hello worldXXX")

# access the unnamed struct fields directly by the inner names
p.id    # => 1
p.len   # => 19

# access the body
p.body  # => "hello world"

Parameters:

  • name —

    name of the attribute, optional for unnamed fields

  • type (CTypes::Type) (defaults to: nil) —

    type of the field



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# File 'lib/ctypes/struct/builder.rb', line 141

def attribute(name, type = nil)
  # handle a named field
  if type
    name = name.to_sym
    if @default.has_key?(name)
      raise Error, "duplicate field name: %p" % name
    end

    @fields << [name, type]
    @schema << Dry::Types::Schema::Key.new(type.dry_type, name)
    @default[name] = type.default_value

  # handle the unnamed field by adding the child fields to our type
  else
    type = name
    dry_keys = type.dry_type.keys or
      raise Error, "unsupported type for unnamed field: %p" % [type]
    names = dry_keys.map(&:name)

    if (duplicate = names.any? { |n| @default.has_key?(n) })
      raise Error, "duplicate field name %p in unnamed field: %p" %
        [duplicate, type]
    end

    @fields << [names, type]
    @schema += dry_keys
    @default.merge!(type.default_value)
  end

  # adjust the byte count for this type
  if @bytes && type.fixed_size?
    @bytes += type.size
  else
    @bytes = nil
  end

  self
end