Module protocol::attributes

Documentation about the attributes available to #[derive(Protocol)].

Here is an example of #[derive(Protocol)].

#[macro_use] extern crate protocol_derive;

#[derive(Protocol)]
struct Packet {
    version_number: u8,
    magic_number: u8,
    payload: Vec<u8>,
}

Attributes that apply to items

These attributes apply to structs and enums.

#[protocol(length_prefix(<kind>(<length prefix field name>)))]

This attribute allows variable-sized fields to have their sizes specified by an arbitrary integer field in the same struct or enum.

Without this attribute, variable-sized fields default to having 32-bit unsigned integer length prefixes prefixed immediately before the field itself.

Length prefix kinds

bytes

#[macro_use] extern crate protocol_derive;

#[derive(Protocol)]
pub struct Foo {
    /// This field specifes the length of the last field `reason`.
    ///
    /// When values of this type are read, the size of `reason` is
    /// assumed to be `reason_length` bytes.
    pub reason_length: u16,
    pub other_stuff_inbetween: [u16; 16],
    pub thingy: bool,
    /// This field
    #[protocol(length_prefix(bytes(reason_length)))]
    pub reason: String,
}

This attribute can only be used with named fields. This means structs like struct Hello(u32) cannot be supported. This is because the length prefix field must be specified by a name, and therefore only items with named fields can ever have length prefixes.