liberty/libertyxdr.adoc

libertyxdr(7)
=============
:doctype: manpage

Name
----
LibertyXDR - an XDR-derived IDL and data serialization format

Description
-----------
*LibertyXDR* is an interface description language, as well as a data
serialization format.  It is largely derived from XDR, though notably
simplified.

Conventions
~~~~~~~~~~~
User-defined types should be named in *CamelCase*, field names in *snake_case*,
and constants in *SCREAMING_SNAKE_CASE*.  Code generators will convert these to
whatever is appropriate in their target language.

Primitive data types
~~~~~~~~~~~~~~~~~~~~
Like in XDR, all data is serialized in the network byte order, i.e., big-endian.

 * *void*: 0 bytes
+
This is a dummy type that cannot be assigned a field name.

 * *bool*: 1 byte 
+
This is a boolean value: 0 means _false_, any other value means _true_.

 * *u8*, *u16*, *u32*, *u64*: 1, 2, 4, and 8 bytes respectively
+
These are unsigned integers.

 * *i8*, *i16*, *i32*, *i64*: 1, 2, 4, and 8 bytes respectively
+
These are signed integers in two's complement.

 * *string*: implicitly prefixed by its length as a *u32*,
   then immediately followed by its contents, with no trailing NUL byte
+
This is a valid UTF-8 string without a byte order mark.  Note that strings are
always unbounded, unlike in XDR.

Constants
~~~~~~~~~
At the top level of a document, outside other definitions, you can define
typeless integer constants:

 const VERSION = 1;

The value can be either a name of another previously defined constant,
or an immediate decimal value, which may not contain leading zeros.

Enumerations
~~~~~~~~~~~~
An *enum* is an *i8* with uniquely named values, in their own namespace.

Values can be either specified explicitly, in the same way as with a constant,
or they can be left implicit, in which case names assume a value that is one
larger than their predecessor.  Zero is reserved for internal use, thus
enumerations implicitly begin with a value of one.  For example, these form
a sequence from one to three:

 enum Vehicle { CAR, LORRY = 2, PLANE, };

Structures
~~~~~~~~~~
A *struct* is a sequence of fields, specified by their type, and their chosen
name.  You can add a *<>* suffix to change a field to an array, in which case
it is implicitly preceded by a *u32* specifying its length in terms of its
elements.

Unlike in XDR, there is no padding between subsequent fields, and type
definitions can be arbitrarily syntactically nested, as in C.

 struct StockReport {
   u8 version;       // Version of this report.
   struct Item {
     Vehicle kind;   // The vehicle in question.
     i32 count;      // How many vehicle of that kind there are.
   } items<>;        // Reported items.
 };

Unions
~~~~~~
A *union* is a kind of structure whose fields depend on the value of its first
and always-present field, which must be a tag *enum*:

 union VehicleDetails switch (Vehicle kind) {
 case CAR:   void;
 case LORRY: i8 axles;
 case PLANE: i8 engines;
 };

All possible enumeration values must be named, and there is no *case*
fall-through.

Framing
-------
Unless this role is already filled by, e.g., WebSocket, _LibertyXDR_ structures
should be prefixed by their byte length in the *u32* format, once serialized.

See also
--------
_XDR: External Data Representation Standard_, RFC 4506