Waltham IPC Library

Waltham is a network IPC library designed to resemble Wayland both protocol and protocol-API wise. Protocol is described in XML files. A generator translates XML into C code at build time. One designs Waltham protocols exactly the same way as Wayland extensions, you just miss the file descriptor argument type.

The protocol framework is designed to be completely asynchronous and object-oriented, just like Wayland. To make synchronous calls one has to write their protocol definition specifically to do that, there is no shortcut offered.

The goal is to have all protocol definitions to be external, so that a libwaltham compiled once can be used by any project with any protocol definition. Waltham itself would carry just the control protocol definition and public interfaces for wthp_callback and wthp_registry, which each user project would implement on their own. User projects will then run the generator and build in the protocol-specific bits of code and API. The implementation is not this far yet, see TODO.

Waltham does not (yet) support any kind of encryption, so usage in untrusted networks is not advised. Help with securing the communications would be appreciated, starting with planning on how it should be done. In the mean time, secure tunnels are recommended (e.g. ssh).

Waltham ABI and API are not yet stable. Breakage will likely occur until Waltham 1.0.0 has been released. You are still welcome to install Waltham and try it in your projects.

Documentation

In addition to this README, the project can build API documentation with Doxygen. Build the project as described below, and you should find the documentation in your @top_builddir@/doc/html/index.html. This README is mirrored on the front page. If the documentation does not seem to get built, try passing --enable-doc to ./autogen.sh or ./configure and see what it complains about.

You can also read a manually updated snapshot of the Waltham documentation.

Usage guidelines can be found in Related pages in the Doxygen-generated documentation.

Building and testing

To build:

$ ./autogen.sh
$ make

To test, first start the server:

$ ./tests/server

Then in another terminal, run the client:

$ ./tests/client

The client runs for about five seconds excercising the protocol and exits automatically. The server can be stopped with ctrl+c.

If you want to run the example server or client under GDB or Valgrind, use e.g. the following:

$ libtool --mode=execute gdb ./tests/server
$ libtool --mode=execute valgrind -v --leak-check=full --show-reachable=yes --track-origins=yes --num-callers=30 ./tests/server

Design

API

The Waltham API revolves around two fundamental types: wth_connection and wth_object. Both have the same implementation on both server and client side, although the possible operations (functions to call) vary. Once created, both types are tied to being either server or client side instances.

wth_connection represents the TCP connection. For a client it represents the connection to a server, for a server it represents one client.

wth_object represents a protocol object. However, usually user code does not handle wth_objects directly, but works with the API generated from XML files which uses opaque pointer types. These opaque pointer types are simply cast to/from wth_object under the hood. For some uncommon operations that the generator does not create wrappers for, you need to cast to (struct wth_object *) yourself.

Threading considerations

Waltham does not carry features to help making threaded programs easier. However, it should be safe if you restrict all manipulations of a wth_connection and all wth_objects associated with it to a single thread at a time.

Message handling and object lifetimes

Incoming messages, both on the server and client side, are first stored in a raw data buffer. Messages are dispatched directly from the data buffer. This is different from Wayland, which demarshals messages from its data buffer and puts them into queues before dispatching per queue. The additional queueing in Wayland adds some complexity that Waltham avoids.

Waltham does not include any automatic object destruction when a wth_connection is destroyed. User code must track and destroy all associated wth_objects before destroying the wth_connection. This may seem awkward at first, but it gives the user code the opportunity to destroy wth_objects in any specific order it prefers and without the risk of corrupting lists. Compare this to Wayland, where one has to be able to deal with any arbitrary order of destruction between all wl_resources, and the destructor path of one wl_resource cannot remove any other destroy listeners that might be on the same destroy listener list as that would lead to list corruption.

wth_object instances are created automatically during dispatch, which means that messages not yet dispatched have not created any entries in the table that maps between object IDs on the wire and wth_object instances. Therefore objects do not need to be cleaned up until the messages creating them have been dispatched, so there is no need for destructor hooks in wth_object.

Main loop integration

Waltham does not have any kind of event loop implementation itself. For every wth_connection, you get a file descriptor you need to watch and then call the appropriate Waltham functions which are guaranteed to never block (unless the user provides message handlers that block).

This design was chosen to ease porting of Waltham to other operating systems and allow users to use any main loop implementations they wish. Even though the examples use Linuxisms like epoll and timerfd, the library does not. This also avoids imposing a dependency to any particular external event loop library. It makes libwaltham a much thinner library than libwayland-server.