libmultiprocess is a library and code generator that allows calling C++ class interfaces across different processes. For an interface to be available from other processes, it needs two definitions:
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An API definition declaring how the interface is called. Included examples: calculator.h, printer.h, init.h. Bitcoin examples: node.h, wallet.h, echo.h, init.h.
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A data definition declaring how interface calls get sent across the wire. Included examples: calculator.capnp, printer.capnp, init.capnp. Bitcoin examples: node.capnp, wallet.capnp, echo.capnp, init.capnp.
The *.capnp data definition files are consumed by the libmultiprocess code generator and each X.capnp file generates X.capnp.c++, X.capnp.h, X.capnp.proxy-client.c++, X.capnp.proxy-server.c++, X.capnp.proxy-types.c++, X.capnp.proxy-types.h, and X.capnp.proxy.h output files. The generated files include mp::ProxyClient<Interface> and mp::ProxyServer<Interface> class specializations for all the interfaces in the .capnp files. These allow methods on C++ objects in one process to be called from other processes over IPC sockets.
The ProxyServer objects help translate IPC requests from a socket to method calls on a local object. The ProxyServer objects are just used internally by the mp::ServeStream(loop, socket, wrapped_object) and mp::ListenConnections(loop, socket, wrapped_object) functions, and aren't exposed externally. The ProxyClient classes are exposed, and returned from the mp::ConnectStream(loop, socket) function and meant to be used directly. The classes implement methods described in .capnp definitions, and whenever any method is called, a request with the method arguments is sent over the associated IPC connection, and the corresponding wrapped_object method on the other end of the connection is called, with the ProxyClient method blocking until it returns and forwarding back any return value to the ProxyClient method caller.
A simple interface description can be found at test/mp/test/foo.capnp, implementation in test/mp/test/foo.h, and usage in test/mp/test/test.cpp.
A more complete example can be found in example and run with:
make -C build example
build/example/mpexample