6.2.1 Communication style

The most important configurable is SWANK:*COMMUNICATION-STYLE*, which specifies the mechanism by which Lisp reads and processes protocol messages from Emacs. The choice of communication style has a global influence on SLIME’s operation.

The available communication styles are:

NIL

This style simply loops reading input from the communication socket and serves SLIME protocol events as they arise. The simplicity means that the Lisp cannot do any other processing while under SLIME’s control.

:FD-HANDLER

This style uses the classical Unix-style “select()-loop.” Swank registers the communication socket with an event-dispatching framework (such as SERVE-EVENT in CMUCL and SBCL) and receives a callback when data is available. In this style requests from Emacs are only detected and processed when Lisp enters the event-loop. This style is simple and predictable.

:SIGIO

This style uses signal-driven I/O with a SIGIO signal handler. Lisp receives requests from Emacs along with a signal, causing it to interrupt whatever it is doing to serve the request. This style has the advantage of responsiveness, since Emacs can perform operations in Lisp even while it is busy doing other things. It also allows Emacs to issue requests concurrently, e.g. to send one long-running request (like compilation) and then interrupt that with several short requests before it completes. The disadvantages are that it may conflict with other uses of SIGIO by Lisp code, and it may cause untold havoc by interrupting Lisp at an awkward moment.

:SPAWN

This style uses multiprocessing support in the Lisp system to execute each request in a separate thread. This style has similar properties to :SIGIO, but it does not use signals and all requests issued by Emacs can be executed in parallel.

The default request handling style is chosen according to the capabilities of your Lisp system. The general order of preference is :SPAWN, then :SIGIO, then :FD-HANDLER, with NIL as a last resort. You can check the default style by calling SWANK-BACKEND::PREFERRED-COMMUNICATION-STYLE. You can also override the default by setting SWANK:*COMMUNICATION-STYLE* in your Swank init file.