MLton 20070826 MLtonSignal
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signature MLTON_SIGNAL =
   sig
      type t
      type signal = t

      structure Handler:
         sig
            type t

            val default: t
            val handler: (Thread.Runnable.t -> Thread.Runnable.t) -> t
            val ignore: t
            val isDefault: t -> bool
            val isIgnore: t -> bool
            val simple: (unit -> unit) -> t
         end

      structure Mask:
         sig
            type t
               
            val all: t
            val allBut: signal list -> t
            val block: t -> unit
            val getBlocked: unit -> t
            val isMember: t * signal -> bool
            val none: t
            val setBlocked: t -> unit
            val some: signal list -> t
            val unblock: t -> unit
         end

      val getHandler: t -> Handler.t
      val handled: unit -> Mask.t
      val prof: t
      val restart: bool ref
      val setHandler: t * Handler.t -> unit
      val suspend: Mask.t -> unit
      val vtalrm: t
   end

Signals handlers are functions from (runnable) threads to (runnable) threads. When a signal arrives, the corresponding signal handler is invoked, its argument being the thread that was interrupted by the signal. The signal handler runs asynchronously, in its own thread. The signal handler returns the thread that it would like to resume execution (this is often the thread that it was passed). It is an error for a signal handler to raise an exception that is not handled within the signal handler itself.

A signal handler is never invoked while the running thread is in a critical section (see MLtonThread). Invoking a signal handler implicitly enters a critical section and the normal return of a signal handler implicitly exits the critical section; hence, a signal handler is never interrupted by another signal handler.

Interruptible System Calls

Signal handling interacts in a non-trivial way with those functions in the Basis Library that correspond directly to interruptible system calls (a subset of those functions that may raise OS.SysError). The desire is that these functions should have predictable semantics. The principal concerns are:

  1. System calls that are interrupted by signals should, by default, be restarted; the alternative is to raise

     OS.SysError (Posix.Error.errorMsg Posix.Error.intr, 
                  SOME Posix.Error.intr)
    This behavior is determined dynamically by the value of Signal.restart.

  2. Signal handlers should always get a chance to run (when outside a critical region). If a system call is interrupted by a signal, then the signal handler will run before the call is restarted or OS.SysError is raised; that is, before the Signal.restart check.

  3. A system call that must be restarted while in a critical section will be restarted with the handled signals blocked (and the previously blocked signals remembered). This encourages the system call to complete, allowing the program to make progress towards leaving the critical section where the signal can be handled. If the system call completes, the set of blocked signals are restored to those previously blocked.


Last edited on 2007-08-23 03:45:18 by MatthewFluet.