Threads, Mutexes, Asyncs and Dynamic Roots
******************************************

[FIXME: This is pasted in from Tom Lord's original guile.texi chapter plus the Cygnus programmer's manual; it should be *very* carefully reviewed and largely reorganized.]

Arbiters
========

 -- primitive: make-arbiter name
     Return an object of type arbiter and name NAME. Its state is initially unlocked.  Arbiters are a way to achieve process synchronization.

 -- primitive: try-arbiter arb
     Return #t and lock the arbiter ARB if the arbiter was unlocked. Otherwise, return #f.

 -- primitive: release-arbiter arb
     Return #t and unlock the arbiter ARB if the arbiter was locked. Otherwise, return #f.

Asyncs
======

 -- primitive: async thunk
     Create a new async for the procedure THUNK.

 -- primitive: system-async thunk
     Create a new async for the procedure THUNK.  Also add it to the system's list of active async objects.

 -- primitive: async-mark a
     Mark the async A for future execution.

 -- primitive: system-async-mark a
     Mark the async A for future execution.

 -- primitive: run-asyncs list_of_a
     Execute all thunks from the asyncs of the list LIST_OF_A.

 -- primitive: noop . args
     Do nothing.  When called without arguments, return #f, otherwise return the first argument.

 -- primitive: unmask-signals
     Unmask signals. The returned value is not specified.

 -- primitive: mask-signals
     Mask signals. The returned value is not specified.

Dynamic Roots
=============

A dynamic root is a root frame of Scheme evaluation. The top-level repl, for example, is an instance of a dynamic root.

Each dynamic root has its own chain of dynamic-wind information.  Each has its own set of continuations, jump-buffers, and pending CATCH statements which are inaccessible from the dynamic scope of any other dynamic root.

In a thread-based system, each thread has its own dynamic root.  Therefore, continuations created by one thread may not be invoked by another.

Even in a single-threaded system, it is sometimes useful to create a new dynamic root.  For example, if you want to apply a procedure, but to not allow that procedure to capture the current continuation, calling the procedure under a new dynamic root will do the job.

 -- primitive: call-with-dynamic-root thunk handler
     Evaluate (thunk) in a new dynamic context, returning its value.

     If an error occurs during evaluation, apply HANDLER to the arguments to the throw, just as throw would.  If this happens, HANDLER is called outside the scope of the new root -- it is called in the same dynamic context in which call-with-dynamic-root was evaluated.

     If THUNK captures a continuation, the continuation is rooted at the call to THUNK.  In particular, the call to call-with-dynamic-root is not captured.  Therefore, call-with-dynamic-root always returns at most one time.

     Before calling THUNK, the dynamic-wind chain is un-wound back to the root and a new chain started for THUNK.  Therefore, this call may not do what you expect:

          ;; Almost certainly a bug:
          (with-output-to-port
           some-port

           (lambda ()
             (call-with-dynamic-root
              (lambda ()
                (display 'fnord)
                (newline))
              (lambda (errcode) errcode))))

     The problem is, on what port will fnord be displayed?  You might expect that because of the with-output-to-port that it will be displayed on the port bound to some-port.  But it probably won't -- before evaluating the thunk, dynamic winds are unwound, including those created by with-output-to-port. So, the standard output port will have been re-set to its default value before display is evaluated.

     (This function was added to Guile mostly to help calls to functions in C libraries that can not tolerate non-local exits or calls that return multiple times.  If such functions call back to the interpreter, it should be under a new dynamic root.)

 -- primitive: dynamic-root
     Return an object representing the current dynamic root.

     These objects are only useful for comparison using eq?. They are currently represented as numbers, but your code should in no way depend on this.

 -- procedure: quit [exit_val]
     Throw back to the error handler of the current dynamic root.

     If integer EXIT_VAL is specified and if Guile is being used stand-alone and if quit is called from the initial dynamic-root, EXIT_VAL becomes the exit status of the Guile process and the process exits.

When Guile is run interactively, errors are caught from within the read-eval-print loop.  An error message will be printed and abort called.  A default set of signal handlers is installed, e.g., to allow user interrupt of the interpreter.

It is possible to switch to a "batch mode", in which the interpreter will terminate after an error and in which all signals cause their default actions.  Switching to batch mode causes any handlers installed from Scheme code to be removed.  An example of where this is useful is after forking a new process intended to run non-interactively.

 -- procedure: batch-mode?
     Returns a boolean indicating whether the interpreter is in batch mode.

 -- procedure: set-batch-mode?! arg
     If ARG is true, switches the interpreter to batch mode. The #f case has not been implemented.

Threads
=======

[NOTE: this chapter was written for Cygnus Guile and has not yet been updated for the Guile 1.x release.]

Here is a the reference for Guile's threads.  In this chapter I simply quote verbatim Tom Lord's description of the low-level primitives written in C (basically an interface to the POSIX threads library) and Anthony Green's description of the higher-level thread procedures written in scheme.

When using Guile threads, keep in mind that each guile thread is executed in a new dynamic root.

Low level thread primitives
---------------------------

 -- primitive: call-with-new-thread thunk error-handler
     Evaluate (thunk) in a new thread, and new dynamic context, returning a new thread object representing the thread.

     If an error occurs during evaluation, call error-handler, passing it an error code describing the condition.  [Error codes are currently meaningless integers.  In the future, real values will be specified.] If this happens, the error-handler is called outside the scope of the new root -- it is called in the same dynamic context in which with-new-thread was evaluated, but not in the caller's thread.

     All the evaluation rules for dynamic roots apply to threads.

 -- primitive: join-thread thread
     Suspend execution of the calling thread until the target THREAD terminates, unless the target THREAD has already terminated.

 -- primitive: yield
     If one or more threads are waiting to execute, calling yield forces an immediate context switch to one of them. Otherwise, yield has no effect.

 -- primitive: make-mutex
     Create a new mutex object.

 -- primitive: lock-mutex mutex
     Lock MUTEX. If the mutex is already locked, the calling thread blocks until the mutex becomes available. The function returns when the calling thread owns the lock on MUTEX.

 -- primitive: unlock-mutex mutex
     Unlocks MUTEX if the calling thread owns the lock on MUTEX. Calling unlock-mutex on a mutex not owned by the current thread results in undefined behaviour. Once a mutex has been unlocked, one thread blocked on MUTEX is awakened and grabs the mutex lock.

 -- primitive: make-condition-variable

 -- primitive: wait-condition-variable cond-var mutex

 -- primitive: signal-condition-variable cond-var

Higher level thread procedures
------------------------------

Higher level thread procedures are available by loading the (ice-9 threads) module.  These provide standardized thread creation and mutex interaction.

 -- primitive: %thread-handler tag args...

     This procedure is specified as the standard error-handler for make-thread and begin-thread.  If the number of ARGS is three or more, use display-error, otherwise display a message "uncaught throw to TAG".  All output is sent to the port specified by current-error-port.

     Before display, global var the-last-stack is set to #f and signals are unmasked with unmask-signals.

     [FIXME: Why distinguish based on number of args?!  Cue voodoo music here.]

 -- macro: make-thread proc [args...]
     Apply PROC to ARGS in a new thread formed by call-with-new-thread using %thread-handler as the error handler.

 -- macro: begin-thread first [rest...]
     Evaluate forms FIRST and REST in a new thread formed by call-with-new-thread using %thread-handler as the error handler.

 -- macro: with-mutex m [body...]
     Lock mutex M, evaluate BODY, and then unlock M. These sub-operations form the branches of a dynamic-wind.

 -- macro: monitor first [rest...]
     Evaluate forms FIRST and REST under a newly created anonymous mutex, using with-mutex.

     [FIXME: Is there any way to access the mutex?]

Fluids
======

 -- primitive: make-fluid
     Return a newly created fluid. Fluids are objects of a certain type (a smob) that can hold one SCM value per dynamic root.  That is, modifications to this value are only visible to code that executes within the same dynamic root as the modifying code.  When a new dynamic root is constructed, it inherits the values from its parent.  Because each thread executes in its own dynamic root, you can use fluids for thread local storage.

 -- primitive: fluid? obj
     Return #t iff OBJ is a fluid; otherwise, return #f.

 -- primitive: fluid-ref fluid
     Return the value associated with FLUID in the current dynamic root.  If FLUID has not been set, then return #f.

 -- primitive: fluid-set! fluid value
     Set the value associated with FLUID in the current dynamic root.

 -- primitive: with-fluids* fluids values thunk
     Set FLUIDS to VALUES temporary, and call THUNK. FLUIDS must be a list of fluids and VALUES must be the same number of their values to be applied.  Each substitution is done one after another.  THUNK must be a procedure with no argument.