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[bpt/guile.git] / doc / ref / scheme-scheduling.texi

@page
@node Scheduling
@chapter 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.]

@menu
* Arbiters::            Synchronization primitives.
* Asyncs::              Asynchronous procedure invocation.
* Dynamic Roots::       Root frames of execution.
* Threads::             Multiple threads of execution.
* Fluids::              Dynamically scoped variables.
@end menu


@node Arbiters
@section Arbiters

@cindex arbiters

@c FIXME::martin: Review me!

Arbiters are synchronization objects.  They are created with
@code{make-arbiter}.  Two or more threads can synchronize on an arbiter
by trying to lock it using @code{try-arbiter}.  This call will succeed
if no other thread has called @code{try-arbiter} on the arbiter yet,
otherwise it will fail and return @code{#f}.  Once an arbiter is
successfully locked, it cannot be locked by another thread until the
thread holding the arbiter calls @code{release-arbiter} to unlock it.

@deffn {Scheme Procedure} make-arbiter name
@deffnx {C Function} scm_make_arbiter (name)
Return an object of type arbiter and name @var{name}. Its
state is initially unlocked.  Arbiters are a way to achieve
process synchronization.
@end deffn

@deffn {Scheme Procedure} try-arbiter arb
@deffnx {C Function} scm_try_arbiter (arb)
Return @code{#t} and lock the arbiter @var{arb} if the arbiter
was unlocked. Otherwise, return @code{#f}.
@end deffn

@deffn {Scheme Procedure} release-arbiter arb
@deffnx {C Function} scm_release_arbiter (arb)
Return @code{#t} and unlock the arbiter @var{arb} if the
arbiter was locked. Otherwise, return @code{#f}.
@end deffn


@node Asyncs
@section Asyncs

@cindex asyncs
@cindex system asyncs

@c FIXME::martin: Review me!

An async is a pair of one thunk (a parameterless procedure) and a mark.
Setting the mark on an async guarantees that the thunk will be executed
somewhen in the future (@dfn{asynchronously}).  Setting the mark more
than once is satisfied by one execution of the thunk.

Guile supports two types of asyncs: Normal asyncs and system asyncs.
They differ in that marked system asyncs are executed implicitly as soon
as possible, whereas normal asyncs have to be invoked explicitly.
System asyncs are held in an internal data structure and are maintained
by Guile.

Normal asyncs are created with @code{async}, system asyncs with
@code{system-async}.  They are marked with @code{async-mark} or
@code{system-async-mark}, respectively.

@deffn {Scheme Procedure} async thunk
@deffnx {C Function} scm_async (thunk)
Create a new async for the procedure @var{thunk}.
@end deffn

@deffn {Scheme Procedure} system-async thunk
@deffnx {C Function} scm_system_async (thunk)
Create a new async for the procedure @var{thunk}.  Also
add it to the system's list of active async objects.
@end deffn

@deffn {Scheme Procedure} async-mark a
@deffnx {C Function} scm_async_mark (a)
Mark the async @var{a} for future execution.
@end deffn

@deffn {Scheme Procedure} system-async-mark a
@deffnx {C Function} scm_system_async_mark (a)
Mark the async @var{a} for future execution.
@end deffn

As already mentioned above, system asyncs are executed automatically.
Normal asyncs have to be explicitly invoked by storing one or more of
them into a list and passing them to @code{run-asyncs}.

@deffn {Scheme Procedure} run-asyncs list_of_a
@deffnx {C Function} scm_run_asyncs (list_of_a)
Execute all thunks from the asyncs of the list @var{list_of_a}.
@end deffn

Automatic invocation of system asyncs can be temporarily disabled by
calling @code{mask-signals} and @code{unmask-signals}.  Setting the mark
while async execution is disabled will nevertheless cause the async to
run once execution is enabled again.  Please note that calls to these
procedures should always be paired, and they must not be nested, e.g. no
@code{mask-signals} is allowed if another one is still active.

@deffn {Scheme Procedure} mask-signals
@deffnx {C Function} scm_mask_signals ()
Mask signals. The returned value is not specified.
@end deffn

@deffn {Scheme Procedure} unmask-signals
@deffnx {C Function} scm_unmask_signals ()
Unmask signals. The returned value is not specified.
@end deffn

@c FIXME::martin: Find an example for usage of `noop'.  What is that
@c procedure for anyway?

@deffn {Scheme Procedure} noop . args
@deffnx {C Function} scm_noop (args)
Do nothing.  When called without arguments, return @code{#f},
otherwise return the first argument.
@end deffn


@node Dynamic Roots
@section Dynamic Roots
@cindex dynamic roots

A @dfn{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.

@deffn {Scheme Procedure} call-with-dynamic-root thunk handler
@deffnx {C Function} scm_call_with_dynamic_root (thunk, handler)
Evaluate @code{(thunk)} in a new dynamic context, returning its value.

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

If @var{thunk} captures a continuation, the continuation is rooted at
the call to @var{thunk}.  In particular, the call to
@code{call-with-dynamic-root} is not captured.  Therefore,
@code{call-with-dynamic-root} always returns at most one time.

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

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

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

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


@deffn {Scheme Procedure} dynamic-root
@deffnx {C Function} scm_dynamic_root ()
Return an object representing the current dynamic root.

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

@c begin (scm-doc-string "boot-9.scm" "quit")
@deffn {Scheme Procedure} quit [exit_val]
Throw back to the error handler of the current dynamic root.

If integer @var{exit_val} is specified and if Guile is being used
stand-alone and if quit is called from the initial dynamic-root,
@var{exit_val} becomes the exit status of the Guile process and the
process exits.
@end deffn

When Guile is run interactively, errors are caught from within the
read-eval-print loop.  An error message will be printed and @code{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.

@c begin (scm-doc-string "boot-9.scm" "batch-mode?")
@deffn {Scheme Procedure} batch-mode?
Returns a boolean indicating whether the interpreter is in batch mode.
@end deffn

@c begin (scm-doc-string "boot-9.scm" "set-batch-mode?!")
@deffn {Scheme Procedure} set-batch-mode?! arg
If @var{arg} is true, switches the interpreter to batch mode.
The @code{#f} case has not been implemented.
@end deffn

@node Threads
@section Threads
@cindex threads
@cindex Guile threads

@strong{[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.
@cindex posix threads
@cindex Lord, Tom
@cindex Green, Anthony

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

@menu
* Low level thread primitives::
* Higher level thread procedures::
@end menu


@node Low level thread primitives
@subsection Low level thread primitives

@c NJFIXME no current mechanism for making sure that these docstrings
@c are in sync.

@c begin (texi-doc-string "guile" "call-with-new-thread")
@deffn {Scheme Procedure} call-with-new-thread thunk error-handler
Evaluate @code{(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.
@end deffn

@c begin (texi-doc-string "guile" "join-thread")
@deffn {Scheme Procedure} join-thread thread
Suspend execution of the calling thread until the target @var{thread}
terminates, unless the target @var{thread} has already terminated.
@end deffn

@c begin (texi-doc-string "guile" "yield")
@deffn {Scheme Procedure} 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.
@end deffn

@c begin (texi-doc-string "guile" "make-mutex")
@deffn {Scheme Procedure} make-mutex
Create a new mutex object.
@end deffn

@c begin (texi-doc-string "guile" "lock-mutex")
@deffn {Scheme Procedure} lock-mutex mutex
Lock @var{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 @var{mutex}.
@end deffn

@c begin (texi-doc-string "guile" "unlock-mutex")
@deffn {Scheme Procedure} unlock-mutex mutex
Unlocks @var{mutex} if the calling thread owns the lock on @var{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 @var{mutex} is awakened and grabs the mutex lock.
@end deffn

@c begin (texi-doc-string "guile" "make-condition-variable")
@deffn {Scheme Procedure} make-condition-variable
@end deffn

@c begin (texi-doc-string "guile" "wait-condition-variable")
@deffn {Scheme Procedure} wait-condition-variable cond-var mutex
@end deffn

@c begin (texi-doc-string "guile" "signal-condition-variable")
@deffn {Scheme Procedure} signal-condition-variable cond-var
@end deffn


@node Higher level thread procedures
@subsection Higher level thread procedures

@c new by ttn, needs review

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

@deffn {Scheme Procedure} %thread-handler tag args@dots{}

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

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

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

@deffn macro make-thread proc [args@dots{}]
Apply @var{proc} to @var{args} in a new thread formed by
@code{call-with-new-thread} using @code{%thread-handler} as the error
handler.
@end deffn

@deffn macro begin-thread first [rest@dots{}]
Evaluate forms @var{first} and @var{rest} in a new thread formed by
@code{call-with-new-thread} using @code{%thread-handler} as the error
handler.
@end deffn

@deffn macro with-mutex m [body@dots{}]
Lock mutex @var{m}, evaluate @var{body}, and then unlock @var{m}.
These sub-operations form the branches of a @code{dynamic-wind}.
@end deffn

@deffn macro monitor first [rest@dots{}]
Evaluate forms @var{first} and @var{rest} under a newly created
anonymous mutex, using @code{with-mutex}.

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


@node Fluids
@section Fluids

@cindex fluids

@c FIXME::martin: Review me!

Fluids are objects to store values in.  They have a few properties which
make them useful in certain situations: Fluids can have one value per
dynamic root (@pxref{Dynamic Roots}), so that changes to the value in a
fluid are only visible in the same dynamic root.  Since threads are
executed in separate dynamic roots, fluids can be used for thread local
storage (@pxref{Threads}).

Fluids can be used to simulate dynamically scoped variables.  These are
used in several (especially in older) dialects of lisp, such as in Emacs
Lisp, and they work a bit like global variables in that they can be
modified by the caller of a procedure, and the called procedure will see
the changes.  With lexically scoped variables---which are normally used
in Scheme---this cannot happen.  See the description of
@code{with-fluids*} below for details.

New fluids are created with @code{make-fluid} and @code{fluid?} is used
for testing whether an object is actually a fluid.

@deffn {Scheme Procedure} make-fluid
@deffnx {C Function} scm_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.
@end deffn

@deffn {Scheme Procedure} fluid? obj
@deffnx {C Function} scm_fluid_p (obj)
Return @code{#t} iff @var{obj} is a fluid; otherwise, return
@code{#f}.
@end deffn

The values stored in a fluid can be accessed with @code{fluid-ref} and
@code{fluid-set!}.

@deffn {Scheme Procedure} fluid-ref fluid
@deffnx {C Function} scm_fluid_ref (fluid)
Return the value associated with @var{fluid} in the current
dynamic root.  If @var{fluid} has not been set, then return
@code{#f}.
@end deffn

@deffn {Scheme Procedure} fluid-set! fluid value
@deffnx {C Function} scm_fluid_set_x (fluid, value)
Set the value associated with @var{fluid} in the current dynamic root.
@end deffn

@code{with-fluids*} temporarily changes the values of one or more fluids,
so that the given procedure and each procedure called by it access the
given values.  After the procedure returns, the old values are restored.

@deffn {Scheme Procedure} with-fluids* fluids values thunk
@deffnx {C Function} scm_with_fluids (fluids, values, thunk)
Set @var{fluids} to @var{values} temporary, and call @var{thunk}.
@var{fluids} must be a list of fluids and @var{values} must be the same
number of their values to be applied.  Each substitution is done
one after another.  @var{thunk} must be a procedure with no argument.
@end deffn


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