guile/doc/ref/api-scheduling.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@page
@node Scheduling
@section 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::                      Thread-local variables.
* Futures::                     Delayed execution in new threads.
* Parallel Forms::              Parallel execution of forms.
* Mutexes::                     Synchronization primitives.
@end menu


@node Arbiters
@subsection Arbiters
@cindex arbiters

Arbiters are synchronization objects, they can be used by threads to
control access to a shared resource.  An arbiter can be locked to
indicate a resource is in use, and unlocked when done.

An arbiter is like a light-weight mutex (@pxref{Mutexes}).  It uses
less memory and may be faster, but there's no way for a thread to
block waiting on an arbiter, it can only test and get the status
returned.

@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)
@deffnx {C Function} scm_try_arbiter (arb)
If @var{arb} is unlocked, then lock it and return @code{#t}.
If @var{arb} is already locked, then do nothing and return
@code{#f}.
@end deffn

@deffn {Scheme Procedure} release-arbiter arb
@deffnx {C Function} scm_release_arbiter (arb)
If @var{arb} is locked, then unlock it and return @code{#t}.  If
@var{arb} is already unlocked, then do nothing and return @code{#f}.

Typical usage is for the thread which locked an arbiter to later
release it, but that's not required, any thread can release it.
@end deffn


@node Asyncs
@subsection Asyncs

@cindex asyncs
@cindex user asyncs
@cindex system asyncs

Asyncs are a means of deferring the excution of Scheme code until it is
safe to do so.

Guile provides two kinds of asyncs that share the basic concept but are
otherwise quite different: system asyncs and user asyncs.  System asyncs
are integrated into the core of Guile and are executed automatically
when the system is in a state to allow the execution of Scheme code.
For example, it is not possible to execute Scheme code in a POSIX signal
handler, but such a signal handler can queue a system async to be
executed in the near future, when it is safe to do so.

System asyncs can also be queued for threads other than the current one.
This way, you can cause threads to asynchronously execute arbitrary
code.

User asyncs offer a convenient means of queueing procedures for future
execution and triggering this execution.  They will not be executed
automatically.

@menu
* System asyncs::               
* User asyncs::                 
@end menu

@node System asyncs
@subsubsection System asyncs

To cause the future asynchronous execution of a procedure in a given
thread, use @code{system-async-mark}.

Automatic invocation of system asyncs can be temporarily disabled by
calling @code{call-with-blocked-asyncs}.  This function works by
temporarily increasing the @emph{async blocking level} of the current
thread while a given procedure is running.  The blocking level starts
out at zero, and whenever a safe point is reached, a blocking level
greater than zero will prevent the execution of queued asyncs.

Analogously, the procedure @code{call-with-unblocked-asyncs} will
temporarily decrease the blocking level of the current thread.  You
can use it when you want to disable asyncs by default and only allow
them temporarily.

In addition to the C versions of @code{call-with-blocked-asyncs} and
@code{call-with-unblocked-asyncs}, C code can use
@code{scm_with_blocked_asyncs} and @code{scm_with_unblocked_asyncs}
inside a @dfn{frame} (@pxref{Frames}) to block or unblock system asyncs
temporarily.

@deffn {Scheme Procedure} system-async-mark proc [thread]
@deffnx {C Function} scm_system_async_mark (proc)
@deffnx {C Function} scm_system_async_mark_for_thread (proc, thread)
Mark @var{proc} (a procedure with zero arguments) for future execution
in @var{thread}.  When @var{proc} has already been marked for
@var{thread} but has not been executed yet, this call has no effect.
When @var{thread} is omitted, the thread that called
@code{system-async-mark} is used.

This procedure is not safe to be called from signal handlers.  Use
@code{scm_sigaction} or @code{scm_sigaction_for_thread} to install
signal handlers.
@end deffn

@c  FIXME: The use of @deffnx for scm_c_call_with_blocked_asyncs and
@c  scm_c_call_with_unblocked_asyncs puts "void" into the function
@c  index.  Would prefer to use @deftypefnx if makeinfo allowed that,
@c  or a @deftypefn with an empty return type argument if it didn't
@c  introduce an extra space.

@deffn {Scheme Procedure} call-with-blocked-asyncs proc
@deffnx {C Function} scm_call_with_blocked_asyncs (proc)
@deffnx {C Function} void *scm_c_call_with_blocked_asyncs (void * (*proc) (void *data), void *data)
@findex scm_c_call_with_blocked_asyncs
Call @var{proc} and block the execution of system asyncs by one level
for the current thread while it is running.  Return the value returned
by @var{proc}.  For the first two variants, call @var{proc} with no
arguments; for the third, call it with @var{data}.
@end deffn

@deffn {Scheme Procedure} call-with-unblocked-asyncs proc
@deffnx {C Function} scm_call_with_unblocked_asyncs (proc)
@deffnx {C Function} void *scm_c_call_with_unblocked_asyncs (void *(*p) (void *d), void *d)
@findex scm_c_call_with_unblocked_asyncs
Call @var{proc} and unblock the execution of system asyncs by one
level for the current thread while it is running.  Return the value
returned by @var{proc}.  For the first two variants, call @var{proc}
with no arguments; for the third, call it with @var{data}.
@end deffn

@deftypefn {C Function} void scm_frame_block_asyncs ()
This function must be used inside a pair of calls to
@code{scm_frame_begin} and @code{scm_frame_end} (@pxref{Frames}).
During the dynamic extent of the frame, asyncs are blocked by one level.
@end deftypefn

@deftypefn {C Function} void scm_frame_unblock_asyncs ()
This function must be used inside a pair of calls to
@code{scm_frame_begin} and @code{scm_frame_end} (@pxref{Frames}).
During the dynamic extent of the frame, asyncs are unblocked by one
level.
@end deftypefn

@node User asyncs
@subsubsection User asyncs

A user async is a pair of a thunk (a parameterless procedure) and a
mark.  Setting the mark on a user async will cause the thunk to be
executed when the user async is passed to @code{run-asyncs}.  Setting
the mark more than once is satisfied by one execution of the thunk.

User asyncs are created with @code{async}.  They are marked with
@code{async-mark}.

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

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

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


@node Dynamic Roots
@subsection 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
@subsection Threads
@cindex threads
@cindex Guile threads
@cindex POSIX threads

Guile threads are implemented using POSIX threads, they run
pre-emptively and concurrently through both Scheme code and system
calls.  The only exception is for garbage collection, where all
threads must rendezvous.

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


@node Low level thread primitives
@subsubsection Low level thread primitives

@deffn {Scheme Procedure} all-threads
@deffnx {C Function} scm_all_threads ()
Return a list of all threads.
@end deffn

@deffn {Scheme Procedure} current-thread
@deffnx {C Function} scm_current_thread ()
Return the thread that called this function.
@end deffn

@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.  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

@deffn {Scheme Procedure} thread-exited? thread
@deffnx {C Function} scm_thread_exited_p (thread)
Return @code{#t} iff @var{thread} has exited.
@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-condition-variable")
@deffn {Scheme Procedure} make-condition-variable
Make a new condition variable.
@end deffn

@deffn {Scheme Procedure} make-fair-condition-variable
@deffnx {C Function} scm_make_fair_condition_variable ()
Make a new fair condition variable.
@end deffn

@c begin (texi-doc-string "guile" "wait-condition-variable")
@deffn {Scheme Procedure} wait-condition-variable cond-var mutex [time]
Wait until @var{cond-var} has been signalled.  While waiting,
@var{mutex} is atomically unlocked (as with @code{unlock-mutex}) and
is locked again when this function returns.  When @var{time} is given,
it specifies a point in time where the waiting should be aborted.  It
can be either a integer as returned by @code{current-time} or a pair
as returned by @code{gettimeofday}.  When the waiting is aborted,
@code{#f} is returned.  When the condition variable has in fact been
signalled, @code{#t} is returned.  The mutex is re-locked in any case
before @code{wait-condition-variable} returns.

When a system async is activated for a thread that is blocked in a
call to @code{wait-condition-variable}, the waiting is interrupted,
the mutex is locked, and the async is executed.  When the async
returns, the mutex is unlocked again and the waiting is resumed.
@end deffn

@c begin (texi-doc-string "guile" "signal-condition-variable")
@deffn {Scheme Procedure} signal-condition-variable cond-var
Wake up one thread that is waiting for @var{cv}.
@end deffn

@c begin (texi-doc-string "guile" "broadcast-condition-variable")
@deffn {Scheme Procedure} broadcast-condition-variable cond-var
Wake up all threads that are waiting for @var{cv}.
@end deffn

@node Higher level thread procedures
@subsubsection 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.

@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 a default error handler that display
the error to the current error port.
@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 a default error handler that display
the error to the current error port.
@end deffn

@node C level thread interface
@subsubsection C level thread interface

You can create and manage threads
with the C versions of the primitives above.
These
functions and data types are only available from C and can not be
mixed with the first set from above.  However, they might be more
efficient and can be used in situations where Scheme data types are
not allowed or are inconvenient to use.

Furthermore, they are the primitives that Guile relies on for its own
higher level threads.  By reimplementing them, you can adapt Guile to
different low-level thread implementations.

C code in a thread must call a libguile function periodically.  When
one thread finds garbage collection is required, it waits for all
threads to rendezvous before doing that GC.  Such a rendezvous is
checked within libguile functions.  If C code wants to sleep or block
in a thread it should use one of the libguile functions provided.

Only threads created by Guile can use the libguile functions.  Threads
created directly with say @code{pthread_create} are unknown to Guile
and they cannot call libguile.  The stack in such foreign threads is
not scanned during GC, so @code{SCM} values generally cannot be held
there.

@c  FIXME:
@c
@c  Describe SCM_TICK which can be called if no other libguile
@c  function is being used by a C function.
@c
@c  Describe "Guile mode", which a thread can enter and exit.  There
@c  are no functions for doing this yet.
@c
@c  When in guile mode a thread can call libguile, is subject to the
@c  tick rule, and its stack is scanned.  When not in guile mode it
@c  cannot call libguile, it doesn't have to tick, and its stack is
@c  not scanned.  The strange guile control flow things like
@c  exceptions, continuations and asyncs only occur when in guile
@c  mode.
@c
@c  When guile mode is exited, the portion of the stack allocated
@c  while it was in guile mode is still scanned.  This portion may not
@c  be modified when outside guile mode.  The stack ends up
@c  partitioned into alternating guile and non-guile regions.
@c
@c  Leaving guile mode is convenient when running an extended
@c  calculation not involving guile, since one doesn't need to worry
@c  about SCM_TICK calls.


@deftp {C Data Type} scm_t_thread
This data type represents a thread, to be used with scm_thread_create,
etc.
@end deftp

@deftypefn {C Function} int scm_thread_create (scm_t_thread *t, void (*proc)(void *), void *data)
Create a new thread that will start by calling @var{proc}, passing it
@var{data}.  A handle for the new thread is stored in @var{t}, which
must be non-NULL.  The thread terminated when @var{proc} returns.
When the thread has not been detached, its handle remains valid after
is has terminated so that it can be used with @var{scm_thread_join},
for example.  When it has been detached, the handle becomes invalid as
soon as the thread terminates.
@end deftypefn

@deftypefn {C Function} void scm_thread_detach (scm_t_thread t)
Detach the thread @var{t}.  See @code{scm_thread_create}.
@end deftypefn

@deftypefn {C Function} void scm_thread_join (scm_t_thread t)
Wait for thread @var{t} to terminate.  The thread must not have been
detached at the time that @code{scm_thread_join} is called, but it
might have been detached by the time it terminates.
@end deftypefn

@deftypefn {C Function} scm_t_thread scm_thread_self ()
Return the handle of the calling thread.
@end deftypefn

@deftp {C Data Type} scm_t_cond
This data type represents a condition variable, to be used with
scm_cond_init, etc.
@end deftp

@deftypefn {C Function} void scm_cond_init (scm_t_cond *c)
Initialize the condition variable structure pointed to by @var{c}.
@end deftypefn

@deftypefn {C Function} void scm_cond_destroy (scm_t_cond *c)
Deallocate all resources associated with @var{c}.
@end deftypefn

@deftypefn {C Function} void scm_cond_wait (scm_t_cond *c, scm_t_mutex *m)
Wait for @var{c} to be signalled.  While waiting @var{m} is unlocked
and locked again before @code{scm_cond_wait} returns.
@end deftypefn

@deftypefn {C Function} void scm_cond_timedwait (scm_t_cond *c, scm_t_mutex *m, timespec *abstime)
Wait for @var{c} to be signalled as with @code{scm_cond_wait} but
don't wait longer than the point in time specified by @var{abstime}.
when the waiting is aborted, zero is returned; non-zero else.
@end deftypefn

@deftypefn {C Function} void scm_cond_signal (scm_t_cond *c)
Signal the condition variable @var{c}.  When one or more threads are
waiting for it to be signalled, select one arbitrarily and let its
wait succeed.
@end deftypefn

@deftypefn {C Function} void scm_cond_broadcast (scm_t_cond *c)
Signal the condition variable @var{c}.  When there are threads waiting
for it to be signalled, wake them all up and make all their waits
succeed.
@end deftypefn

@deftp {C Type} scm_t_key
This type represents a key for a thread-specific value.
@end deftp

@deftypefn {C Function} void scm_key_create (scm_t_key *keyp)
Create a new key for a thread-specific value.  Each thread has its own
value associated to such a handle.  The new handle is stored into
@var{keyp}, which must be non-NULL.
@end deftypefn

@deftypefn {C Function} void scm_key_delete (scm_t_key key)
This function makes @var{key} invalid as a key for thread-specific data.
@end deftypefn

@deftypefn {C Function} void scm_key_setspecific (scm_t_key key, const void *value)
Associate @var{value} with @var{key} in the calling thread.
@end deftypefn

@deftypefn {C Function} int scm_key_getspecific (scm_t_key key)
Return the value currently associated with @var{key} in the calling
thread.  When @code{scm_key_setspecific} has not yet been called in
this thread with this key, @code{NULL} is returned.
@end deftypefn

@deftypefn {C Function} int scm_thread_select (...)
This function does the same thing as the system's @code{select}
function, but in a way that is friendly to the thread implementation.
You should call it in preference to the system @code{select}.
@end deftypefn

@node Fluids
@subsection Fluids

@cindex fluids

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 the desirable effects of dynamically
scoped variables.  Dynamically scoped variables are useful when you
want to set a variable to a value during some dynamic extent in the
execution of your program and have them revert to their original value
when the control flow is outside of this dynamic extent.  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.  The values
stored in a fluid can be accessed with @code{fluid-ref} and
@code{fluid-set!}.

@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

@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-fluid* fluid value thunk
@deffnx {C Function} scm_with_fluid (fluid, value, thunk)
Set @var{fluid} to @var{value} temporarily, and call @var{thunk}.
@var{thunk} must be a procedure with no argument.
@end deffn

@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
in the order given.  @var{thunk} must be a procedure with no argument.
it is called inside a @code{dynamic-wind} and the fluids are
set/restored when control enter or leaves the established dynamic
extent.
@end deffn

@deffn {Scheme Macro} with-fluids ((fluid value) ...) body...
Execute @var{body...} while each @var{fluid} is set to the
corresponding @var{value}.  Both @var{fluid} and @var{value} are
evaluated and @var{fluid} must yield a fluid.  @var{body...} is
executed inside a @code{dynamic-wind} and the fluids are set/restored
when control enter or leaves the established dynamic extent.
@end deffn

@deftypefn {C Function} SCM scm_c_with_fluids (SCM fluids, SCM vals, SCM (*cproc)(void *), void *data)
@deftypefnx {C Function} SCM scm_c_with_fluid (SCM fluid, SCM val, SCM (*cproc)(void *), void *data)
The function @code{scm_c_with_fluids} is like @code{scm_with_fluids}
except that it takes a C function to call instead of a Scheme thunk.

The function @code{scm_c_with_fluid} is similar but only allows one
fluid to be set instead of a list.
@end deftypefn

@deftypefn {C Function} void scm_frame_fluid (SCM fluid, SCM val)
This function must be used inside a pair of calls to
@code{scm_frame_begin} and @code{scm_frame_end} (@pxref{Frames}).
During the dynamic extent of the frame, the fluid @var{fluid} is set
to @var{val}.

More precisely, the value of the fluid is swapped with a `backup'
value whenever the frame is entered or left.  The backup value is
initialized with the @var{val} argument.
@end deftypefn

@node Futures
@subsection Futures
@cindex futures

Futures are a convenient way to run a calculation in a new thread, and
only wait for the result when it's actually needed.

Futures are similar to promises (@pxref{Delayed Evaluation}), in that
they allow mainline code to continue immediately.  But @code{delay}
doesn't evaluate at all until forced, whereas @code{future} starts
immediately in a new thread.

@deffn {syntax} future expr
Begin evaluating @var{expr} in a new thread, and return a ``future''
object representing the calculation.
@end deffn

@deffn {Scheme Procedure} make-future thunk
@deffnx {C Function} scm_make_future (thunk)
Begin evaluating the call @code{(@var{thunk})} in a new thread, and
return a ``future'' object representing the calculation.
@end deffn

@deffn {Scheme Procedure} future-ref f
@deffnx {C Function} scm_future_ref (f)
Return the value computed by the future @var{f}.  If @var{f} has not
yet finished executing then wait for it to do so.
@end deffn


@node Parallel Forms
@subsection Parallel forms
@cindex parallel forms

The functions described in this section are available from

@example
(use-modules (ice-9 threads))
@end example

@deffn syntax parallel expr1 @dots{} exprN
Evaluate each @var{expr} expression in parallel, each in a new thread.
Return the results as a set of @var{N} multiple values
(@pxref{Multiple Values}).
@end deffn

@deffn syntax letpar ((var1 expr1) @dots{} (varN exprN)) body@dots{}
Evaluate each @var{expr} in parallel, each in a new thread, then bind
the results to the corresponding @var{var} variables and evaluate
@var{body}.

@code{letpar} is like @code{let} (@pxref{Local Bindings}), but all the
expressions for the bindings are evaluated in parallel.
@end deffn

@deffn {Scheme Procedure} par-map proc lst1 @dots{} lstN
@deffnx {Scheme Procedure} par-for-each proc lst1 @dots{} lstN
Call @var{proc} on the elements of the given lists.  @code{par-map}
returns a list comprising the return values from @var{proc}.
@code{par-for-each} returns an unspecified value, but waits for all
calls to complete.

The @var{proc} calls are @code{(@var{proc} @var{elem1} @dots{}
@var{elemN})}, where each @var{elem} is from the corresponding
@var{lst}.  Each @var{lst} must be the same length.  The calls are
made in parallel, each in a new thread.

These functions are like @code{map} and @code{for-each} (@pxref{List
Mapping}), but make their @var{proc} calls in parallel.
@end deffn

@deffn {Scheme Procedure} n-par-map n proc lst1 @dots{} lstN
@deffnx {Scheme Procedure} n-par-for-each n proc lst1 @dots{} lstN
Call @var{proc} on the elements of the given lists, in the same way as
@code{par-map} and @code{par-for-each} above, but use no more than
@var{n} new threads at any one time.  The order in which calls are
initiated within that threads limit is unspecified.

These functions are good for controlling resource consumption if
@var{proc} calls might be costly, or if there are many to be made.  On
a dual-CPU system for instance @math{@var{n}=4} might be enough to
keep the CPUs utilized, and not consume too much memory.
@end deffn

@deffn {Scheme Procedure} n-for-each-par-map n sproc pproc lst1 @dots{} lstN
Apply @var{pproc} to the elements of the given lists, and apply
@var{sproc} to each result returned by @var{pproc}.  The final return
value is unspecified, but all calls will have been completed before
returning.

The calls made are @code{(@var{sproc} (@var{pproc} @var{elem1} @dots{}
@var{elemN}))}, where each @var{elem} is from the corresponding
@var{lst}.  Each @var{lst} must have the same number of elements.

The @var{pproc} calls are made in parallel, in new threads.  No more
than @var{n} new threads are used at any one time.  The order in which
@var{pproc} calls are initiated within that limit is unspecified.

The @var{sproc} calls are made serially, in list element order, one at
a time.  @var{pproc} calls on later elements may execute in parallel
with the @var{sproc} calls.  Exactly which thread makes each
@var{sproc} call is unspecified.

This function is designed for individual calculations that can be done
in parallel, but with results needing to be handled serially, for
instance to write them to a file.  The @var{n} limit on threads
controls system resource usage when there are many calculations or
when they might be costly.

It will be seen that @code{n-for-each-par-map} is like a combination
of @code{n-par-map} and @code{for-each},

@example
(for-each sproc (n-par-map pproc lst1 ... lstN))
@end example

@noindent
But the actual implementation is more efficient since each @var{sproc}
call, in turn, can be initiated once the relevant @var{pproc} call has
completed, it doesn't need to wait for all to finish.
@end deffn


@node Mutexes
@subsection Mutexes
@cindex mutex

A mutex is a thread synchronization object, it can be used by threads
to control access to a shared resource.  A mutex can be locked to
indicate a resource is in use, and other threads can then block on the
mutex to wait for the resource (or can just test and do something else
if not available).  ``Mutex'' is short for ``mutual exclusion''.

There are two types of mutexes, ``standard'' and ``fair''.  They're
created by @code{make-mutex} and @code{make-fair-mutex} respectively,
the operation functions are then common to both.

Note that for both types of mutex there's no protection against a
``deadly embrace''.  For instance if one thread has locked mutex A and
is waiting on mutex B, but another thread owns B and is waiting on A,
then an endless wait will occur (in the current implementation).
Acquiring requisite mutexes in a fixed order (like always A before B)
in all threads is one way to avoid such problems.

@sp 1
@deffn {Scheme Procedure} make-mutex
@deffnx {Scheme Procedure} make-fair-mutex
Return a new mutex object.

@code{make-mutex} creates a standard mutex.  This is fast, but its
features are restricted.  Recursive locking (multiple lock calls by
one thread) is not permitted, and an unlock can be done only when
already locked and only by the owning thread.  When multiple threads
are blocked waiting to acquire the mutex, it's unspecified which will
get it next.

@code{make-fair-mutex} creates a fair mutex.  This has more features
and error checking.  Recursive locking is allowed, a given thread can
make multiple lock calls and the mutex is released when a balancing
number of unlocks are done.  Other threads blocked waiting to acquire
the mutex form a queue and the one waiting longest will be the next to
acquire it.
@end deffn

@deffn {Scheme Procedure} lock-mutex mutex
Lock @var{mutex}.  If the mutex is already locked by another thread
then block and return only when @var{mutex} has been acquired.

For standard mutexes (@code{make-mutex}), if the thread has itself
already locked @var{mutex} it must not call @code{lock-mutex} on it a
further time.  Behaviour is unspecified if this is done.

For a fair mutex (@code{make-fair-mutex}), if the thread has itself
already locked @var{mutex}, then a further @code{lock-mutex} call
increments the lock count.  An additional @code{unlock-mutex} will be
required to finally release.

When a system async (@pxref{System asyncs}) is activated for a thread
blocked in @code{lock-mutex}, the wait is interrupted and the async is
executed.  When the async returns the wait resumes.
@end deffn

@deffn {Scheme Procedure} try-mutex mutex
Try to lock @var{mutex} as per @code{lock-mutex}.  If @var{mutex} can
be acquired immediately then this is done and the return is @code{#t}.
If @var{mutex} is locked by some other thread then nothing is done and
the return is @code{#f}.
@end deffn

@deffn {Scheme Procedure} unlock-mutex mutex
Unlock @var{mutex}.

For a standard mutex (@code{make-mutex}), if @var{mutex} is not locked
by the calling thread then behaviour is unspecified.

For a fair mutex (@code{make-fair-mutex}), if @var{mutex} is not
locked by the calling thread then an error is thrown.
@end deffn

@sp 1
The following are higher level operations on mutexes.  These are
available from

@example
(use-modules (ice-9 threads))
@end example

@deffn macro with-mutex mutex [body@dots{}]
Lock @var{mutex}, evaluate the @var{body} forms, then unlock
@var{mutex}.  The return value is the return from the last @var{body}
form.

The lock, body and unlock form the branches of a @code{dynamic-wind}
(@pxref{Dynamic Wind}), so @var{mutex} is automatically unlocked if an
error or new continuation exits @var{body}, and is re-locked if
@var{body} is re-entered by a captured continuation.
@end deffn

@deffn macro monitor body@dots{}
Evaluate the @var{body} forms, with a mutex locked so only one thread
can execute that code at any one time.  The return value is the return
from the last @var{body} form.

Each @code{monitor} form has its own private mutex and the locking and
evaluation is as per @code{with-mutex} above.  A standard mutex
(@code{make-mutex}) is used, which means @var{body} must not
recursively re-enter the @code{monitor} form.

The term ``monitor'' comes from operating system theory, where it
means a particular bit of code managing access to some resource and
which only ever executes on behalf of one process at any one time.
@end deffn

@sp 1
The following provide access to standard mutexes from C code.

@deftp {C Data Type} scm_t_mutex
A mutex, to be used with @code{scm_mutex_init}, etc.
@end deftp

@deftypefn {C Function} void scm_mutex_init (scm_t_mutex *m)
Initialize the mutex structure pointed to by @var{m}.
@end deftypefn

@deftypefn {C Function} void scm_mutex_destroy (scm_t_mutex *m)
Free all resources associated with @var{m}.
@end deftypefn

@deftypefn {C Function} void scm_mutex_lock (scm_t_mutex *m)
Lock the mutex @var{m}.  This is as per @code{lock-mutex} above on a
standard mutex.
@end deftypefn

@deftypefn {C Function} int scm_mutex_trylock (scm_t_mutex *m)
Attempt to lock mutex @var{m}, as per @code{scm_mutex_lock}.  If
@var{m} is unlocked then this is done and the return is non-zero.  If
@var{m} is already locked by another thread then do nothing and return
zero.
@end deftypefn

@deftypefn {C Function} void scm_mutex_unlock (scm_t_mutex *m)
Unlock the mutex @var{m}.  The mutex must have been locked by the
current thread, otherwise the behavior is undefined.
@end deftypefn


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