@c -*-texinfo-*- @c This is part of the GNU Guile Reference Manual. @c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2007 @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. * Continuation Barriers:: Protection from non-local control flow. * Threads:: Multiple threads of execution. * Mutexes and Condition Variables:: Synchronization primitives. * Blocking:: How to block properly in guile mode. * Critical Sections:: Avoiding concurrency and reentries. * Fluids and Dynamic States:: Thread-local variables, etc. * Parallel Forms:: Parallel execution of forms. @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 and Condition Variables}). 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_dynwind_block_asyncs} and @code{scm_dynwind_unblock_asyncs} inside a @dfn{dynamic context} (@pxref{Dynamic Wind}) 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_dynwind_block_asyncs () This function must be used inside a pair of calls to @code{scm_dynwind_begin} and @code{scm_dynwind_end} (@pxref{Dynamic Wind}). During the dynwind context, asyncs are blocked by one level. @end deftypefn @deftypefn {C Function} void scm_dynwind_unblock_asyncs () This function must be used inside a pair of calls to @code{scm_dynwind_begin} and @code{scm_dynwind_end} (@pxref{Dynamic Wind}). During the dynwind context, 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 Continuation Barriers @subsection Continuation Barriers The non-local flow of control caused by continuations might sometimes not be wanted. You can use @code{with-continuation-barrier} etc to errect fences that continuations can not pass. @deffn {Scheme Procedure} with-continuation-barrier proc @deffnx {C Function} scm_with_continuation_barrier (proc) Call @var{proc} and return its result. Do not allow the invocation of continuations that would leave or enter the dynamic extent of the call to @code{with-continuation-barrier}. Such an attempt causes an error to be signaled. Throws (such as errors) that are not caught from within @var{proc} are caught by @code{with-continuation-barrier}. In that case, a short message is printed to the current error port and @code{#f} is returned. Thus, @code{with-continuation-barrier} returns exactly once. @end deffn @deftypefn {C Function} {void *} scm_c_with_continuation_barrier (void *(*func) (void *), void *data) Like @code{scm_with_continuation_barrier} but call @var{func} on @var{data}. When an error is caught, @code{NULL} is returned. @end deftypefn @node Threads @subsection Threads @cindex threads @cindex Guile threads @cindex POSIX threads @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 [handler] Call @code{thunk} in a new thread and with a new dynamic state, returning the new thread. The procedure @var{thunk} is called via @code{with-continuation-barrier}. When @var{handler} is specified, then @var{thunk} is called from within a @code{catch} with tag @code{#t} that has @var{handler} as its handler. This catch is established inside the continuation barrier. Once @var{thunk} or @var{handler} returns, the return value is made the @emph{exit value} of the thread and the thread is terminated. @end deffn @deftypefn {C Function} SCM scm_spawn_thread (scm_t_catch_body body, void *body_data, scm_t_catch_handler handler, void *handler_data) Call @var{body} in a new thread, passing it @var{body_data}, returning the new thread. The function @var{body} is called via @code{scm_c_with_continuation_barrier}. When @var{handler} is non-@code{NULL}, @var{body} is called via @code{scm_internal_catch} with tag @code{SCM_BOOL_T} that has @var{handler} and @var{handler_data} as the handler and its data. This catch is established inside the continuation barrier. Once @var{body} or @var{handler} returns, the return value is made the @emph{exit value} of the thread and the thread is terminated. @end deftypefn @deffn {Scheme Procedure} thread? obj @deffnx {C Function} scm_thread_p (obj) Return @code{#t} iff @var{obj} is a thread; otherwise, return @code{#f}. @end deffn @c begin (texi-doc-string "guile" "join-thread") @deffn {Scheme Procedure} join-thread thread [timeout [timeoutval]] @deffnx {C Function} scm_join_thread (thread) @deffnx {C Function} scm_join_thread_timed (thread, timeout, timeoutval) Wait for @var{thread} to terminate and return its exit value. Threads that have not been created with @code{call-with-new-thread} or @code{scm_spawn_thread} have an exit value of @code{#f}. When @var{timeout} is given, it specifies a point in time where the waiting should be aborted. It can be either an integer as returned by @code{current-time} or a pair as returned by @code{gettimeofday}. When the waiting is aborted, @var{timeoutval} is returned (if it is specified; @code{#f} is returned otherwise). @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 @deffn {Scheme Procedure} cancel-thread thread @deffnx {C Function} scm_cancel_thread (thread) Asynchronously notify @var{thread} to exit. Immediately after receiving this notification, @var{thread} will call its cleanup handler (if one has been set) and then terminate, aborting any evaluation that is in progress. Because Guile threads are isomorphic with POSIX threads, @var{thread} will not receive its cancellation signal until it reaches a cancellation point. See your operating system's POSIX threading documentation for more information on cancellation points; note that in Guile, unlike native POSIX threads, a thread can receive a cancellation notification while attempting to lock a mutex. @end deffn @deffn {Scheme Procedure} set-thread-cleanup! thread proc @deffnx {C Function} scm_set_thread_cleanup_x (thread, proc) Set @var{proc} as the cleanup handler for the thread @var{thread}. @var{proc}, which must be a thunk, will be called when @var{thread} exits, either normally or by being canceled. Thread cleanup handlers can be used to perform useful tasks like releasing resources, such as locked mutexes, when thread exit cannot be predicted. The return value of @var{proc} will be set as the @emph{exit value} of @var{thread}. To remove a cleanup handler, pass @code{#f} for @var{proc}. @end deffn @deffn {Scheme Procedure} thread-cleanup thread @deffnx {C Function} scm_thread_cleanup (thread) Return the cleanup handler currently installed for the thread @var{thread}. If no cleanup handler is currently installed, thread-cleanup returns @code{#f}. @end deffn 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. The @var{args@dots{}} expressions are evaluated in the new thread. @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 Mutexes and Condition Variables @subsection Mutexes and Condition Variables @cindex mutex @cindex condition variable 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 in Guile, ``standard'' and ``recursive''. They're created by @code{make-mutex} and @code{make-recursive-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 . flags @deffnx {C Function} scm_make_mutex () @deffnx {C Function} scm_make_mutex_with_flags (SCM flags) Return a new mutex. It is initially unlocked. If @var{flags} is specified, it must be a list of symbols specifying configuration flags for the newly-created mutex. The supported flags are: @table @code @item unchecked-unlock Unless this flag is present, a call to `unlock-mutex' on the returned mutex when it is already unlocked will cause an error to be signalled. @item allow-external-unlock Allow the returned mutex to be unlocked by the calling thread even if it was originally locked by a different thread. @item recursive The returned mutex will be recursive. @end table @end deffn @deffn {Scheme Procedure} mutex? obj @deffnx {C Function} scm_mutex_p (obj) Return @code{#t} iff @var{obj} is a mutex; otherwise, return @code{#f}. @end deffn @deffn {Scheme Procedure} make-recursive-mutex @deffnx {C Function} scm_make_recursive_mutex () Create a new recursive mutex. It is initially unlocked. Calling this function is equivalent to calling `make-mutex' and specifying the @code{recursive} flag. @end deffn @deffn {Scheme Procedure} lock-mutex mutex [timeout [owner]] @deffnx {C Function} scm_lock_mutex (mutex) @deffnx {C Function} scm_lock_mutex_timed (mutex, timeout, owner) Lock @var{mutex}. If the mutex is already locked, then block and return only when @var{mutex} has been acquired. When @var{timeout} is given, it specifies a point in time where the waiting should be aborted. It can be either an 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 @var{owner} is given, it specifies an owner for @var{mutex} other than the calling thread. @var{owner} may also be @code{#f}, indicating that the mutex should be locked but left unowned. For standard mutexes (@code{make-mutex}), and error is signalled if the thread has itself already locked @var{mutex}. For a recursive mutex (@code{make-recursive-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. If @var{mutex} was locked by a thread that exited before unlocking it, the next attempt to lock @var{mutex} will succeed, but @code{abandoned-mutex-error} will be signalled. 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 @deftypefn {C Function} void scm_dynwind_lock_mutex (SCM mutex) Arrange for @var{mutex} to be locked whenever the current dynwind context is entered and to be unlocked when it is exited. @end deftypefn @deffn {Scheme Procedure} try-mutex mx @deffnx {C Function} scm_try_mutex (mx) 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 [condvar [timeout]] @deffnx {C Function} scm_unlock_mutex (mutex) @deffnx {C Function} scm_unlock_mutex_timed (mutex, condvar, timeout) Unlock @var{mutex}. An error is signalled if @var{mutex} is not locked and was not created with the @code{unchecked-unlock} flag set, or if @var{mutex} is locked by a thread other than the calling thread and was not created with the @code{allow-external-unlock} flag set. If @var{condvar} is given, it specifies a condition variable upon which the calling thread will wait to be signalled before returning. (This behavior is very similar to that of @code{wait-condition-variable}, except that the mutex is left in an unlocked state when the function returns.) When @var{timeout} is also given, it specifies a point in time where the waiting should be aborted. It can be either an integer as returned by @code{current-time} or a pair as returned by @code{gettimeofday}. When the waiting is aborted, @code{#f} is returned. Otherwise the function returns @code{#t}. @end deffn @deffn {Scheme Procedure} mutex-owner mutex @deffnx {C Function} scm_mutex_owner (mutex) Return the current owner of @var{mutex}, in the form of a thread or @code{#f} (indicating no owner). Note that a mutex may be unowned but still locked. @end deffn @deffn {Scheme Procedure} mutex-level mutex @deffnx {C Function} scm_mutex_level (mutex) Return the current lock level of @var{mutex}. If @var{mutex} is currently unlocked, this value will be 0; otherwise, it will be the number of times @var{mutex} has been recursively locked by its current owner. @end deffn @deffn {Scheme Procedure} mutex-locked? mutex @deffnx {C Function} scm_mutex_locked_p (mutex) Return @code{#t} if @var{mutex} is locked, regardless of ownership; otherwise, return @code{#f}. @end deffn @deffn {Scheme Procedure} make-condition-variable @deffnx {C Function} scm_make_condition_variable () Return a new condition variable. @end deffn @deffn {Scheme Procedure} condition-variable? obj @deffnx {C Function} scm_condition_variable_p (obj) Return @code{#t} iff @var{obj} is a condition variable; otherwise, return @code{#f}. @end deffn @deffn {Scheme Procedure} wait-condition-variable condvar mutex [time] @deffnx {C Function} scm_wait_condition_variable (condvar, mutex, time) Wait until @var{condvar} 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. When the thread block while re-acquiring the mutex, execution of asyncs is blocked. @end deffn @deffn {Scheme Procedure} signal-condition-variable condvar @deffnx {C Function} scm_signal_condition_variable (condvar) Wake up one thread that is waiting for @var{condvar}. @end deffn @deffn {Scheme Procedure} broadcast-condition-variable condvar @deffnx {C Function} scm_broadcast_condition_variable (condvar) Wake up all threads that are waiting for @var{condvar}. @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 @node Blocking @subsection Blocking in Guile Mode A thread must not block outside of a libguile function while it is in guile mode. The following functions can be used to temporily leave guile mode or to perform some common blocking operations in a supported way. @deftypefn {C Function} {void *} scm_without_guile (void *(*func) (void *), void *data) Leave guile mode, call @var{func} on @var{data}, enter guile mode and return the result of calling @var{func}. While a thread has left guile mode, it must not call any libguile functions except @code{scm_with_guile} or @code{scm_without_guile} and must not use any libguile macros. Also, local variables of type @code{SCM} that are allocated while not in guile mode are not protected from the garbage collector. When used from non-guile mode, calling @code{scm_without_guile} is still allowed: it simply calls @var{func}. In that way, you can leave guile mode without having to know whether the current thread is in guile mode or not. @end deftypefn @deftypefn {C Function} int scm_pthread_mutex_lock (pthread_mutex_t *mutex) Like @code{pthread_mutex_lock}, but leaves guile mode while waiting for the mutex. @end deftypefn @deftypefn {C Function} int scm_pthread_cond_wait (pthread_cond_t *cond, pthread_mutex_t *mutex) @deftypefnx {C Function} int scm_pthread_cond_timedwait (pthread_cond_t *cond, pthread_mutex_t *mutex, struct timespec *abstime) Like @code{pthread_cond_wait} and @code{pthread_cond_timedwait}, but leaves guile mode while waiting for the condition variable. @end deftypefn @deftypefn {C Function} int scm_std_select (int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout) Like @code{select} but leaves guile mode while waiting. Also, the delivery of a system async causes this function to be interrupted with error code @code{EINTR}. @end deftypefn @deftypefn {C Function} {unsigned int} scm_std_sleep ({unsigned int} seconds) Like @code{sleep}, but leaves guile mode while sleeping. Also, the delivery of a system async causes this function to be interrupted. @end deftypefn @deftypefn {C Function} {unsigned long} scm_std_usleep ({unsigned long} usecs) Like @code{usleep}, but leaves guile mode while sleeping. Also, the delivery of a system async causes this function to be interrupted. @end deftypefn @node Critical Sections @subsection Critical Sections @deffn {C Macro} SCM_CRITICAL_SECTION_START @deffnx {C Macro} SCM_CRITICAL_SECTION_END These two macros can be used to delimit a critical section. Syntactically, they are both statements and need to be followed immediately by a semicolon. Executing @code{SCM_CRITICAL_SECTION_START} will lock a recursive mutex and block the executing of system asyncs. Executing @code{SCM_CRITICAL_SECTION_END} will unblock the execution of system asyncs and unlock the mutex. Thus, the code that executes between these two macros can only be executed in one thread at any one time and no system asyncs will run. However, because the mutex is a recursive one, the code might still be reentered by the same thread. You must either allow for this or avoid it, both by careful coding. On the other hand, critical sections delimited with these macros can be nested since the mutex is recursive. You must make sure that for each @code{SCM_CRITICAL_SECTION_START}, the corresponding @code{SCM_CRITICAL_SECTION_END} is always executed. This means that no non-local exit (such as a signalled error) might happen, for example. @end deffn @deftypefn {C Function} void scm_dynwind_critical_section (SCM mutex) Call @code{scm_dynwind_lock_mutex} on @var{mutex} and call @code{scm_dynwind_block_asyncs}. When @var{mutex} is false, a recursive mutex provided by Guile is used instead. The effect of a call to @code{scm_dynwind_critical_section} is that the current dynwind context (@pxref{Dynamic Wind}) turns into a critical section. Because of the locked mutex, no second thread can enter it concurrently and because of the blocked asyncs, no system async can reenter it from the current thread. When the current thread reenters the critical section anyway, the kind of @var{mutex} determines what happens: When @var{mutex} is recursive, the reentry is allowed. When it is a normal mutex, an error is signalled. @end deftypefn @node Fluids and Dynamic States @subsection Fluids and Dynamic States @cindex fluids A @emph{fluid} is an object that can store one value per @emph{dynamic state}. Each thread has a current dynamic state, and when accessing a fluid, this current dynamic state is used to provide the actual value. In this way, fluids can be used for thread local storage, but they are in fact more flexible: dynamic states are objects of their own and can be made current for more than one thread at the same time, or only be made current temporarily, for example. Fluids can also 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 that can hold one value per dynamic state. That is, modifications to this value are only visible to code that executes with the same dynamic state as the modifying code. When a new dynamic state is constructed, it inherits the values from its parent. Because each thread normally executes with its own dynamic state, 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_dynwind_fluid (SCM fluid, SCM val) This function must be used inside a pair of calls to @code{scm_dynwind_begin} and @code{scm_dynwind_end} (@pxref{Dynamic Wind}). During the dynwind context, the fluid @var{fluid} is set to @var{val}. More precisely, the value of the fluid is swapped with a `backup' value whenever the dynwind context is entered or left. The backup value is initialized with the @var{val} argument. @end deftypefn @deffn {Scheme Procedure} make-dynamic-state [parent] @deffnx {C Function} scm_make_dynamic_state (parent) Return a copy of the dynamic state object @var{parent} or of the current dynamic state when @var{parent} is omitted. @end deffn @deffn {Scheme Procedure} dynamic-state? obj @deffnx {C Function} scm_dynamic_state_p (obj) Return @code{#t} if @var{obj} is a dynamic state object; return @code{#f} otherwise. @end deffn @deftypefn {C Procedure} int scm_is_dynamic_state (SCM obj) Return non-zero if @var{obj} is a dynamic state object; return zero otherwise. @end deftypefn @deffn {Scheme Procedure} current-dynamic-state @deffnx {C Function} scm_current_dynamic_state () Return the current dynamic state object. @end deffn @deffn {Scheme Procedure} set-current-dynamic-state state @deffnx {C Function} scm_set_current_dynamic_state (state) Set the current dynamic state object to @var{state} and return the previous current dynamic state object. @end deffn @deffn {Scheme Procedure} with-dynamic-state state proc @deffnx {C Function} scm_with_dynamic_state (state, proc) Call @var{proc} while @var{state} is the current dynamic state object. @end deffn @deftypefn {C Procedure} void scm_dynwind_current_dynamic_state (SCM state) Set the current dynamic state to @var{state} for the current dynwind context. @end deftypefn @deftypefn {C Procedure} {void *} scm_c_with_dynamic_state (SCM state, void *(*func)(void *), void *data) Like @code{scm_with_dynamic_state}, but call @var{func} with @var{data}. @end deftypefn @c @node Futures @c @subsection Futures @c @cindex futures @c -- Futures are disabled for the time being, see futures.h for an @c -- explanation. @c Futures are a convenient way to run a calculation in a new thread, and @c only wait for the result when it's actually needed. @c Futures are similar to promises (@pxref{Delayed Evaluation}), in that @c they allow mainline code to continue immediately. But @code{delay} @c doesn't evaluate at all until forced, whereas @code{future} starts @c immediately in a new thread. @c @deffn {syntax} future expr @c Begin evaluating @var{expr} in a new thread, and return a ``future'' @c object representing the calculation. @c @end deffn @c @deffn {Scheme Procedure} make-future thunk @c @deffnx {C Function} scm_make_future (thunk) @c Begin evaluating the call @code{(@var{thunk})} in a new thread, and @c return a ``future'' object representing the calculation. @c @end deffn @c @deffn {Scheme Procedure} future-ref f @c @deffnx {C Function} scm_future_ref (f) @c Return the value computed by the future @var{f}. If @var{f} has not @c yet finished executing then wait for it to do so. @c @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 its own 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 its own 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 its own 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} 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 separate threads. No more than @var{n} 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 n 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 @c Local Variables: @c TeX-master: "guile.texi" @c End: