For Gauche 0.9.5

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9.32 gauche.threads - Threads

If enabled at compilation time, Gauche can use threads built on top of either POSIX threads (pthreads) or Windows threads.

Module: gauche.threads

Provides thread API. You can ’use’ this module regardless whether the thread support is compiled in or not; if threads are not supported, many thread-related procedures simply signals a "not supported" error.

If you want to switch code depending on whether pthreads are available or not, you can use a feature identifier gauche.sys.threads with cond-expand form (see Feature conditional).

   ;; Code that uses thread API (gauche.threads is automatically
   ;; loaded at this moment).
   ;; Code that doesn't use thread API

There are also feature identifiers gauche.sys.pthreads and gauche.sys.wthreads defined for pthreads and Windows threads platforms, respectively. In Scheme level, however, you hardly need to distinguish the underlying implementations. It is recommended to use gauche.sys.threads to switch the code according to thread availability.

To check if threads are available at runtime, instead of compile time, use the following procedure.

Function: gauche-thread-type

Returns a symbol that indicates the supported thread type. It can be one of the following symbols.


Threads are not supported.


Threads are built on top of POSIX pthreads.


Threads are built on top of Win32 threads.

(Note: On pthreads platforms, it should return pthreads instead of pthread; then the returned symbol would correspond to the value given to --enable-threads option at configuration time. It’s a historical overlook, stuck for the backward compatibility.)

Scheme-level thread API conforms SRFI-18, "Multithreading support" ([SRFI-18]), wrapped around Gauche’s object interface.

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9.32.1 Thread programming tips

What’s Gauche threads for

Although the surface API of threads looks simple and portable, you need to know how the threads are implemented in order to utilize the feature’s potential. Some languages support threads as language’s built-in construct and encourage programmers to express the calculation in terms of threads. However, it should be noted that in many cases there are alternative ways than threads to implement the desired algorithm, and you need to compare advantages and disadvantages of using threads depending on how the threads are realized in the underlying system.

In Gauche, the primary purpose of threads is to write programs that require preemptive scheduling, therefore are difficult to express in other ways. Preemptive threads may be required, for example, when you have to call a module that does blocking I/O which you can’t intercept, or may spend nondeterministic amount of calculation time that you want to interrupt.

For each Gauche’s thread, an individual VM is allocated and it is run by the dedicated POSIX thread. Thus the cost of context switch is the same as the native thread, but the creation of threads costs much higher than, say, lightweight threads built on top of call/cc. So Gauche’s preemptive threads are not designed for applications that want to create thousands of threads for fine-grained calculation.

The recommended usage is the technique so called "thread pool", that you create a set of threads and keep them around for long time and dispatch jobs to them as needed. Gauche provides a thread pool implementation in control.thread-pool module (see Thread pools).

Preemptive threads have other difficulties (e.g. see FairThreads), and sometimes the alternatives may be a better fit than the native preemptive threads.

Of course, these technique are not mutually exclusive with native threads. You can use dispatcher with "thread pool" technique, for example. Just keep it in your mind that the native threads are not only but one of the ways to realize those features.

Uncaught errors in a thread body

When you run a single-thread program that raises an unexpected (unhandled) error, Gauche prints an error message and a stack trace by default. So sometimes it perplexes programmers when a thread doesn’t print anything when it dies because of an unhandled error.

What’s happneing is this: An unhandled error in a thread body would cause the thread to terminate, and the error itself will propagate to the thread who’s expecting the result of the terminated thread. So, you get the error (wrapped by <uncaught-exception>) when you call thread-join! on a thread which is terminated because of an unhandled error. The behavior is defined in SRFI-18.

If you fire a thread for one-shot calculation, expecting to receive the result by thread-join!, then this is reasonable—you can handle the error situation in the “parent” thread. However, if you run a thread to loop indefinitely to process something and not expect to retrieve its result via thread-join!, this becomes a pitfall; the thread may die unexpectedly but you wouldn’t know it. (If such a thread is garbage-collected, a warning is printed. However you wouldn’t know when that happens so you can’t count on it.)

For such threads, you should always wrap the body of such thread with guard, and handles the error explicitly. You can call report-error to display the default error message and a stack trace.

 (make-thread (^[] (guard (e [else (report-error e) #f])
                     ... thread body ...))))

See Thread exceptions, for the details of thread exception handling.

Note: As of 0.9.5, Gauche has a known bug that the tail call of error handling clauses of guard doesn’t become a proper tail call. So, the following code, which should run safely in Scheme, could eat up a stack:

 (make-thread (^[] (let loop ()
                     (guard (e [else (report-error e) (loop)])
                       ... thread body ...)))))

For the time being, you can lift the call to loop outside of guard as workaround.

 (make-thread (^[] (let loop ()
                     (guard (e [else (report-error e)])
                       ... thread body ...)

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9.32.2 Thread procedures

Builtin Class: <thread>

A thread. Each thread has an associated thunk which is evaluated by a POSIX thread. When thunk returns normally, the result is stored in the internal ’result’ slot, and can be retrieved by thread-join!. When thunk terminates abnormally, either by raising an exception or terminated by thread-terminate!, the exception condition is stored in their internal ’result exception’ slot, and will be passed to the thread calling thread-join! on the terminated thread.

Each thread has its own dynamic environment and dynamic handler stack. When a thread is created, its dynamic environment is initialized by the creator’s dynamic environment. The thread’s dynamic handler stack is initially empty.

A thread is in one of the following four states at a time. You can query the thread state by the thread-state procedure.


A thread hasn’t started yet. A thread returned from make-thread is in this state. Once a thread is started it will never be in this state again. At this point, no POSIX thread has been created; thread-start! creates a POSIX thread to run the Gauche thread.


When a thread is started by thread-start!, it becomes to this state. Note that a thread blocked by a system call is still in runnable state.


A thread becomes in this state when it is stopped by thread-stop!. A thread in this state can go back to runnable state by thread-cont!, resuming execution from the point when it is stopped.


When the thread finished executing associated code, or is terminated by thread-terminate!, it becomes in this state. Once a thread is in this state, the state can no longer be changed.

Access to the resouces shared by multiple threads must be protected explicitly by synchronization primitives. See Synchronization primitives.

Access to ports are serialized by Gauche. If multiple threads attempt to write to a port, their output may be interleaved but no output will be lost, and the state of the port is kept consistent. If multiple threads attempt to read from a port, a single read primitive (e.g. read, read-char or read-line) works atomically.

Signal handlers are shared by all threads, but each thread has its own signal mask. See Signals and threads, for details.

A thread object has the following external slots.

Instance Variable of <thread>: name

A name can be associated to a thread. This is just for the convenience of the application. The primordial thread has the name "root".

Instance Variable of <thread>: specific

A thread-local slot for use of the application.

Function: current-thread

[SRFI-18], [SRFI-21] Returns the current thread.

Function: thread? obj

[SRFI-18], [SRFI-21] Returns #t if obj is a thread, #f otherwise.

Function: make-thread thunk :optional name

[SRFI-18], [SRFI-21] Creates and returns a new thread to execute thunk. To run the thread, you need to call thread-start!. The result of thunk may be retrieved by calling thread-join!.

You can provide the name of the thread by the optional argument name.

The created thread inherits the signal mask of the calling thread (see Signals and threads), and has a copy of parameters of the calling thread at the time of creation (see Parameters).

Other than those initial setups, there will be no relationship between the new thread and the calling thread; there’s no parent-child relationship like Unix process. Any thread can call thread-join! on any other thread to receive the result. If nobody issues thread-join! and nobody holds a reference to the created thread, it will be garbage collected after the execution of the thread terminates.

If a thread execution is terminated because of uncaught exception, and its result is never retrieved by thread-join!, a warning will be printed to the standard error port notifying “thread dies a lonely death”: It usually indicates some coding error. If you don’t collect the result of threads, you have to make sure that all the exceptions are captured and handled within thunk.

Internally, this procedure just allocates and initializes a Scheme thread object; the POSIX thread is not created until thread-start! is called.

Function: thread-state thread

Returns one of symbols new, runnable, stopped or terminated, indicating the state of thread.

Function: thread-name thread

[SRFI-18], [SRFI-21] Returns the value of name slot of thread.

Function: thread-specific thread
Function: thread-specific-set! thread value

[SRFI-18], [SRFI-21] Gets/sets the value of the thread’s specific slot.

Function: thread-start! thread

[SRFI-18], [SRFI-21] Starts the thread. It is an error if thread is already started. Returns thread.

Function: thread-yield!

[SRFI-18], [SRFI-21] Suspends the execution of the calling thread and yields CPU to other waiting runnable threads, if any.

Function: thread-sleep! timeout

[SRFI-18], [SRFI-21] Suspends the calling thread for the period specified by timeout, which must be either a <time> object (see Time) that specifies absolute point of time, or a real number that specifies relative point of time from the time this procedure is called in number of seconds.

After the specified time passes, thread-sleep! returns with unspecified value.

If timeout points a past time, thread-sleep! returns immediately.

Function: thread-stop! thread :optional timeout timeout-val

Stops execution of the target thread temporarily. You can resume the execution of the thread by thread-cont!.

The stop request is handled synchronously; that is, Gauche VM only checks the request at the “safe” point of the VM and stops itself. It means if the thread is blocked by a system call, it won’t become stopped state until the system call returns.

By default, thread-stop! returns after the target thread stops. Since it may take indefinitely, you can give optional timeout argument to specify timeout. The timeout argument can be #f, which means no timeout, or a <time> object that specifies an absolute point of time, or a real number specifying the number of seconds to wait.

The return value of thread-stop! is thread if it could successfully stop the target, or timeout-val if timeout reached. When timeout-val is omitted, #f is assumed.

If the target thread has already been stopped by the caller thread, this procedure returns thread immediately.

When thread-stop! is timed out, the request remains effective even after thread-stop! returns. That is, the target thread may stop at some point in future. The caller thread is expected to call thread-stop! again to complete the stop operation.

An error is signaled if the target thread has already been stopped by another thread (including the “pending” stop request issued by other threads), or the target thread is in neither runnable nor stopped state.

Function: thread-cont! thread

Resumes execution of thread which has been stopped by thread-stop!. An error is raised if thread is not in stopped state, or it is stopped by another thread.

If the caller thread has already requested to stop the target thread but timed out, calling thread-cont! cancels the request.

Function: thread-terminate! thread

[SRFI-18], [SRFI-21] Terminates the specified thread thread. The thread is terminated and an instance of <terminated-thread-exception> is stored in the result exception field of thread.

If thread is the same as the calling thread, this procedure won’t return. Otherwise, this procedure returns unspecified value.

This procedure should be used with care, since thread won’t have a chance to call cleanup procedures (such as ’after’ thunks of dynamic-wind). If thread is in a critical section, it can leave some state inconsistent. However, once a thread is terminated, any mutex that the thread has kept becomes ’abandoned’ state, and an attempt to lock such a mutex by other thread raises an ’abandoned mutex exception’, so that you will know the situation. See Synchronization primitives.

Function: thread-join! thread :optional timeout timeout-val

[SRFI-18], [SRFI-21] Waits termination of thread, or until the timeout is reached if timeout is given.

Timeout must be either a <time> object (see Time) that specifies absolute point of time, or a real number that specifies relative point of time from the time this procedure is called in number of seconds, or #f that indicates no timeout (default).

If thread terminates normally, thread-join! returns a value which is stored in the result field of thread. If thread terminates abnormally, thread-join! raises an exception which is stored in the result exception field of thread. It can be either a <terminated-thread-exception> or <uncaught-exception>.

If the timeout is reached, thread-join! returns timeout-val if given, or raises <join-timeout-exception>.

See Thread exceptions, for the details of these exceptions.

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9.32.3 Synchronization primitives


Builtin Class: <mutex>

A primitive synchronization device. It can take one of four states: locked/owned, locked/not-owned, unlocked/abandoned and unlocked/not-abandoned. A mutex can be locked (by mutex-lock!) only if it is in unlocked state. An ’owned’ mutex keeps a thread that owns it. Typically an owner thread is the one that locked the mutex, but you can make a thread other than the locking thread own a mutex. A mutex becomes unlocked either by mutex-unlock! or the owner thread terminates. In the former case, a mutex becomes unlocked/not-abandoned state. In the latter case, a mutex becomes unlocked/abandoned state.

A mutex has the following external slots.

Instance Variable of <mutex>: name

The name of the mutex.

Instance Variable of <mutex>: state

The state of the mutex. This is a read-only slot. See the description of mutex-state below.

Instance Variable of <mutex>: specific

A slot an application can keep arbitrary data. For example, an application can implement a ’recursive’ mutex using the specific field.

Function: mutex? obj

[SRFI-18], [SRFI-21] Returns #t if obj is a mutex, #f otherwise.

Function: make-mutex :optional name

[SRFI-18], [SRFI-21] Creates and returns a new mutex object. When created, the mutex is in unlocked/not-abandoned state. Optionally, you can give a name to the mutex.

Function: mutex-name mutex

[SRFI-18], [SRFI-21] Returns the name of the mutex.

Function: mutex-specific mutex
Function: mutex-specific-set! mutex value

[SRFI-18], [SRFI-21] Gets/sets the specific value of the mutex.

Function: mutex-state mutex

[SRFI-18], [SRFI-21] Returns the state of mutex, which may be one of the followings:

a thread

The mutex is locked/owned, and the owner is the returned thread.

symbol not-owned

The mutex is locked/not-owned.

symbol abandoned

The mutex is unlocked/abandoned.

symbol not-abandoned

The mutex is unlocked/not-abandoned.

Function: mutex-lock! mutex :optional timeout thread

[SRFI-18], [SRFI-21] Locks mutex. If mutex is in unlocked/not-abandoned state, this procedure changes its state to locked state exclusively. By default, mutex becomes locked/owned state, owned by the calling thread. You can give other owner thread as thread argument. If thread argument is given and #f, the mutex becomes locked/not-owned state.

If mutex is in unlocked/abandoned state, that is, some other thread has been terminated without unlocking it, this procedure signals ’abandoned mutex exception’ (see Thread exceptions) after changing the state of mutex.

If mutex is in locked state and timeout is omitted or #f, this procedure blocks until mutex becomes unlocked. If timeout is specified, mutex-lock! returns when the specified time reaches in case it couldn’t obtain a lock. You can give timeout an absolute point of time (by <time> object, see Time), or a relative time (by a real number).

Mutex-lock! returns #t if mutex is successfully locked, or #f if timeout reached.

Note that mutex itself doesn’t implements a ’recursive lock’ feature; that is, if a thread that has locked mutex tries to lock mutex again, the thread blocks. It is not difficult, however, to implement a recursive lock semantics on top of this mutex. The following example is taken from SRFI-18 document:

(define (mutex-lock-recursively! mutex)
  (if (eq? (mutex-state mutex) (current-thread))
      (let ((n (mutex-specific mutex)))
        (mutex-specific-set! mutex (+ n 1)))
        (mutex-lock! mutex)
        (mutex-specific-set! mutex 0))))

(define (mutex-unlock-recursively! mutex)
  (let ((n (mutex-specific mutex)))
    (if (= n 0)
        (mutex-unlock! mutex)
        (mutex-specific-set! mutex (- n 1)))))
Function: mutex-unlock! mutex :optional condition-variable timeout

[SRFI-18], [SRFI-21] Unlocks mutex. The state of mutex becomes unlocked/not-abandoned. It is allowed to unlock a mutex that is not owned by the calling thread.

If optional condition-variable is given, mutex-unlock! serves the "condition variable wait" operation (e.g. pthread_cond_wait in POSIX threads). The current thread atomically wait on condition-variable and unlocks mutex. The thread will be unblocked when other thread signals on condition-variable (see condition-variable-signal! and condition-variable-broadcast! below), or timeout reaches if it is supplied. The timeout argument can be either a <time> object to represent an absolute time point (see Time), a real number to represent a relative time in seconds, or #f which means never. The calling thread may be unblocked prematurely, so it should reacquire the lock of mutex and checks the condition, as in the following example (it is taken from SRFI-18 document):

(let loop ()
  (mutex-lock! m)
  (if (condition-is-true?)
        (mutex-unlock! m))
        (mutex-unlock! m cv)

The return value of mutex-unlock! is #f when it returns because of timeout, and #t otherwise.

Function: mutex-locker mutex
Function: mutex-unlocker mutex

Returns (lambda () (mutex-lock! mutex)) and (lambda () (mutex-unlock! mutex)), respectively. Each closure is created at most once per mutex, thus it is lighter than using literal lambda forms in a tight loop.

Function: with-locking-mutex mutex thunk

Calls thunk with locking a mutex mutex. This is defined as follows.

(define (with-locking-mutex mutex thunk)
   (mutex-locker mutex)
   (mutex-unlocker mutex)))

Condition variable

Builtin Class: <condition-variable>

A condition variable keeps a set of threads that are waiting for a certain condition to be true. When a thread modifies the state of the concerned condition, it can call condition-variable-signal! or condition-variable-broadcast!, which unblock one or more waiting threads so that they can check if the condition is satisfied.

A condition variable object has the following slots.

Instance Variable of <condition-variable>: name

The name of the condition variable.

Instance Variable of <condition-variable>: specific

A slot an application can keep arbitrary data.

Note that SRFI-18 doesn’t have a routine equivalent to pthreads’ pthread_cont_wait. If you want to wait on condition variable, you can pass a condition variable to mutex-unlock! as an optional argument (see above), then acquire mutex again by mutex-lock!. This design is for flexibility; see SRFI-18 document for the details.

This is the common usage of pthreads’ condition variable:

while (some_condition != TRUE) {
  pthread_cond_wait(condition_variable, mutex);

And it can be translated to SRFI-18 as follows:

(let loop ()
  (unless some-condition
    (mutex-unlock! mutex condition-variable)
    (mutex-lock! mutex)
Function: condition-variable? obj

[SRFI-18], [SRFI-21] Returns #t if obj is a condition variable, #f otherwise.

Function: make-condition-variable :optional name

[SRFI-18], [SRFI-21] Returns a new condition variable. You can give its name by optional name argument.

Function: condition-variable-name cv

[SRFI-18], [SRFI-21] Returns the name of the condition variable.

Function: condition-variable-specific cv
Function: condition-variable-specific-set! cv value

[SRFI-18], [SRFI-21] Gets/sets the specific value of the condition variable.

Function: condition-variable-signal! cv

[SRFI-18], [SRFI-21] If there are threads waiting on cv, causes the scheduler to select one of them and to make it runnable.

Function: condition-variable-broadcast! cv

[SRFI-18], [SRFI-21] Unblocks all the threads waiting on cv.


An atom is a convenient wrapper to make operations on a given set of objects thread-safe. Instead of defining thread-safe counterparts of every structure, you can easily wrap an existing data structures to make it thread-safe.

Function: atom val …

Creates and returns an atom object with val … as the initial values.

Function: atom? obj

Returns #t iff obj is an atom.

The following procedures can be used to atomically access and update the content of an atom. They commonly takes optional timeout and timeout-val arguments, both are defaulted to #f, which causes those procedures to block until they acquire a lock.

Those arguments can be used to modify the behavior when the lock cannot be acquired in timely manner. timeout may be a <time> object (see Time) to specify an absolute point of time, or a real number to specify the relative time in seconds. If timeout is expired, those procedures give up acquiring the lock, and the value given to timeout-val is returned.

Function: atom-ref atom :optional index timeout timeout-val

Returns index-th value of atom. See above for timeout and timeout-val arguments.

(define a (atom 'a 'b))

(atom-ref a 0) ⇒ a
(atom-ref a 1) ⇒ b
Function: atomic atom proc :optional timeout timeout-val

Calls proc with the current values in atom, while locking atom. proc must take as many arguments as the number of values atom has.

The returned value(s) of proc is the result of atomic, unless timeout occurs. See above for timeout and timeout-val arguments.

For example, the ref/count procedure in the following example counts the number of times the hashtable is referenced in thread-safe way.

(define a (atom (make-hash-table 'eq?) (list 0)))

(define (ref/count a key)
  (atomic a
   (lambda (ht count-cell)
     (inc! (car count-cell))
     (hash-table-get h key))))
Function: atomic-update! atom proc :optional timeout timeout-val

Calls proc with the current values in atom while locking atom, and updates the values in atom by the returned values from proc. proc must take as many arguments as the number of values atom has, and must return the same number of values.

The returned value(s) of proc is the result of atomic, unless timeout occurs. See above for timeout and timeout-val arguments.

The following example shows a thread-safe counter.

(define a (atom 0))

(atomic-update! a (cut + 1 <>))

Note: The term atom in historical Lisps meant objects that are not a cons cell (pair). Back then cons cells were the only aggregated datatype and there were few other datatypes (numbers and symbols), so having a complementary term to cells made sense.

Although it still appears in introductory Lisp tutorials, modern Lisps, including Scheme, has so many datatypes and it makes little sense to have a specific term for non-aggregate types.

Clojure adopted the term atom for thread-safe (atomic) primitive data, and we followed it.

Note: The constructor of atom is not make-atom but atom, following the convention of list/make-list, vector/make-vector, and string/make-string; that is, the name without make- takes its elements as variable number of arguments.

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9.32.4 Thread exceptions

Some types of exceptions may be thrown from thread-related procedures. These exceptions can be handled by Gauche’s exception mechanism (see Exceptions).

Builtin Class: <thread-exception>

A base class of thread-related exceptions. Inherits <exception> class. It has one slot.

Instance Variable of <thread-exception>: thread

A thread that threw this exception.

Builtin Class: <join-timeout-exception>

An exception thrown by thread-join! when a timeout reaches before the waited thread returns. Inherits <thread-exception>.

Builtin Class: <abandoned-mutex-exception>

An exception thrown by mutex-lock! when a mutex to be locked is in unlocked/abandoned state. Inherits <thread-exception>. It has one additional slot.

Instance Variable of <abandoned-mutex-exception>: mutex

A mutex that caused this exception.

Builtin Class: <terminated-thread-exception>

An exception thrown by thread-join! when the waited thread is terminated abnormally (by thread-terminate!). Inherits <thread-exception>. It has one additional slot.

Instance Variable of <terminated-thread-exception>: terminator

A thread that terminated the thread that causes this exception.

Builtin Class: <uncaught-exception>

An exception thrown by thread-join! when the waited thread is terminated by an uncaught exception. Inherits <thread-exception>. It has one additional slot.

Instance Variable of <uncaught-exception>: reason

An exception that caused the termination of the thread.

Function: join-timeout-exception? obj
Function: abandoned-mutex-exception? obj
Function: terminated-thread-exception? obj
Function: uncaught-exception? obj

[SRFI-18], [SRFI-21] These procedures checks if obj is a certain type of exception. Provided for the compatibility to SRFI-18.

Function: uncaught-exception-reason exc

[SRFI-18], [SRFI-21] Returns the value of reason slot of <uncaught-exception> object. Provided for the compatibility to SRFI-18.

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