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thread(n) thread(n) 





NAME


thread - Extension for script access to Tcl threading






SYNOPSIS


package require Tcl 8.4


package require Thread ?2.8?


thread::create ?-joinable? ?-preserved? ?script?


thread::preserve ?id?


thread::release ?-wait? ?id?


thread::id


thread::errorproc ?procname?


thread::cancel ?-unwind? id ?result?


thread::unwind


thread::exit ?status?


thread::names


thread::exists id


thread::send ?-async? ?-head? id script
    ?varname?


thread::broadcast script


thread::wait


thread::eval ?-lock mutex? arg ?arg ...?


thread::join id


thread::configure id ?option? ?value? ?...?


thread::transfer id channel


thread::detach channel


thread::attach channel


thread::mutex


thread::mutex create ?-recursive?


thread::mutex destroy mutex


thread::mutex lock mutex


thread::mutex unlock mutex


thread::rwmutex


thread::rwmutex create


thread::rwmutex destroy mutex


thread::rwmutex rlock mutex


thread::rwmutex wlock mutex


thread::rwmutex unlock mutex


thread::cond


thread::cond create


thread::cond destroy cond


thread::cond notify cond


thread::cond wait cond mutex ?ms?










DESCRIPTION


The thread extension creates threads that contain Tcl
    interpreters, and it lets you send scripts to those threads for evaluation.
    Additionally, it provides script-level access to basic thread
    synchronization primitives, like mutexes and condition variables.






COMMANDS


This section describes commands for creating and destroying
    threads and sending scripts to threads for evaluation.



  

  
This command creates a thread that contains a Tcl interpreter. The Tcl
      interpreter either evaluates the optional script, if specified, or
      it waits in the event loop for scripts that arrive via the
      thread::send command. Both of them would take place simultaneously
      with the return of command thread::create to the caller thread.
      Neither the caller is waiting for the finishing of optional script,
      nor the result, if any, of the script is returned to the caller.
      The result of thread::create is the ID of the thread. This is the
      opaque handle which identifies the newly created thread for all other
      package commands. The handle of the thread goes out of scope automatically
      when thread is marked for exit (see the thread::release command
      below).
    
If the optional script argument contains the
        thread::wait command the thread will enter into the event loop.
        If such command is not found in the script the thread will run
        the script to the end and exit. In that case, the handle may be
        safely ignored since it refers to a thread which does not exists any
        more at the time when the command returns.

    
Using flag -joinable it is possible to create a
        joinable thread, i.e. one upon whose exit can be waited upon by using
        thread::join command. Note that failure to join a thread created
        with -joinable flag results in resource and memory leaks.

    
Threads created by the thread::create cannot be
        destroyed forcefully. Consequently, there is no corresponding thread
        destroy command. A thread may only be released using the
        thread::release and if its internal reference count drops to
        zero, the thread is marked for exit. This kicks the thread out of the
        event loop servicing and the thread continues to execute commands passed
        in the script argument, following the thread::wait
        command. If this was the last command in the script, as usually the
        case, the thread will exit.

    
It is possible to create a situation in which it may be
        impossible to terminate the thread, for example by putting some endless
        loop after the thread::wait or entering the event loop again by
        doing an vwait-type of command. In such cases, the thread may never
        exit. This is considered to be a bad practice and should be avoided if
        possible. This is best illustrated by the example below:

  








    # You should never do ...


    set tid [thread::create {


        package require Http


        thread::wait


        vwait forever ; # <-- this!


    }]






  

  
The thread created in the above example will never be able to exit. After
      it has been released with the last matching thread::release call,
      the thread will jump out of the thread::wait and continue to
      execute commands following. It will enter vwait command and wait
      endlessly for events. There is no way one can terminate such thread, so
      you wouldn't want to do this!
    
Each newly created has its internal reference counter set to 0
        (zero), i.e. it is unreserved. This counter gets incremented by a call
        to thread::preserve and decremented by a call to
        thread::release command. These two commands implement simple but
        effective thread reservation system and offer predictable and
        controllable thread termination capabilities. It is however possible to
        create initially preserved threads by using flag -preserved of
        the thread::create command. Threads created with this flag have
        the initial value of the reference counter of 1 (one), and are thus
        initially marked reserved.

  





  

  
This command increments the thread reference counter. Each call to this
      command increments the reference counter by one (1). Command returns the
      value of the reference counter after the increment. If called with the
      optional thread id, the command preserves the given thread.
      Otherwise the current thread is preserved.
    
With reference counting, one can implement controlled access
        to a shared Tcl thread. By incrementing the reference counter, the
        caller signalizes that he/she wishes to use the thread for a longer
        period of time. By decrementing the counter, caller signalizes that
        he/she has finished using the thread.

  

  

  
This command decrements the thread reference counter. Each call to this
      command decrements the reference counter by one (1). If called with the
      optional thread id, the command releases the given thread.
      Otherwise, the current thread is released. Command returns the value of
      the reference counter after the decrement. When the reference counter
      reaches zero (0), the target thread is marked for termination. You should
      not reference the thread after the thread::release command returns
      zero or negative integer. The handle of the thread goes out of scope and
      should not be used any more. Any following reference to the same thread
      handle will result in Tcl error.
    
Optional flag -wait instructs the caller thread to wait
        for the target thread to exit, if the effect of the command would result
        in termination of the target thread, i.e. if the return result would be
        zero (0). Without the flag, the caller thread does not wait for the
        target thread to exit. Care must be taken when using the -wait,
        since this may block the caller thread indefinitely. This option has
        been implemented for some special uses of the extension and is
        deprecated for regular use. Regular users should create joinable threads
        by using the -joinable option of the thread::create
        command and the thread::join to wait for thread to exit.

  

  

  
This command returns the ID of the current thread.

  

  
This command sets a handler for errors that occur in scripts sent
      asynchronously, using the -async flag of the thread::send
      command, to other threads. If no handler is specified, the current handler
      is returned. The empty string resets the handler to default (unspecified)
      value. An uncaught error in a thread causes an error message to be sent to
      the standard error channel. This default reporting scheme can be changed
      by registering a procedure which is called to report the error. The
      procname is called in the interpreter that invoked the
      thread::errorproc command. The procname is called like
    this:








    myerrorproc thread_id errorInfo






  

  
This command requires Tcl version 8.6 or higher.
    
Cancels the script being evaluated in the thread given by the
        id parameter. Without the -unwind switch the evaluation
        stack for the interpreter is unwound until an enclosing catch command is
        found or there are no further invocations of the interpreter left on the
        call stack. With the -unwind switch the evaluation stack for the
        interpreter is unwound without regard to any intervening catch command
        until there are no further invocations of the interpreter left on the
        call stack. If result is present, it will be used as the error
        message string; otherwise, a default error message string will be
      used.

  

  

  
Use of this command is deprecated in favour of more advanced thread
      reservation system implemented with thread::preserve and
      thread::release commands. Support for thread::unwind command
      will disappear in some future major release of the extension.
    
This command stops a prior thread::wait command.
        Execution of the script passed to newly created thread will continue
        from the thread::wait command. If thread::wait was the
        last command in the script, the thread will exit. The command returns
        empty result but may trigger Tcl error with the message "target
        thread died" in some situations.

  

  

  
Use of this command is deprecated in favour of more advanced thread
      reservation system implemented with thread::preserve and
      thread::release commands. Support for thread::exit command
      will disappear in some future major release of the extension.
    
This command forces a thread stuck in the thread::wait
        command to unconditionally exit. The thread's exit status defaults to
        666 and can be specified using the optional status argument. The
        execution of thread::exit command is guaranteed to leave the
        program memory in the inconsistent state, produce memory leaks and
        otherwise affect other subsystem(s) of the Tcl application in an
        unpredictable manner. The command returns empty result but may trigger
        Tcl error with the message "target thread died" in some
        situations.

  

  

  
This command returns a list of thread IDs. These are only for threads that
      have been created via thread::create command. If your application
      creates other threads at the C level, they are not reported by this
      command.

  

  
Returns true (1) if thread given by the id parameter exists, false
      (0) otherwise. This applies only for threads that have been created via
      thread::create command.

  

  
This command passes a script to another thread and, optionally,
      waits for the result. If the -async flag is specified, the command
      does not wait for the result and it returns empty string. The target
      thread must enter it's event loop in order to receive scripts sent via
      this command. This is done by default for threads created without a
      startup script. Threads can enter the event loop explicitly by calling
      thread::wait or any other relevant Tcl/Tk command, like
      update, vwait, etc.
    
Optional varname specifies name of the variable to
        store the result of the script. Without the -async flag,
        the command returns the evaluation code, similarly to the standard Tcl
        catch command. If, however, the -async flag is specified,
        the command returns immediately and caller can later vwait on
        ?varname? to get the result of the passed script

  








    set t1 [thread::create]


    set t2 [thread::create]


    thread::send -async $t1 "set a 1" result


    thread::send -async $t2 "set b 2" result


    for {set i 0} {$i < 2} {incr i} {


        vwait result


    }






  

  
In the above example, two threads were fed work and both of them were
      instructed to signalize the same variable "result" in the
      calling thread. The caller entered the event loop twice to get both
      results. Note, however, that the order of the received results may vary,
      depending on the current system load, type of work done, etc, etc.
    
Many threads can simultaneously send scripts to the target
        thread for execution. All of them are entered into the event queue of
        the target thread and executed on the FIFO basis, intermingled with
        optional other events pending in the event queue of the target thread.
        Using the optional ?-head? switch, scripts posted to the thread's event
        queue can be placed on the head, instead on the tail of the queue, thus
        being executed in the LIFO fashion.

  





  

  
This command passes a script to all threads created by the package
      for execution. It does not wait for response from any of the threads.

  

  
This enters the event loop so a thread can receive messages from the
      thread::send command. This command should only be used within the
      script passed to the thread::create. It should be the very last
      command in the script. If this is not the case, the exiting thread will
      continue executing the script lines past the thread::wait which is
      usually not what you want and/or expect.








    set t1 [thread::create {


        #


        # Do some initialization work here


        #


        thread::wait ; # Enter the event loop


    }]






  

  
This command concatenates passed arguments and evaluates the resulting
      script under the mutex protection. If no mutex is specified by using the
      ?-lock mutex? optional argument, the internal static mutex is used.

  

  
This command waits for the thread with ID id to exit and then
      returns it's exit code. Errors will be returned for threads which are not
      joinable or already waited upon by another thread. Upon the join the
      handle of the thread has gone out of scope and should not be used any
      more.

  

  
This command configures various low-level aspects of the thread with ID
      id in the similar way as the standard Tcl command fconfigure
      configures some Tcl channel options. Options currently supported are:
      -eventmark and -unwindonerror.
    
When -eventmark is provided with a value greater than 0
        (zero), that value is the maximum number of asynchronously posted
        scripts that may be pending for the thread. thread::send -async
        blocks until the number of pending scripts in the event loop drops below
        the -eventmark value.

    
When -unwindonerror is provided with a value of true,
        an error result in a script causes the thread to unwind, making it
        unavailable to evaluate additional scripts.

  

  

  
This moves the specified channel from the current thread and
      interpreter to the main interpreter of the thread with the given
      id. After the move the current interpreter has no access to the
      channel any more, but the main interpreter of the target thread will be
      able to use it from now on. The command waits until the other thread has
      incorporated the channel. Because of this it is possible to deadlock the
      participating threads by commanding the other through a synchronous
      thread::send to transfer a channel to us. This easily extends into
      longer loops of threads waiting for each other. Other restrictions: the
      channel in question must not be shared among multiple interpreters running
      in the sending thread. This automatically excludes the special channels
      for standard input, output and error.
    
Due to the internal Tcl core implementation and the
        restriction on transferring shared channels, one has to take extra
        measures when transferring socket channels created by accepting the
        connection out of the socket commands callback procedures:

  








    socket -server _Accept 2200


    proc _Accept {s ipaddr port} {


        after idle [list Accept $s $ipaddr $port]


    }


    proc Accept {s ipaddr port} {


        set tid [thread::create]


        thread::transfer $tid $s


    }






  

  
This detaches the specified channel from the current thread and
      interpreter. After that, the current interpreter has no access to the
      channel any more. The channel is in the parked state until some other (or
      the same) thread attaches the channel again with thread::attach.
      Restrictions: same as for transferring shared channels with the
      thread::transfer command.

  

  
This attaches the previously detached channel in the current
      thread/interpreter. For already existing channels, the command does
      nothing, i.e. it is not an error to attach the same channel more than
      once. The first operation will actually perform the operation, while all
      subsequent operation will just do nothing. Command throws error if the
      channel cannot be found in the list of detached channels and/or in
      the current interpreter.

  

  
Mutexes are most common thread synchronization primitives. They are used
      to synchronize access from two or more threads to one or more shared
      resources. This command provides script-level access to exclusive and/or
      recursive mutexes. Exclusive mutexes can be locked only once by one
      thread, while recursive mutexes can be locked many times by the same
      thread. For recursive mutexes, number of lock and unlock operations must
      match, otherwise, the mutex will never be released, which would lead to
      various deadlock situations.
    
Care has to be taken when using mutexes in an multithreading
        program. Improper use of mutexes may lead to various deadlock
        situations, especially when using exclusive mutexes.

    
The thread::mutex command supports following
        subcommands and options:

  







  

  
Creates the mutex and returns it's opaque handle. This handle should be
      used for any future reference to the newly created mutex. If no optional
      ?-recursive? argument was specified, the command creates the exclusive
      mutex. With the ?-recursive? argument, the command creates a recursive
      mutex.

  

  
Destroys the mutex. Mutex should be in unlocked state before the
      destroy attempt. If the mutex is locked, the command will throw Tcl
    error.

  

  
Locks the mutex. Locking the exclusive mutex may throw Tcl error if
      on attempt to lock the same mutex twice from the same thread. If your
      program logic forces you to lock the same mutex twice or more from the
      same thread (this may happen in recursive procedure invocations) you
      should consider using the recursive mutexes.

  

  
Unlocks the mutex so some other thread may lock it again. Attempt
      to unlock the already unlocked mutex will throw Tcl error.









  

  
This command creates many-readers/single-writer mutexes. Reader/writer
      mutexes allow you to serialize access to a shared resource more optimally.
      In situations where a shared resource gets mostly read and seldom
      modified, you might gain some performance by using reader/writer mutexes
      instead of exclusive or recursive mutexes.
    
For reading the resource, thread should obtain a read lock on
        the resource. Read lock is non-exclusive, meaning that more than one
        thread can obtain a read lock to the same resource, without waiting on
        other readers. For changing the resource, however, a thread must obtain
        a exclusive write lock. This lock effectively blocks all threads from
        gaining the read-lock while the resource is been modified by the writer
        thread. Only after the write lock has been released, the resource may be
        read-locked again.

    
The thread::rwmutex command supports following
        subcommands and options:

  







  

  
Creates the reader/writer mutex and returns it's opaque handle. This
      handle should be used for any future reference to the newly created
    mutex.

  

  
Destroys the reader/writer mutex. If the mutex is already locked,
      attempt to destroy it will throw Tcl error.

  

  
Locks the mutex for reading. More than one thread may read-lock the
      same mutex at the same time.

  

  
Locks the mutex for writing. Only one thread may write-lock the
      same mutex at the same time. Attempt to write-lock same
      mutex twice from the same thread will throw Tcl error.

  

  
Unlocks the mutex so some other thread may lock it again. Attempt
      to unlock already unlocked mutex will throw Tcl error.









  

  
This command provides script-level access to condition variables. A
      condition variable creates a safe environment for the program to test some
      condition, sleep on it when false and be awakened when it might have
      become true. A condition variable is always used in the conjunction with
      an exclusive mutex. If you attempt to use other type of mutex in
      conjunction with the condition variable, a Tcl error will be thrown.
    
The command supports following subcommands and options:

  







  

  
Creates the condition variable and returns it's opaque handle. This handle
      should be used for any future reference to newly created condition
      variable.

  

  
Destroys condition variable cond. Extreme care has to be taken that
      nobody is using (i.e. waiting on) the condition variable, otherwise
      unexpected errors may happen.

  

  
Wakes up all threads waiting on the condition variable cond.

  

  
This command is used to suspend program execution until the condition
      variable cond has been signalled or the optional timer has expired.
      The exclusive mutex must be locked by the calling thread on
      entrance to this command. If the mutex is not locked, Tcl error is thrown.
      While waiting on the cond, the command releases mutex.
      Before returning to the calling thread, the command re-acquires the
      mutex again. Unlocking the mutex and waiting on the
      condition variable cond is done atomically.
    
The ms command option, if given, must be an integer
        specifying time interval in milliseconds the command waits to be
        signalled. Otherwise the command waits on condition notify forever.

    
In multithreading programs, there are many situations where a
        thread has to wait for some event to happen until it is allowed to
        proceed. This is usually accomplished by repeatedly testing a condition
        under the mutex protection and waiting on the condition variable until
        the condition evaluates to true:

  








    set mutex [thread::mutex create]


    set cond  [thread::cond  create]


    thread::mutex lock $mutex


    while {<some_condition_is_true>} {


        thread::cond wait $cond $mutex


    }


    # Do some work under mutex protection


    thread::mutex unlock $mutex






  

  
Repeated testing of the condition is needed since the condition variable
      may get signalled without the condition being actually changed (spurious
      thread wake-ups, for example).










DISCUSSION


The fundamental threading model in Tcl is that there can be one or
    more Tcl interpreters per thread, but each Tcl interpreter should only be
    used by a single thread which created it. A "shared memory"
    abstraction is awkward to provide in Tcl because Tcl makes assumptions about
    variable and data ownership. Therefore this extension supports a simple form
    of threading where the main thread can manage several background, or
    "worker" threads. For example, an event-driven server can pass
    requests to worker threads, and then await responses from worker threads or
    new client requests. Everything goes through the common Tcl event loop, so
    message passing between threads works naturally with event-driven I/O,
    vwait on variables, and so forth. For the transfer of bulk
    information it is possible to move channels between the threads.


For advanced multithreading scripts, script-level access to two
    basic synchronization primitives, mutex and condition variables, is also
    supported.






SEE ALSO


http://www.tcl.tk/doc/howto/thread_model.html, tpool, tsv,
    ttrace






KEYWORDS


events, message passing, mutex, synchronization, thread







  

    2.8
    Tcl Threading