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Threads(3)		    Tcl Library Procedures		    Threads(3)

______________________________________________________________________________

NAME
       Tcl_ConditionNotify, Tcl_ConditionWait, Tcl_ConditionFinalize, Tcl_Get‐
       ThreadData, Tcl_MutexLock, Tcl_MutexUnlock, Tcl_MutexFinalize, Tcl_Cre‐
       ateThread, Tcl_JoinThread - Tcl thread support.

SYNOPSIS
       #include <tcl.h>

       void
       Tcl_ConditionNotify(condPtr)

       void
       Tcl_ConditionWait(condPtr, mutexPtr, timePtr)

       void
       Tcl_ConditionFinalize(condPtr)

       Void *
       Tcl_GetThreadData(keyPtr, size)

       void
       Tcl_MutexLock(mutexPtr)

       void
       Tcl_MutexUnlock(mutexPtr)

       void
       Tcl_MutexFinalize(mutexPtr)

       int
       Tcl_CreateThread(idPtr, threadProc, clientData, stackSize, flags)

       int
       Tcl_JoinThread(id, result)

ARGUMENTS
       Tcl_Condition	   *condPtr  (in)      A   condition  variable,	 which
					       must be associated with a mutex
					       lock.

       Tcl_Condition	   *mutexPtr (in)      A mutex lock.

       Tcl_Time		   *timePtr  (in)      A  time	limit on the condition
					       wait.  NULL  to	wait  forever.
					       Note  that a polling value of 0
					       seconds	 doesn't   make	  much
					       sense.

       Tcl_ThreadDataKey   *keyPtr   (in)      This   identifies  a  block  of
					       thread local storage.  The  key
					       should  be  static and process-
					       wide, yet each thread will  end
					       up   associating	  a  different
					       block of storage with this key.

       int		   *size     (in)      The size of  the	 thread	 local
					       storage	block.	This amount of
					       data is allocated and  initial‐
					       ized  to	 zero  the  first time
					       each  thread   calls   Tcl_Get‐
					       ThreadData.

       Tcl_ThreadId	   *idPtr    (out)     The  referred storage will con‐
					       tain the id of the  newly  cre‐
					       ated  thread as returned by the
					       operating system.

       Tcl_ThreadId	   id	     (in)      Id of the thread waited upon.

       Tcl_ThreadCreateProc	     threadProc(in)
					       This procedure will act as  the
					       main()  of  the	newly  created
					       thread. The  specified  client‐
					       Data will be its sole argument.

       ClientData	   clientData(in)      Arbitrary  information.	Passed
					       as sole argument to the thread‐
					       Proc.

       int		   stackSize (in)      The  size of the stack given to
					       the new thread.

       int		   flags     (in)      Bitmask containing flags allow‐
					       ing the caller to modify behav‐
					       iour of the new thread.

       int		   *result   (out)     The referred storage is used to
					       place  the  exit	 code  of  the
					       thread waited upon into it.
_________________________________________________________________

INTRODUCTION
       Beginning with the 8.1 release, the Tcl	core  is  thread  safe,	 which
       allows  you  to incorporate Tcl into multithreaded applications without
       customizing the Tcl core.  To enable Tcl	 multithreading	 support,  you
       must  include the --enable-threads option to configure when you config‐
       ure and compile your Tcl core.

       An important constraint of the Tcl threads implementation is that  only
       the thread that created a Tcl interpreter can use that interpreter.  In
       other words, multiple threads can not access the same Tcl  interpreter.
       (However,  as  was  the	case in previous releases, a single thread can
       safely create and use multiple interpreters.)

       Tcl does provide Tcl_CreateThread for creating threads. The caller  can │
       determine  the size of the stack given to the new thread and modify the │
       behaviour     through	 the	 supplied     flags.	 The	 value │
       TCL_THREAD_STACK_DEFAULT	 for  the stackSize indicates that the default │
       size as specified by the operating system is to be  used	 for  the  new │
       thread.	 As   for   the	  flags,   currently   are   only  the	values │
       TCL_THREAD_NOFLAGS and TCL_THREAD_JOINABLE defined. The first  of  them │
       invokes	the  default  behaviour	 with no specialties. Using the second │
       value marks the new thread as joinable. This means that another	thread │
       can wait for the such marked thread to exit and join it.		       │

       Restrictions: On some unix systems the pthread-library does not contain │
       the functionality to specify the stacksize of a thread.	The  specified │
       value  for  the stacksize is ignored on these systems. Both Windows and │
       Macintosh currently do not support joinable threads. This flag value is │
       therefore ignored on these platforms.

       Tcl  does provide Tcl_ExitThread and Tcl_FinalizeThread for terminating
       threads and  invoking  optional	per-thread  exit  handlers.   See  the
       Tcl_Exit page for more information on these procedures.

       The  Tcl_JoinThread  function is provided to allow threads to wait upon │
       the exit of another thread, which must have  been  marked  as  joinable │
       through	usage  of the TCL_THREAD_JOINABLE-flag during its creation via │
       Tcl_CreateThread.						       │

       Trying to wait for the exit of a non-joinable thread or a thread	 which │
       is  already waited upon will result in an error. Waiting for a joinable │
       thread which already exited is possible, the  system  will  retain  the │
       necessary  information  until  after  the call to Tcl_JoinThread.  This │
       means that not calling Tcl_JoinThread for a joinable thread will	 cause │
       a memory leak.

       Tcl  provides  Tcl_ThreadQueueEvent  and	 Tcl_ThreadAlert  for handling
       event queueing in multithreaded applications.  See the Notifier	manual
       page for more information on these procedures.

       In this release, the Tcl language itself provides no support for creat‐
       ing multithreaded scripts (for example, scripts that could spawn a  Tcl
       interpreter  in a separate thread).  If you need to add this feature at
       this time, see the tclThreadTest.c file in the Tcl source  distribution
       for an experimental implementation of a Tcl "Thread" package implement‐
       ing thread creation and management commands at the script level.

DESCRIPTION
       A mutex is a lock that is used to serialize all threads through a piece
       of  code	 by  calling Tcl_MutexLock and Tcl_MutexUnlock.	 If one thread
       holds a mutex, any other thread calling Tcl_MutexLock will block	 until
       Tcl_MutexUnlock	is  called.  A mutex can be destroyed after its use by │
       calling Tcl_MutexFinalize.  The result of locking a  mutex  twice  from │
       the  same  thread  is undefined.	 On some platforms it will result in a │
       deadlock.  The  Tcl_MutexLock,  Tcl_MutexUnlock	and  Tcl_MutexFinalize
       procedures  are	defined	 as empty macros if not compiling with threads
       enabled.

       A condition variable is used as a signaling  mechanism:	a  thread  can
       lock  a mutex and then wait on a condition variable with Tcl_Condition‐
       Wait.  This atomically releases the mutex lock and blocks  the  waiting
       thread  until  another thread calls Tcl_ConditionNotify.	 The caller of
       Tcl_ConditionNotify should have the associated mutex held by previously
       calling	Tcl_MutexLock, but this is not enforced.  Notifying the condi‐
       tion variable unblocks all threads waiting on the  condition  variable,
       but  they  do not proceed until the mutex is released with Tcl_MutexUn‐
       lock.  The implementation of Tcl_ConditionWait automatically locks  the
       mutex before returning.

       The caller of Tcl_ConditionWait should be prepared for spurious notifi‐
       cations by calling Tcl_ConditionWait within a  while  loop  that	 tests
       some invariant.

       A condition variable can be destroyed after its use by calling Tcl_Con‐ │
       ditionFinalize.							       │

       The Tcl_ConditionNotify,	 Tcl_ConditionWait  and	 Tcl_ConditionFinalize │
       procedures  are	defined	 as empty macros if not compiling with threads │
       enabled.

       The Tcl_GetThreadData call returns a pointer to a block of  thread-pri‐
       vate  data.   Its argument is a key that is shared by all threads and a
       size for the block of storage.  The storage is automatically  allocated
       and  initialized	 to  all zeros the first time each thread asks for it.
       The storage is automatically deallocated by Tcl_FinalizeThread.

INITIALIZATION
       All of these synchronization objects are self initializing.   They  are
       implemented as opaque pointers that should be NULL upon first use.  The
       mutexes and condition variables are either cleaned up by	 process  exit │
       handlers	 (if  living that long) or explicitly by calls to Tcl_MutexFi‐ │
       nalize or Tcl_ConditionFinalize.	 Thread	 local	storage	 is  reclaimed
       during Tcl_FinalizeThread.

CREATING THREADS
       The  API	 to  create  threads is not finalized at this time.  There are
       private facilities to create threads that  contain  a  new  Tcl	inter‐
       preter,	and  to send scripts among threads.  Dive into tclThreadTest.c
       and tclThread.c for examples.

SEE ALSO
       Tcl_GetCurrentThread, Tcl_ThreadQueueEvent, Tcl_ThreadAlert,  Tcl_Exit‐
       Thread,	       Tcl_FinalizeThread,	  Tcl_CreateThreadExitHandler,
       Tcl_DeleteThreadExitHandler

KEYWORDS
       thread, mutex, condition variable, thread local storage

Tcl				      8.1			    Threads(3)
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