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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_Mutex *mutexPtr (in)			     A mutex lock.

       Tcl_Time *timePtr (in)			     A	time limit on the con‐
						     dition  wait.   NULL   to
						     wait  forever.  Note that
						     a polling value of 0 sec‐
						     onds  does	 not 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 initialized
						     to	 zero  the  first time
						     each     thread	 calls
						     Tcl_GetThreadData.

       Tcl_ThreadId *idPtr (out)		     The referred storage will
						     contain  the  id  of  the
						     newly  created  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 clientData will
						     be its sole argument.

       ClientData clientData (in)		     Arbitrary	  information.
						     Passed  as	 sole argument
						     to the threadProc.

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

       int flags (in)				     Bitmask  containing flags
						     allowing  the  caller  to
						     modify  behaviour	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, a single thread can safely create  and	 use  multiple	inter‐
       preters.)

DESCRIPTION
       Tcl  provides  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 only the values  TCL_THREAD_NOFLAGS
       and  TCL_THREAD_JOINABLE	 are  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 stack size of a thread. The specified
       value for the stack size is ignored on  these  systems.	 Windows  cur‐
       rently  does not support joinable threads. This flag value is therefore
       ignored on this platform.

       Tcl provides the Tcl_ExitThread and  Tcl_FinalizeThread	functions  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.

       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.

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

       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.	 For declaration of mutexes the TCL_DECLARE_MUTEX macro should
       be  used.  This macro assures correct mutex handling even when the core
       is compiled without 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.

   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.

SCRIPT-LEVEL ACCESS TO THREADS
       Tcl provides no built-in commands for scripts to use to create, manage, │
       or join threads, nor any script-level  access  to  mutex	 or  condition │
       variables.   It	provides  such	facilities  only via C interfaces, and │
       leaves it up to packages to expose these matters to the	script	level. │
       One such package is the Thread package.

SEE ALSO
       Tcl_GetCurrentThread(3),	 Tcl_ThreadQueueEvent(3),  Tcl_ThreadAlert(3),
       Tcl_ExitThread(3),      Tcl_FinalizeThread(3),	   Tcl_CreateThreadEx‐
       itHandler(3), Tcl_DeleteThreadExitHandler(3), Thread

KEYWORDS
       thread, mutex, condition variable, thread local storage

Tcl				      8.1			    Threads(3)
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