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.
Tcl_ConditionWait(condPtr, mutexPtr, timePtr)
Tcl_CreateThread(idPtr, threadProc, clientData, stackSize, flags)
Tcl_Condition *condPtr (in) A condition variable, which
must be associated with a mutex
Tcl_Mutex *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
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‐
Tcl_ThreadId *idPtr (out) The referred storage will con‐
tain the id of the newly cre‐
ated thread as returned by the
Tcl_ThreadId id (in) Id of the thread waited upon.
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‐
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.
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 │
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 or use the Tcl "Threading Extension"
package implementing thread creation and management commands at the
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
A condition variable can be destroyed after its use by calling Tcl_Con‐ │
The Tcl_ConditionNotify, Tcl_ConditionWait and Tcl_ConditionFinalize │
procedures are defined as empty macros if not compiling with threads │
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.
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
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.
Tcl_GetCurrentThread, Tcl_ThreadQueueEvent, Tcl_ThreadAlert, Tcl_Exit‐
Thread, Tcl_FinalizeThread, Tcl_CreateThreadExitHandler,
thread, mutex, condition variable, thread local storage
Tcl 8.1 Threads(3)