Threads(3) Tcl Library Procedures Threads(3)
Tcl_ConditionNotify, Tcl_ConditionWait, Tcl_ConditionFinalize, Tcl_GetThreadData,
Tcl_MutexLock, Tcl_MutexUnlock, Tcl_MutexFinalize, Tcl_CreateThread, Tcl_JoinThread - Tcl
Tcl_ConditionWait(condPtr, mutexPtr, timePtr)
Tcl_CreateThread(idPtr, threadProc, clientData, stackSize, flags)
Tcl_Condition *condPtr (in) A condition variable, which must be associ-
ated with a mutex lock.
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 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 ini-
tialized to zero the first time each thread
Tcl_ThreadId *idPtr (out) The referred storage will contain the id of
the newly created thread as returned by the
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 argu-
ment to the threadProc.
int stackSize (in) The size of the stack given to the new
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.
Beginning with the 8.1 release, the Tcl core is thread safe, which allows you to incorpo-
rate 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
configure 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
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 speci-
fied 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 currently 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 informa-
tion 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 possi-
ble, 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-private data. Its argu-
ment 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 multi-
threaded applications. See the Notifier manual page for more information on these proce-
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 com-
piling 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_ConditionWait. 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 condition variable unblocks all
threads waiting on the condition variable, but they do not proceed until the mutex is
released with Tcl_MutexUnlock. The implementation of Tcl_ConditionWait automatically
locks the mutex before returning.
The caller of Tcl_ConditionWait should be prepared for spurious notifications by calling
Tcl_ConditionWait within a while loop that tests some invariant.
A condition variable can be destroyed after its use by calling Tcl_ConditionFinalize.
The Tcl_ConditionNotify, Tcl_ConditionWait and Tcl_ConditionFinalize procedures are
defined as empty macros if not compiling with threads enabled.
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_MutexFinalize or Tcl_ConditionFinalize. Thread local storage is reclaimed
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.
Tcl_GetCurrentThread(3), Tcl_ThreadQueueEvent(3), Tcl_ThreadAlert(3), Tcl_ExitThread(3),
Tcl_FinalizeThread(3), Tcl_CreateThreadExitHandler(3), Tcl_DeleteThreadExitHandler(3),
thread, mutex, condition variable, thread local storage
Tcl 8.1 Threads(3)