Manual Reference Pages - TCL_GETTHREADDATA (3)
Tcl_ConditionNotify, Tcl_ConditionWait, Tcl_ConditionFinalize, Tcl_GetThreadData, Tcl_MutexLock, Tcl_MutexUnlock, Tcl_MutexFinalize, Tcl_CreateThread, 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 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 initialized 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 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.
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 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 multiple interpreters.)
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 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 information on these
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-private 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_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
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
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.
Tcl_GetCurrentThread(3), Tcl_ThreadQueueEvent(3), Tcl_ThreadAlert(3),
Tcl_ExitThread(3), Tcl_FinalizeThread(3), Tcl_CreateThreadExitHandler(3),
thread, mutex, condition variable, thread local storage
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