Threads(3) Tcl Library Procedures Threads(3)
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NAME
Tcl_ConditionNotify, Tcl_ConditionWait, Tcl_ConditionFinalize, Tcl_GetThreadData, Tcl_MutexLock,
Tcl_MutexUnlock, Tcl_MutexFinalize, Tcl_CreateThread, 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 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 thread-Proc. threadProc.
Proc.
int stackSize (in) The size of the stack given to the new thread.
int flags (in) Bitmask containing flags allowing the caller to modify behav-iour behaviour
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.
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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 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, 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 oper- |
ating 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 cre- |
ation 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 creating 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 implementing 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_MutexFi-nalize Tcl_MutexFinalize
nalize 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_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_Condition-Wait Tcl_ConditionWait
Wait automatically locks the mutex before returning.
The caller of Tcl_ConditionWait should be prepared for spurious notifications by calling Tcl_Condi-tionWait Tcl_ConditionWait
tionWait 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.
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 auto-matically automatically
matically 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_MutexFinalize or Tcl_Condi- |
tionFinalize. 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 interpreter, and to send scripts among threads. Dive into tclThread-Test.c tclThreadTest.c
Test.c and tclThread.c for examples.
SEE ALSO
Tcl_GetCurrentThread, Tcl_ThreadQueueEvent, Tcl_ThreadAlert, Tcl_ExitThread, Tcl_FinalizeThread,
Tcl_CreateThreadExitHandler, Tcl_DeleteThreadExitHandler
KEYWORDS
thread, mutex, condition variable, thread local storage
Tcl 8.1 Threads(3)
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