Provided by: tcl8.6-doc_8.6.12+dfsg-1build1_all 
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, proc, 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.
const 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 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 *proc (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 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 behavior 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.
Starting with the 8.6 release, Tcl multithreading support is on by default. To disable Tcl
multithreading support, you must include the --disable-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.)
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 behavior 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 behavior with no special settings. 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 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-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 Tcl_ConditionFinalize.
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_MutexFinalize 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.
EXAMPLE
To create a thread with portable code, its implementation function should be declared as
follows:
static Tcl_ThreadCreateProc MyThreadImplFunc;
It should then be defined like this example, which just counts up to a given value and
then finishes.
static Tcl_ThreadCreateType
MyThreadImplFunc(
ClientData clientData)
{
int i, limit = (int) clientData;
for (i=0 ; i<limit ; i++) {
/* doing nothing at all here */
}
TCL_THREAD_CREATE_RETURN;
}
To create the above thread, make it execute, and wait for it to finish, we would do this:
int limit = 1000000000;
ClientData limitData = (void*)((intptr_t) limit);
Tcl_ThreadId id; /* holds identity of thread created */
int result;
if (Tcl_CreateThread(&id, MyThreadImplFunc, limitData,
TCL_THREAD_STACK_DEFAULT,
TCL_THREAD_JOINABLE) != TCL_OK) {
/* Thread did not create correctly */
return;
}
/* Do something else for a while here */
if (Tcl_JoinThread(id, &result) != TCL_OK) {
/* Thread did not finish properly */
return;
}
/* All cleaned up nicely */
SEE ALSO
Tcl_GetCurrentThread(3tcl), Tcl_ThreadQueueEvent(3tcl), Tcl_ThreadAlert(3tcl),
Tcl_ExitThread(3tcl), Tcl_FinalizeThread(3tcl), Tcl_CreateThreadExitHandler(3tcl),
Tcl_DeleteThreadExitHandler(3tcl), Thread
KEYWORDS
thread, mutex, condition variable, thread local storage