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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, 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.
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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. 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(3), Tcl_ThreadQueueEvent(3), Tcl_ThreadAlert(3),
Tcl_ExitThread(3), Tcl_FinalizeThread(3),
Tcl_CreateThreadExitHandler(3), Tcl_DeleteThreadExitHandler(3), Thread
KEYWORDS
thread, mutex, condition variable, thread local storage
Tcl 8.1 Threads(3)