plucky (3) Tcl_CreateThread.3tcl.gz
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>
Tcl_ConditionNotify(condPtr)
Tcl_ConditionWait(condPtr, mutexPtr, timePtr)
Tcl_ConditionFinalize(condPtr)
void *
Tcl_GetThreadData(keyPtr, size)
Tcl_MutexLock(mutexPtr)
Tcl_MutexUnlock(mutexPtr)
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 recursive 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.
void *clientData (in) Arbitrary information. Passed as sole argument to the proc.
size_t 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.
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. │
Mutexes are reentrant: they can be locked several times from the same thread. However there must be │
exactly one call to Tcl_MutexUnlock for each call to Tcl_MutexLock in order for a thread to release a │
mutex completely. 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(
void *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;
void *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