Threads(3)

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
A condition variable, which must be associated with a mutex lock. A recursive mutex lock. A time limit on the condition wait. NULL to wait forever. Note that a polling value of 0 seconds does not make much sense. 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. 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. The referred storage will contain the id of the newly created thread as returned by the operating system. Id of the thread waited upon. This procedure will act as the main() of the newly created thread. The specified clientData will be its sole argument. Arbitrary information. Passed as sole argument to the proc. The size of the stack given to the new thread. Bitmask containing flags allowing the caller to modify behavior of the new thread. 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. 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(
    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