Threads(3)

NAME
Tcl_ConditionNotify, Tcl_ConditionWait, Tcl_GetThreadData, Tcl_MutexLock, Tcl_MutexUnlock - Tcl thread support.

SYNOPSIS
#include <tcl.h>

void
Tcl_ConditionNotify(condPtr)

void
Tcl_ConditionWait(condPtr, mutexPtr, timePtr)

Void *
Tcl_GetThreadData(keyPtr, size)

void
Tcl_MutexLock(mutexPtr)

void
Tcl_MutexUnlock(mutexPtr)

ARGUMENTS
A condition variable, which must be associated with a mutex lock. A mutex lock. A time limit on the condition wait. NULL to wait forever. Note that a polling value of 0 seconds doesn't 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.

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 contstraint 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 provides no special API for creating threads. When writing multithreaded applications incorporating Tcl, use the standard POSIX threads APIs on Unix systems and the standard Win32 threads APIs on Windows systems.

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.

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. The result of locking a mutex twice from the same thread is undefined. On some platforms it will result in a deadlock. Tcl_MutexLock and Tcl_MutexUnlock 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_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.

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.

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 cleaned up by process exit handlers. 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 tclThreadTest.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