Threads(3) Tcl Library Procedures 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
Tcl_Condition *condPtr (in) A condition variable, which
must be associated with a mutex
lock.
Tcl_Condition *mutexPtr (in) A mutex lock.
Tcl_Time *timePtr (in) A time limit on the condition
wait. NULL to wait forever.
Note that a polling value of 0
seconds doesn't 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 initial‐
ized to zero the first time
each thread calls Tcl_Get‐
ThreadData.
_________________________________________________________________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 config‐
ure 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 multi‐
threaded 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 creat‐
ing 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 implement‐
ing 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_Condition‐
Wait. 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 condi‐
tion variable unblocks all threads waiting on the condition variable,
but they do not proceed until the mutex is released with Tcl_MutexUn‐
lock. The implementation of Tcl_ConditionWait automatically locks the
mutex before returning.
The caller of Tcl_ConditionWait should be prepared for spurious notifi‐
cations by calling Tcl_ConditionWait within a while loop that tests
some invariant.
The Tcl_GetThreadData call returns a pointer to a block of thread-pri‐
vate 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 han‐
dlers. 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 inter‐
preter, and to send scripts among threads. Dive into tclThreadTest.c
and tclThread.c for examples.
SEE ALSO
Tcl_GetCurrentThread, Tcl_ThreadQueueEvent, Tcl_ThreadAlert, Tcl_Exit‐
Thread, Tcl_FinalizeThread, Tcl_CreateThreadExitHandler,
Tcl_DeleteThreadExitHandler
KEYWORDS
thread, mutex, condition variable, thread local storage
Tcl 8.1 Threads(3)