path: root/doc/Thread.3

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'\" RCS: @(#) $Id: Thread.3,v 1.5 1999/04/30 22:45:00 stanton Exp $
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.so man.macros
.TH Threads 3 "8.1" Tcl "Tcl Library Procedures"
.BS
.SH NAME
Tcl_ConditionNotify, Tcl_ConditionWait, Tcl_GetThreadData, Tcl_MutexLock, Tcl_MutexUnlock \- Tcl thread support.
.SH SYNOPSIS
.nf
\fB#include <tcl.h>\fR
.sp
void
\fBTcl_ConditionNotify\fR(\fIcondPtr\fR)
.sp
void
\fBTcl_ConditionWait\fR(\fIcondPtr, mutexPtr, timePtr\fR)
.sp
Void *
\fBTcl_GetThreadData\fR(\fIkeyPtr, size\fR)
.sp
void
\fBTcl_MutexLock\fR(\fImutexPtr\fR)
.sp
void
\fBTcl_MutexUnlock\fR(\fImutexPtr\fR)
.SH ARGUMENTS
.AS Tcl_ThreadDataKey *keyPtr
.AP Tcl_Condition *condPtr in
A condition variable, which must be associated with a mutex lock.
.AP Tcl_Condition *mutexPtr in
A mutex lock.
.AP 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.
.AP 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.
.AP 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 \fBTcl_GetThreadData\fR.
.BE
.SH 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 \fB--enable-threads\fR option to \fBconfigure\fR
when you configure and compile your Tcl core.
.PP
An important contstraint of the Tcl threads implementation is that
\fIonly the thread that created a Tcl interpreter can use that
interpreter\fR.  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.)
.PP
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.
.PP
Tcl does provide \fBTcl_ExitThread\fR and \fBTcl_FinalizeThread\fR
for terminating threads and invoking optional per-thread exit
handlers.  See the \fBTcl_Exit\fR page for more information on these
procedures.
.PP
Tcl provides \fBTcl_ThreadQueueEvent\fR and \fBTcl_ThreadAlert\fR
for handling event queueing in multithreaded applications.  See
the \fBNotifier\fR manual page for more information on these procedures.
.PP
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 \fItclThreadTest.c\fR
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.
.SH DESCRIPTION
A mutex is a lock that is used to serialize all threads through a piece
of code by calling \fBTcl_MutexLock\fR and \fBTcl_MutexUnlock\fR.
If one thread holds a mutex, any other thread calling \fBTcl_MutexLock\fR will
block until \fBTcl_MutexUnlock\fR is called.
.VS
The result of locking a mutex twice from the same thread is undefined.
On some platforms it will result in a deadlock.
.VE
\fBTcl_MutexLock\fR and \fBTcl_MutexUnlock\fR
procedures are defined as empty macros if not compiling with threads enabled.
.PP
A condition variable is used as a signaling mechanism:
a thread can lock a mutex and then wait on a condition variable
with \fBTcl_ConditionWait\fR.  This atomically releases the mutex lock
and blocks the waiting thread until another thread calls
\fBTcl_ConditionNotify\fR.  The caller of \fBTcl_ConditionNotify\fR should
have the associated mutex held by previously calling \fBTcl_MutexLock\fR,
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 \fBTcl_MutexUnlock\fR.
The implementation of \fBTcl_ConditionWait\fR automatically locks
the mutex before returning.
.PP
The caller of \fBTcl_ConditionWait\fR should be prepared for spurious
notifications by calling \fBTcl_ConditionWait\fR within a while loop
that tests some invariant.
.PP
The \fBTcl_GetThreadData\fR 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 \fBTcl_FinalizeThread\fR.
.SH INITIALIZATION
.PP
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 \fBTcl_FinalizeThread\fR.
.SH 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.
.SH "SEE ALSO"
Tcl_GetCurrentThread, Tcl_ThreadQueueEvent, Tcl_ThreadAlert,
Tcl_ExitThread, Tcl_FinalizeThread,
Tcl_CreateThreadExitHandler, Tcl_DeleteThreadExitHandler
.SH KEYWORDS
thread, mutex, condition variable, thread local storage