path: root/doc/Thread.3

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'\" Copyright (c) 1998 Sun Microsystems, Inc.
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'\" RCS: @(#) $Id: Thread.3,v 1.12 2000/05/02 22:02:33 kupries 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_ConditionFinalize, Tcl_GetThreadData, Tcl_MutexLock, Tcl_MutexUnlock, Tcl_MutexFinalize \- 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_ConditionFinalize\fR(\fIcondPtr\fR)
.sp
Void *
\fBTcl_GetThreadData\fR(\fIkeyPtr, size\fR)
.sp
void
\fBTcl_MutexLock\fR(\fImutexPtr\fR)
.sp
void
\fBTcl_MutexUnlock\fR(\fImutexPtr\fR)
.sp
void
\fBTcl_MutexFinalize\fR(\fImutexPtr\fR)
.sp
int
\fBTcl_CreateThread\fR(\fIidPtr, threadProc, clientData, stackSize, flags\fR)
.sp
int
\fBTcl_JoinThread\fR(\fIid, result\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.
.AP Tcl_ThreadId *idPtr out
The refered storage will contain the id of the newly created thread as
returned by the operating system.
.AP Tcl_ThreadId id in
Id of the thread waited upon.
.AP Tcl_ThreadCreateProc threadProc in
This procedure will act as the \fBmain()\fR of the newly created
thread. The specified \fIclientData\fR will be its sole argument.
.AP ClientData clientData in
Arbitrary information. Passed as sole argument to the \fIthreadProc\fR.
.AP int stackSize in
The size of the stack given to the new thread.
.AP int flags in
Bitmask containing flags allowing the caller to modify behaviour of
the new thread.
.AP int *result out
The refered storage is used to place the exit code of the thread
waited upon into it.
.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
.VS 8.3.1
Tcl does provide \fBTcl_CreateThread\fR for creating threads. The
caller can determine the size of the stack given to the new thread and
modify the behaviour through the supplied \fIflags\fR. The value
\fBTCL_THREAD_STACK_DEFAULT\fR for the \fIstackSize\fR indicates that
the default size as specified by the operating system is to be used
for the new thread. As for the flags, currently are only the values
\fBTCL_THREAD_NOFLAGS\fR and \fBTCL_THREAD_JOINABLE\fR defined. The
first of them invokes the default behaviour with no
specialities. Using the second value marks the new thread as
\fIjoinable\fR. This means that another thread can wait for the such
marked thread to exit and join it.
.PP
Restrictions: On some unix systems the pthread-library does not
contain the functionality to specify the stacksize of a thread. The
specified value for the stacksize is ignored on these systems. Both
Windows and Macintosh currently do not support joinable threads. This
flag value is therefore ignored on these platforms.
.VE
.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
.VS
The \fBTcl_JoinThread\fR 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 \fBTCL_THREAD_JOINABLE\fR-flag during
its creation via \fBTcl_CreateThread\fR.
.PP
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 \fBTcl_JoinThread\fR.
This means that not calling \fBTcl_JoinThread\fR for a joinable thread
will cause a memory leak.
.VE
.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
A mutex can be destroyed after its use by calling \fBTcl_MutexFinalize\fR.
The result of locking a mutex twice from the same thread is undefined.
On some platforms it will result in a deadlock.
.VE
The \fBTcl_MutexLock\fR, \fBTcl_MutexUnlock\fR and \fBTcl_MutexFinalize\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
.VS
A condition variable can be destroyed after its use by calling
\fBTcl_ConditionFinalize\fR.
.PP
The \fBTcl_ConditionNotify\fR, \fBTcl_ConditionWait\fR and
\fBTcl_ConditionFinalize\fR procedures are defined as empty macros if
not compiling with threads enabled.
.VE
.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
.VS
either cleaned up by process exit handlers (if living that long) or
explicitly by calls to \fBTcl_MutexFinalize\fR or
\fBTcl_ConditionFinalize\fR.
.VE
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