Tribblix: manual page: Tcl

Threads(3) Tcl Library Procedures Threads(3)

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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

Tcl_Condition *condPtr (in) A condition variable, which
must be associated with a
mutex lock.

Tcl_Mutex *mutexPtr (in) A mutex lock.

const Tcl_Time *timePtr (in) A time limit on the condition
wait. NULL to wait forever.
Note that a polling value of
0 seconds does not 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
initialized to zero the first
time each thread calls
Tcl_GetThreadData.

Tcl_ThreadId *idPtr (out) The referred storage will
contain the id of the newly
created thread as returned by
the operating system.

Tcl_ThreadId id (in) Id of the thread waited upon.

Tcl_ThreadCreateProc *proc (in) This procedure will act as
the main() of the newly
created thread. The specified
clientData will be its sole
argument.

ClientData clientData (in) Arbitrary information. Passed
as sole argument to the proc.

int stackSize (in) The size of the stack given
to the new thread.

int flags (in) Bitmask containing flags
allowing the caller to modify
behavior of the new thread.

int *result (out) The referred storage is used
to place the exit code of the
thread waited upon into it.
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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. The result of locking a
mutex twice from the same thread is undefined. On some platforms it
will result in a deadlock. 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 */

KEYWORDS

thread, mutex, condition variable, thread local storage

Tcl 8.1 Threads(3)