Ubuntu Manpage: Tcl_ConditionNotify, Tcl_ConditionWait, Tcl_ConditionFinalize, Tcl_GetThreadData, Tcl

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, threadProc, 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.

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 threadProc (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
threadProc.

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

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

int *result (out) The referred storage is used to place the exit code of the
thread waited upon into it.
________________________________________________________________________________________________________________

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 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 behaviour 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 behaviour with no specialties. 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.

KEYWORDS

       thread, mutex, condition variable, thread local storage