Threads(3) Tcl Library Procedures Threads(3)
NAME
Tcl_ConditionNotify, Tcl_ConditionWait, Tcl_ConditionFinalize, Tcl_Get-
ThreadData, Tcl_MutexLock, Tcl_MutexUnlock, Tcl_MutexFinalize, Tcl_Cre-
ateThread, 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
A condition variable, which must be associated with a mutex lock. A
mutex lock. A time limit on the condition wait. NULL to wait forever.
Note that a polling value of 0 seconds does not make much sense. 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. 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. The referred storage
will contain the id of the newly created thread as returned by the
operating system. Id of the thread waited upon. This procedure will
act as the main() of the newly created thread. The specified clientData
will be its sole argument. Arbitrary information. Passed as sole argu-
ment to the threadProc. The size of the stack given to the new thread.
Bitmask containing flags allowing the caller to modify behaviour of the
new thread. 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 config-
ure 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 inter-
preters.)
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 cur-
rently 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-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.
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_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.
A condition variable can be destroyed after its use by calling Tcl_Con-
ditionFinalize.
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_MutexFi-
nalize 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.
SEE ALSO
Tcl_GetCurrentThread(3), Tcl_ThreadQueueEvent(3), Tcl_ThreadAlert(3),
Tcl_ExitThread(3), Tcl_FinalizeThread(3), Tcl_CreateThreadEx-
itHandler(3), Tcl_DeleteThreadExitHandler(3), Thread
KEYWORDS
thread, mutex, condition variable, thread local storage
Tcl 8.1 Threads(3)
Thread 8.5.4 - Generated Thu Aug 21 08:46:58 CDT 2008