Tcl 9.0/Tk9.0 Documentation > Tcl C API, version 9.0.0 > Thread

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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>

Tcl_ConditionNotify(condPtr)

Tcl_ConditionWait(condPtr, mutexPtr, timePtr)

Tcl_ConditionFinalize(condPtr)

void *

Tcl_GetThreadData(keyPtr, size)

Tcl_MutexLock(mutexPtr)

Tcl_MutexUnlock(mutexPtr)

Tcl_MutexFinalize(mutexPtr)

int

Tcl_CreateThread(idPtr, proc, clientData, stackSize, flags)

int

Tcl_JoinThread(id, result)

ARGUMENTS

INTRODUCTION

DESCRIPTION

SYNCHRONIZATION AND COMMUNICATION

INITIALIZATION

SCRIPT-LEVEL ACCESS TO THREADS

EXAMPLE

SEE ALSO

KEYWORDS

NAME

SYNOPSIS

ARGUMENTS

Tcl_Condition *condPtr (in)

A condition variable, which must be associated with a mutex lock.

Tcl_Mutex *mutexPtr (in)

A recursive 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.

void *clientData (in)

Arbitrary information. Passed as sole argument to the proc.

size_t 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.

INTRODUCTION

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

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

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. Mutexes are reentrant: they can be locked several times from the same thread. However there must be exactly one call to Tcl_MutexUnlock for each call to Tcl_MutexLock in order for a thread to release a mutex completely. 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

SCRIPT-LEVEL ACCESS TO THREADS

EXAMPLE

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(
    void *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;
void *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 */

SEE ALSO

KEYWORDS