BlitzMax/Modules/System/Threads

Welcome to the weird and wonderful world of multithreading!

Multithreading effectively allows your programs to do several things at the same time. The word 'thread' in this context means 'thread of execution' - or, the series of instructions, branches and so on executed by your program. Most programs are 'single threaded', meaning there is only one thread of execution. However, more and more programs are using multiple threads.

Multithreading used to be achieved by software trickery, which made threading useful but not really faster - there was still only one CPU pretending to do multiple things at the same time! But these days, multicore CPUs mean that threading can be used to truly do multiple things at the same time (or 'in parallel').

Creating a thread is easy - just call CreateThread. You will need to provide a function for the thread to use as its 'entry point'. Once the thread is created, this function will start executing in parallel with the code that called CreateThread. When the thread function returns, that thread will be 'terminated'.

Alas, threading turns out to be rather tricky due to an issue known as 'synchronization'. Synchronization is required when you need to prevent multiple threads from modifying or accessing the same data at the same time. Synchronization usually involves a thread 'blocking'. When a thread blocks, it completely halts execution until another thread does something that causes it to 'unblock' and resume execution.

BlitzMax provides 2 primitives known as 'mutexes' and 'semaphores' to assist with synchronization:


 * Mutexes provide a simple locking mechanism. Only one thread at a time can lock a mutex (using LockMutex or TryLockMutex), so this is an easy way to protect resources from simultaneous access. If a thread calls LockMutex and the mutex is already locked by another thread, the current thread will block until the other thread releases the mutex using UnlockMutex. So don't forget to UnlockMutex a mutex after you are finished with it!


 * Semaphores provide a synchronized counting mechanism, and contain an internal integer counter. There are 2 operations you can perform on a semaphore - post and wait. Posting a semaphore (using PostSemaphore) causes the semaphore's internal counter to be incremented, while waiting for a semaphore (using WaitSemaphore) will cause the current thread to block until the semaphore's internal counter is greater than 0. When it is, the counter is decremented and the thread unblocks. Semaphores are very useful for producer/consumer type situations.

=Types=

TThread
Thread Type


 * Detach
 * Wait
 * Running


 * Create
 * Main
 * Current

TThread: Methods
Method Detach

Description: Detach this thread

Method Wait:Object

Description: Wait For this thread To finish

Returns: The Object returned by the thread.

Method Running

Description: Check if this thread is running

TThread: Functions
Function Create:TThread( entry:Object( data:Object),data:Object )

Description: Create a new thread

Function Main:TThread

Description: Get Main thread

Returns: A thread object representing the main application thread.

Function Current:TThread

Description: Get current thread

Returns: A thread object representing the current thread.

TThreadData
ThreadData type


 * SetValue
 * GetValue


 * Create

TThreadData: Methods
Method SetValue( value:Object )

Description: Set thread data value

Method GetValue:Object

Description: Get thread data value

TThreadData: Functions
Function Create:TThreadData

Description: Create thread data

TMutex
Mutex type


 * Close
 * Lock
 * TryLock
 * Unlock


 * Create

TMutex: Methods
Method Close

Description: Close the mutex

Method Lock

Description: Lock the mutex

Method TryLock

Description: Try to lock the mutex

Returns: True if mutex was successfully locked; False if mutex was already locked by another thread.

Method Unlock

Description: Unlock the mutex

TMutex: Functions
Function Create:TMutex

Description: Create a new mutex

TSemaphore
Semaphore Type


 * Close
 * Wait
 * Post


 * Create

TSemaphore: Methods
Method Close

Description: Close the semaphore

Method Wait

Description: Wait for the semaphore

Method Post

Description: Post the semaphore

TSemaphore: Functions
Function Create:TSemaphore( count )

Description: Create a new semaphore

TCondVar
CondVar type


 * Close
 * Wait
 * Signal
 * Broadcast


 * Create

TCondVar: Methods
Method Close

Description: Close the condvar

Method Wait( mutex:TMutex )

Description: Wait for the condvar

Method Signal

Description: Signal the condvar

Method Broadcast

Description: Broadcast the condvar

TCondVar: Functions
Function Create:TCondVar

Description: Create a new condvar

=Functions=

CreateThread
Function CreateThread:TThread( entry:Object( data:Object ),data:Object )

Description: Create a thread

Returns: A new thread object.

Information: Creates a thread and returns a thread object.

The value returned by the thread entry routine can be later retrieved using WaitThread.

To 'close' a thread, call either DetachThread or WaitThread. This isn't strictly necessary as the thread will eventually be closed when it is garbage collected, however, it may be a good idea if you are creating many threads very often, as some operating systems have a limit on the number of threads that can be allocated at once.

Example:

'Make sure to have 'Threaded build' enabled! ' Strict

'Custom print that shows which thread is doing the printing Function MyPrint( t$ ) If CurrentThread=MainThread Print "Main thread: "+t Else Print "Child thread: "+t EndIf End Function

'Our thread function Function MyThread:Object( data:Object )

'show data we were passed Myprint data.ToString

'do some work For Local i=1 To 1000 MyPrint "i="+i Next 'return a value from the thread Return "Data returned from child thread." End Function

MyPrint "About to start child thread."

'create a thread! Local thread:TThread=CreateThread( MyThread,"Data passed to child thread." )

'wait for thread to finish and print value returned from thread MyPrint WaitThread( Thread ).ToString

MainThread
Function MainThread:TThread

Description: Get Main thread

Returns: A thread Object representing the Main application thread.

CurrentThread
Function CurrentThread:TThread

Description: Get current thread

Returns: A thread object representing the current thread.

DetachThread
Function DetachThread( thread:TThread )

Description: Detach a thread

Information: DetachThread closes a thread's handle, but does not halt or otherwise affect the target thread.

Once one a thread has been detached, it wil no longer be possible to use WaitThread to get its return value.

This allows the thread to run without your program having to continually check whether it has completed in order to close it.

WaitThread
Function WaitThread:Object( thread:TThread )

Description: Wait for a thread to finish

Returns: The object returned by the thread entry routine.

Information: WaitThread causes the calling thread to block until the target thread has completed execution.

If the target thread has already completed execution, WaitThread returns immediately.

The returned object is the object returned by the thread's entry routine, as passed to CreateThread.

ThreadRunning
Function ThreadRunning( thread:TThread )

Description: Check if a thread is running

Returns: True if thread is still running, otherwise False.

CreateThreadData
Function CreateThreadData:TThreadData

Description: Create thread data

Returns: A new thread data object.

SetThreadDataValue
Function SetThreadDataValue( data:TThreadData,value:Object )

Description: Set thread data value

GetThreadDataValue
Function GetThreadDataValue:Object( data:TThreadData )

Description: Get thread data value

CreateMutex
Function CreateMutex:TMutex

Description: Create a mutex

Returns: A new mutex object

Example:

'Make sure to have 'Threaded build' enabled! ' Strict

'a global list that multiple threads want to modify Global list:TList=New TList

'a mutex controlling access to the global list Global listMutex:TMutex=CreateMutex

Function MyThread:Object( data:Object )

For Local item=1 To 10 'simulate 'other' processing... Delay Rand( 10,50 )

'lock mutex so we can safely modify global list LockMutex listMutex

'modify list list.AddLast "Thread "+data.ToString+" added item "+item

'unlock mutex UnlockMutex listMutex Next End Function

Local threads:TThread[10]

'Create worker threads For Local i=0 Until 10 threads[i]=CreateThread( MyThread,String( i+1 ) ) Next

Print "Waiting for worker threads..."

'Wait for worker threads to finish For Local i=0 Until 10 WaitThread threads[i] Next

'Show the resulting list ' 'Note: We don't really have to lock the mutex here, as there are no other threads running. 'Still, it's a good habit to get into. LockMutex listMutex For Local t$=EachIn list Print t Next UnlockMutex listMutex

CloseMutex
Function CloseMutex( mutex:TMutex )

Description: Close a mutex

LockMutex
Function LockMutex( mutex:TMutex )

Description: Lock a mutex

TryLockMutex
Function TryLockMutex( mutex:TMutex )

Description: Try to lock a mutex

Returns: True if mutex was successfully locked; False if mutex was already locked by another thread.

UnlockMutex
Function UnlockMutex( mutex:TMutex )

Description: Unlock a mutex

CreateSemaphore
Function CreateSemaphore:TSemaphore( count )

Description: Create a semaphore

Returns: A new semaphore object

Example:

'Make sure to have 'Threaded build' enabled! ' Strict

'a simple queue Global queue$[100],put,get

'a counter semaphore Global counter:TSemaphore=CreateSemaphore( 0 )

Function MyThread:Object( data:Object )

'process 100 items For Local item=1 To 100 'add an item to the queue queue[put]="Item "+item put:+1 'increment semaphore count. PostSemaphore counter Next End Function

'create worker thread Local thread:TThread=CreateThread( MyThread,Null )

'receive 100 items For Local i=1 To 100

'Wait for semaphore count to be non-0, then decrement. WaitSemaphore counter 'Get an item from the queue Local item$=queue[get] get:+1 Print item

Next

CloseSemaphore
Function CloseSemaphore( semaphore:TSemaphore )

Description: Close a semaphore

WaitSemaphore
Function WaitSemaphore( semaphore:TSemaphore )

Description: Wait for a semaphore

PostSemaphore
Function PostSemaphore( semaphore:TSemaphore )

Description: Post a semaphore

CreateCondVar
Function CreateCondVar:TCondVar

Description: Create a condvar

Returns: A new condvar object

CloseCondVar
Function CloseCondVar( condvar:TCondVar )

Description: Close a condvar

WaitCondVar
Function WaitCondVar( condvar:TCondVar,mutex:TMutex )

Description: Wait for a condvar

SignalCondVar
Function SignalCondVar( condvar:TCondVar )

Description: Signal a condvar

BroadcastCondVar
Function BroadcastCondVar( condvar:TCondVar )

Description: Broadcast a condvar

CompareAndSwap
Function CompareAndSwap( target Var,oldValue,newValue )

Description: Compare and swap

Returns: True if target was updated

Information: Atomically replace target with new_value if target equals old_value.

AtomicAdd
Function AtomicAdd( target Var,value )

Description: Atomic add

Returns: Previous value of target

Information: Atomically add value to target.

AtomicSwap
Function AtomicSwap( target Var,value )

Description: Atomically swap values

Returns: The old value of target