Creates a conditional variable to be used in synchronizing threads

declare function Condcreate ( ) as any ptr

result = Condcreate

Return Value:
A handle to a newly created conditional variable, or the null pointer (0) on failure.

Once the conditional is Condcreated and the threads are started, one or more of them (including the main thread executing main program) can be set to Condwait for the conditional, they will be stopped until some other thread Condsignals that the waiting thread can restart. Condbroadcast can be used to restart all threads waiting for the conditional. At the end of the program Conddestroy must be used to avoid leaking resources in the OS.

See also Condwait and Condsignal

'' make newly-created threads wait until all threads are ready, then start them all at once

Dim Shared hcondstart As Any Ptr
Dim Shared hmutexstart As Any Ptr
Dim Shared start As Integer = 0

Dim Shared threadcount As Integer
Dim Shared hmutexready As Any Ptr
Dim Shared hcondready As Any Ptr

Sub mythread(ByVal id_ptr As Any Ptr)
    Dim id As Integer = Cast(Integer, id_ptr)
    '' signal that this thread is ready
    MutexLock hmutexready
    threadcount += 1
    Print "Thread #" & id & " is waiting..."
    CondSignal hcondready
    MutexUnlock hmutexready
    '' wait for the start signal
    MutexLock hmutexstart
    Do While start = 0  
        CondWait hcondstart, hmutexstart

    '' now this thread holds the lock on hmutexstart
    MutexUnlock hmutexstart

    '' print out the number of this thread
    For i As Integer = 1 To 40
        Print id;
    Next i
End Sub

Dim threads(1 To 9) As Any Ptr

hcondstart = CondCreate()
hmutexstart = MutexCreate()

hcondready = CondCreate()
hmutexready = MutexCreate()

threadcount = 0

For i As Integer = 1 To 9
    threads(i) = ThreadCreate(@mythread, Cast(Any Ptr, i))
    If threads(i) = 0 Then
        Print "unable to create thread"
    End If
Next i

Print "Waiting until all threads are ready..."

Do Until threadcount = 9
    CondWait(hcondready, hmutexready)

Print "Go!"

MutexLock hmutexstart
start = 1
CondBroadcast hcondstart
MutexUnlock hmutexstart

'' wait for all threads to complete
For i As Integer = 1 To 9
    If threads(i) <> 0 Then
        ThreadWait threads(i)
    End If
Next i

MutexDestroy hmutexready
CondDestroy hcondready

MutexDestroy hmutexstart
CondDestroy hcondstart

'Visual example of mutual exclusion + mutual synchronization between 2 threads
'by using Mutex and CondVar:
'the "user-defined thread" computes the points coordinates on a circle,
'and the "main thread" plots the points.
'Principle of mutual exclusion + mutual synchronisation
'          Thread#A               XOR + <==>             Thread#B
'.....                                          .....
'MutexLock(mut)                                 MutexLock(mut)
'  Do_something_with_exclusion                    Do_something_with_exclusion
'  While bool#1 <> true <------------------------ bool#1 = true
'    CondWait(cond#1, mut) <--------------------- CondSignal(cond#1)
'  Wend <-----------------------------------.     Do_something_with_exclusion
'  bool#1 = false               .---------- | --> While bool#2 <> true
'  Do_something_with_exclusion  |   .------ | ----> CondWait(cond#2, mut)
'  bool#2 = true ---------------'   |   .-- | --> Wend
'  CondSignal(cond#2) --------------'   |   |     bool#2 = false
'  Do_something_with_exclusion          |   |     Do_something_with_exclusion
'MutexUnlock(mut) ----------------------'   '-- MutexUnlock(mut)
'.....                                          .....
'- Unnecessary to pre-calculate the first point.
'- Each calculated point is plotted one time only.
'If you comment out the lines containing "MutexLock" and "MutexUnlock",
'"CondWait" and "CondSignal", ".ready"
'(inside "user-defined thread" or/and "main thread"),
'there will be no longer mutual exclusion nor mutual synchronization
'between computation of coordinates and plotting of points,
'and many points will not be plotted on circle (due to non coherent coordinates).


Type ThreadUDT                                   'Generic user thread UDT
    Dim handle As Any Ptr                        'Any Ptr handle to user thread
    Dim sync As Any Ptr                          'Any Ptr handle to mutex
    Dim cond1 As Any Ptr                         'Any Ptr handle to conditional1
    Dim cond2 As Any Ptr                         'Any Ptr handle to conditional2
    Dim ready1 As Byte                           'Boolean to coordinates ready1
    Dim ready2 As Byte                           'Boolean to coordinates ready2
    Dim quit As Byte                             'Boolean to end user thread
    Declare Static Sub Thread (ByVal As Any Ptr) 'Generic user thread procedure
    Dim procedure As Sub (ByVal As Any Ptr)      'Procedure(Any Ptr) to be executed by user thread
    Dim p As Any Ptr                             'Any Ptr to pass to procedure executed by user thread
    Const false As Byte = 0                      'Constante "false"
    Const true As Byte = Not false               'Constante "true"
End Type

Static Sub ThreadUDT.Thread (ByVal param As Any Ptr) 'Generic user thread procedure
    Dim tp As ThreadUDT Ptr = param                  'Casting to generic user thread UDT
        Static As Integer I
        MutexLock(tp->sync)                          'Mutex (Lock) for user thread
        tp->procedure(tp->p)                         'Procedure(Any Ptr) to be executed by user thread
        I += 1
        Locate 30, 38
        Print I;
        tp->ready1 = true                            'Set ready1
        CondSignal(tp->cond1)                        'CondSignal to send signal1 to main thread
        While tp->ready2 <> true                     'Process loop against spurious wakeups
            CondWait(tp->cond2, tp->sync)            'CondWait to receive signal2 from main-thread
        tp->ready2 = false
        If tp->quit = tp->true Then                  'Test for ending user thread
            MutexUnlock(tp->sync)                    'Mutex (Unlock) for user thread
            Exit Do
        End If
        MutexUnlock(tp->sync)                        'Mutex (Unlock) for user thread
        Sleep 5
End Sub


Type Point2D
    Dim x As Integer
    Dim y As Integer
End Type

Const x0 As Integer = 640 / 2
Const y0 As Integer = 480 / 2
Const r0 As Integer = 200

Const pi As Single = 4 * Atn(1)

Sub PointOnCircle (ByVal p As Any Ptr)
    Dim pp As Point2D Ptr = p
    Dim teta As Single = 2 * pi * Rnd
    pp->x = x0 + r0 * Cos(teta)
    Sleep 5                            'To increase possibility of uncorrelated data occurrence
    pp->y = y0 + r0 * Sin(teta)
End Sub

Screen 12
Locate 30, 2
Print "<any_key> : exit";
Locate 30, 27
Print "calculated:";
Locate 30, 54
Print "plotted:";

Dim Pptr As Point2D Ptr = New Point2D

Dim Tptr As ThreadUDT Ptr = New ThreadUDT
Tptr->sync = MutexCreate
Tptr->cond1 = CondCreate
Tptr->cond2 = CondCreate
Tptr->procedure = @PointOnCircle
Tptr->p = Pptr
Tptr->handle = ThreadCreate(@ThreadUDT.Thread, Tptr)

    Static As Integer I
    MutexLock(Tptr->sync)                 'Mutex (Lock) for main thread
    While Tptr->ready1 <> Tptr->true      'Process loop against spurious wakeups
        CondWait(Tptr->cond1, Tptr->sync) 'CondWait to receive signal1 from user-thread
    Tptr->ready1 = Tptr->false
    PSet (Pptr->x, Pptr->y)               'Plotting one point
    I += 1
    Locate 30, 62
    Print I;
    Tptr->ready2 = Tptr->true             'Set ready2
    CondSignal(Tptr->cond2)               'CondSignal to send signal2 to user thread
    If Inkey <> "" Then
        Tptr->quit = Tptr->true           'Set quit
        MutexUnlock(Tptr->sync)           'Mutex (Unlock) for main thread
        Exit Do
    End If
    MutexUnlock(Tptr->sync)               'Mutex (Unlock) for main thread
    Sleep 5

Delete Tptr
Delete Pptr

See also the similar Mutexcreate example

Platform Differences:
Dialect Differences:
Differences from QB:
See also:
Back to Threading Support Functions
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