Critical Sections FAQ - FreeBASIC Wiki Manual

When accessing to shared variables between several threads, all their accesses must be generally put inside blocks Mutexlock...Mutexunlock, in all threads:

- When the shared variable is only one simple predefined numeric type of size <= sizeof(integer) (only one assembler instruction for access), the mutex use may be not mandatory.
- But if this is for example one shared variable LongInt with a win32 compilation, it is advised here to use a mutex (otherwise the reading phase by a thread may be interlaced with the writing phase of another thread).

That is because to access a variable in memory (for reading or for writing), a processor uses its internal registers.
A N-bit processor has N-bit registers but none greater:

- So one only assembler instruction allows it to access a N-bit variable in memory.
- At opposite, to access a 2N-bit variable, it must use 2 assembler instructions.
- If between these two assembler instructions (for writing), another thread accesses this same variable (for reading), the got value may be incoherent (N-bit highest and N-bit lowest incoherent together).

This behavior can be checked with a graphic program using two threads and a shared LongInt (64-bit) without mutex:

- by compiling in 32-bit, many read values are incoherent.
- by compiling in 64-bit, no read value is incoherent.

Compile the below test program:

- in 32-bit => many erroneous points not on the circle but anywhere in the square containing the circle (if you uncomment the four lines 37/39/58/60 to activate the mutex, then all the got points are now on the circle only),
- in 64-bit => all points are valid, on the circle only, even if the mutex is not activated.

' - The "user-defined thread" computes the points coordinates on a circle,
' and write those in a LongInt (32-bit & 32-bit = 64-bit)
' - The "main thread" plots the points from the LongInt value.
'
' Behavior:
' - The first point must be pre-determined.
' - Nothing prevents that a same calculated point could be plotted several times
' (depends on execution times of the loops between main thread and user thread).
' - Nothing prevents that a calculated point could be not plotted
' (same remark on the loop times).
'
' Remark:
' Voluntarily, there is no Sleep in the loop of each thread (normally strongly discouraged),
' but this is just in this special case to amplify the behavior effects to observe.

Union Point2D
Dim As LongInt xy
Type
Dim As Long y
Dim As Long x
End Type
End Union

Dim As Any Ptr handle
Dim Shared As Any Ptr mutex
Dim Shared As Integer quit

Sub Thread (ByVal param As Any Ptr)
Const pi As Single = 4 * Atn(1)
Dim As Point2D Ptr p = param
Do
Dim As Point2D P2D0
Dim As Single teta = 2 * pi * Rnd
P2D0.x = 320 + 200 * Cos(teta)
P2D0.y = 240 + 200 * Sin(teta)
' Mutexlock(mutex)
p->xy = P2D0.xy
' Mutexunlock(mutex)
' Sleep 5, 1
Loop Until quit = 1
End Sub

Screen 12

Dim As Point2D P2D
P2D.x = 520
P2D.y = 240

mutex = MutexCreate
handle = ThreadCreate(@Thread, @P2D)

Dim As Integer c

Do
Dim As Point2D P2D0
' Mutexlock(mutex)
P2D0.xy = P2D.xy
' Mutexunlock(mutex)
PSet (P2D0.x, P2D0.y), c
c = (c Mod 15) + 1
' Sleep 5, 1
Loop Until Inkey <> ""

quit = 1
ThreadWait(handle)
MutexDestroy(mutex)

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Chronology for one thread signaling which occurs:

while another thread is waiting (within a While loop on predicate),
before another thread is waiting (within a While loop on predicate).

#define while_loop_on_predicate

Dim As Any Ptr handle
Dim Shared As Any Ptr mutex
Dim Shared As Any Ptr cond
Dim As Integer sleep0
Dim As Integer sleep1
#ifdef while_loop_on_predicate
Dim Shared As Integer ready
#endif

Sub Thread1 (ByVal param As Any Ptr)
Sleep *Cast(Integer Ptr, param), 1
MutexLock(mutex)
Color 11 : Print " Thread#1 locks the mutex"
Color 11 : Print " Thread#1 executes code with exclusion"
#ifdef while_loop_on_predicate
ready = 1
#endif
Color 11 : Print " Thread#1 is signaling"
CondSignal(cond)
Color 11 : Print " Thread#1 executes post-code with exclusion"
Color 11 : Print " Thread#1 unlocks the mutex"
MutexUnlock(mutex)
End Sub

Sub Thread0 (ByVal param As Any Ptr)
Sleep *Cast(Integer Ptr, param), 1
MutexLock(mutex)
Color 10 : Print " Thread#0 locks the mutex"
Color 10 : Print " Thread#0 executes pre-code with exclusion"
#ifdef while_loop_on_predicate
While ready <> 1
#endif
Color 10 : Print " Thread#0 is waiting"
CondWait(cond, mutex)
Color 10 : Print " Thread#0 is waked"
#ifdef while_loop_on_predicate
Wend
#endif
Color 10 : Print " Thread#0 executes code with exclusion"
#ifdef while_loop_on_predicate
ready = 0
#endif
Color 10 : Print " Thread#0 unlocks the mutex"
MutexUnlock(mutex)
End Sub

mutex = MutexCreate
cond = CondCreate

sleep0 = 0
sleep1 = 1000
Color 7 : Print "Chronology for Thread#1 signaling while Thread#0 is waiting:"
handle = ThreadCreate(@Thread1, @sleep1)
Thread0(@sleep0)
ThreadWait(handle)
Color 7 : Print "Thread#1 finished": Print
Sleep 1000, 1

sleep0 = 1000
sleep1 = 0
Color 7 : Print "Chronology for Thread#1 signaling before Thread#0 is waiting:"
handle = ThreadCreate(@Thread1, @sleep1)
Thread0(@sleep0)
ThreadWait(handle)
Color 7 : Print "Thread#1 finished": Print

MutexDestroy(mutex)
CondDestroy(cond)
Sleep

Output part a - Chronology for Thread#1 signaling while Thread#0 is waiting:

Chronology for Thread#1 signaling while Thread#0 is waiting:
	Thread#0 locks the mutex
	Thread#0 executes pre-code with exclusion
	Thread#0 is waiting
		Thread#1 locks the mutex
		Thread#1 executes code with exclusion
		Thread#1 is signaling
		Thread#1 executes post-code with exclusion
		Thread#1 unlocks the mutex
	Thread#0 is waked
	Thread#0 executes code with exclusion
	Thread#0 unlocks the mutex
Thread#1 finished

Output part b - Chronology for Thread#1 signaling before Thread#0 is waiting:

Chronology for Thread#1 signaling before Thread#0 is waiting:
		Thread#1 locks the mutex
		Thread#1 executes code with exclusion
		Thread#1 is signaling
		Thread#1 executes post-code with exclusion
		Thread#1 unlocks the mutex
	Thread#0 locks the mutex
	Thread#0 executes pre-code with exclusion
	Thread#0 executes code with exclusion
	Thread#0 unlocks the mutex
Thread#1 finished

Note: If CondWait is not within a While loop on predicate (by putting in comment the first line of above program), one can check in the second case (thread#1 signaling before thread#0 waiting), that thread#0 remains blocked in its waiting phase (Ctrl-C to quit).

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Referring to the example 2 on the Critical Sections, one takes this opportunity to recall that:

- The mutex must always be also locked while executing Condsignal() or Condbroadcast() to wake up a thread (it may be unlocked but only after Condsignal() or Condbroadcast()).
- If the mutex is not locked (or even if the mutex is unlocked only just before executing Condsignal() or Condbroadcast()), the behavior may become unpredictable (it may work or not, depending on the threads configuration and execution real time).

In the example 2 on the Critical Sections "Synchronous method example using a condwait then a condbroadcast (and a mutex) for all threads":

- If one at least Mutexunlock() is moved just before its Condbroadcast(), the program hangs very quickly.
- Although some users certify that the mutex can always be unlocked just before Condsignal() or Condbroadcast(), and others more cautious assert that one can do it only for a Condbroadcast(), experiment shows the opposite!

The general rule is that:

- The condition must not be signaled (by Condsignal() or Condbroadcast()) between the time a thread locks the mutex and the time it waits on the condition variable (CondWait()), otherwise it seems that it may damage the waiting queue of threads on that condition variable.
- Thus to avoid that and follow this rule, it is necessary that the mutex remains locked when the condition is signaled.

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While predicate <> True Condwait(conditionalid, mutexid) Wend predicate = False

In all documentations, it is highly advisable to do so, mainly justified to fight against eventual spurious wake-ups.

This is probably true, but it is also advisable to do so to avoid to loose a CondSignal (or CondBroadcast) if it is prematurely activated while the receiving thread is not yet waiting on CondWait (the signal is lost forever):

- In that case, the receiving thread has even not yet locked the mutex before that CondSignal (or CondBroadcast) is activated.
- So the predicate will already true before the receiving thread reaches the While loop, inducing that CondWait is downright skipped, so avoiding a definitive blocking phenomenon.

Let two threads (thread #0 in main program, thread #1 in a user procedure, each that prints its number in a loop), having about the same execution time, and each one synchronizing the other in order to well interlace their numbers (by using one mutex, two condition variables and CondSignal/CondWait):

- Without a While loop on predicate, the program hangs quickly (Ctrl-C to quit):

'             Thread#0               XOR + <==>             Thread#1
'   .....                                          .....
'   MutexLock(mut)                                 MutexLock(mut)
'     Do_something_with_exclusion                    Do_something_with_exclusion
'     CondWait(cond#1, mut) <----------------------- CondSignal(cond#1)
'     Do_something_with_exclusion <---------.        Do_something_with_exclusion
'     CondSignal(cond#2) ------------------ | -----> CondWait(cond#2, mut)
'     Do_something_with_exclusion     .---- | -----> Do_something_with_exclusion
'   MutexUnlock(mut) -----------------'     '----- MutexUnlock(mut)
'   .....                                          .....

Dim As Any Ptr handle
Dim Shared As Any Ptr mutex
Dim Shared As Any Ptr cond1
Dim Shared As Any Ptr cond2
Dim Shared As Integer quit

Sub Thread (ByVal param As Any Ptr)
Do
MutexLock(mutex)
Print "1";
CondSignal(cond1)
CondWait(cond2, mutex)
If quit = 1 Then
MutexUnlock(mutex)
Exit Do
End If
MutexUnlock(mutex)
Sleep 1, 1
Loop
End Sub

mutex = MutexCreate
cond1 = CondCreate
cond2 = CondCreate
handle = ThreadCreate(@Thread)

Do
MutexLock(mutex)
CondWait(cond1, mutex)
Print "0";
CondSignal(cond2)
If Inkey <> "" Then
quit = 1
MutexUnlock(mutex)
Exit Do
End If
MutexUnlock(mutex)
Sleep 1, 1
Loop

ThreadWait(handle)
MutexDestroy(mutex)
CondDestroy(cond1)
CondDestroy(cond2)
Print

Sleep

- With a While loop on predicate around each CondWait, no blocking phenomenon:

'             Thread#0               XOR + <==>             Thread#1
'   .....                                          .....
'   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)
'   .....                                          .....

Dim As Any Ptr handle
Dim Shared As Any Ptr mutex
Dim Shared As Any Ptr cond1
Dim Shared As Any Ptr cond2
Dim Shared As Integer new1
Dim Shared As Integer new2
Dim Shared As Integer quit

Sub Thread (ByVal param As Any Ptr)
Do
MutexLock(mutex)
Print "1";
new1 = 1
CondSignal(cond1)
While new2 <> 1
CondWait(cond2, mutex)
Wend
new2 = 0
If quit = 1 Then
MutexUnlock(mutex)
Exit Do
End If
MutexUnlock(mutex)
Sleep 1, 1
Loop
End Sub

mutex = MutexCreate
cond1 = CondCreate
cond2 = CondCreate
handle = ThreadCreate(@Thread)

Do
MutexLock(mutex)
While new1 <> 1
CondWait(cond1, mutex)
Wend
new1 = 0
Print "0";
new2 = 1
CondSignal(cond2)
If Inkey <> "" Then
quit = 1
MutexUnlock(mutex)
Exit Do
End If
MutexUnlock(mutex)
Sleep 1, 1
Loop

ThreadWait(handle)
MutexDestroy(mutex)
CondDestroy(cond1)
CondDestroy(cond2)
Print

Sleep

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The Input keyword may be not thread-safe, when another thread must also access to input/output resource:

- When executing the Input statement, the other running threads must not change the position of the text cursor, which prohibits instructions such as Locate, Print, ...
- Moreover, one cannot enclosed the Input keyword inside a mutex locking (as we can do it for the Inkey keyword), because while the inputting line would be not completed and validated, the other threads that want to also access to input/output would be fully blocked (waiting for mutex unlocking).

Thread-safe input-line function (versus input/output resource):

Input position, prompt message, sleeping time, line-blanking command, mutex pointer can be passed to the following threadInput() function that simulates a simplified input function, but thread-safe, by using a looping around the Inkey keyword (all input/output keywords must be enclosed inside a mutex locking block, and the cursor position must be restored at each mutex locking block ending):

Function threadInput (ByVal row As Integer, ByVal column As Integer, ByRef prompt As String = "", ByVal sleeptime As Integer = 15, ByVal blank As Integer = 0, ByVal mutex As Any Ptr = 0) As String
Dim As Integer r
Dim As Integer c
Dim As String inputchr
Dim As String inputline

MutexLock(mutex)
r = CsrLin()
c = Pos()
Locate row, column
Print prompt & "? _";
Locate r, c
MutexUnlock(mutex)

Do
MutexLock(mutex)
r = CsrLin()
c = Pos()
Locate row, column + Len(inputline) + Len(prompt) + 2
inputchr = Inkey
If Len(inputchr) = 1 Then
If Asc(inputchr) >= 32 Then
Print inputchr & Chr(95);
Locate , Pos - 1
inputline &= inputchr
ElseIf Asc(inputchr) = 08 And Len(inputline) > 0 Then
Locate , Pos - 1
Print Chr(95) & " ";
Locate , Pos() - 2
inputline = Left(inputline, Len(inputline) - 1)
End If
End If
Locate r, c
MutexUnlock(mutex)
Sleep sleeptime, 1
Loop Until inputchr = Chr(13)

If blank <> 0 Then
MutexLock(mutex)
r = CsrLin()
c = Pos()
Locate row, column + Len(prompt) + 2
Print Space(Len(inputline) + 1);
Locate r, c
MutexUnlock(mutex)
End If

Return inputline
End Function

From the example 1 on the Critical Sections page "Asynchronous method example using a mutex for all threads", now the running multi-threading code is waiting for the "quit" command in order to exit the program:

'   User thread algorithm:
'
'     Do
'     |  Mutexlock
'     |  | .....
'     |  | Critical section of code
'     |  | .....
'     |  Mutexunlock
'     |  Sleep my_tempo
'     Loop Until quit = true
'
'   There is no any advantage or disadvantage between threads for running their critical sections.

Function threadInput (ByVal row As Integer, ByVal column As Integer, ByRef prompt As String = "", ByVal sleeptime As Integer = 15, ByVal blank As Integer = 0, ByVal mutex As Any Ptr = 0) As String
Dim As Integer r
Dim As Integer c
Dim As String inputchr
Dim As String inputline

MutexLock(mutex)
r = CsrLin()
c = Pos()
Locate row, column
Print prompt & "? _";
Locate r, c
MutexUnlock(mutex)

Do
MutexLock(mutex)
r = CsrLin()
c = Pos()
Locate row, column + Len(inputline) + Len(prompt) + 2
inputchr = Inkey
If Len(inputchr) = 1 Then
If Asc(inputchr) >= 32 Then
Print inputchr & Chr(95);
Locate , Pos - 1
inputline &= inputchr
ElseIf Asc(inputchr) = 08 And Len(inputline) > 0 Then
Locate , Pos - 1
Print Chr(95) & " ";
Locate , Pos() - 2
inputline = Left(inputline, Len(inputline) - 1)
End If
End If
Locate r, c
MutexUnlock(mutex)
Sleep sleeptime, 1
Loop Until inputchr = Chr(13)

If blank <> 0 Then
MutexLock(mutex)
r = CsrLin()
c = Pos()
Locate row, column + Len(prompt) + 2
Print Space(Len(inputline) + 1);
Locate r, c
MutexUnlock(mutex)
End If

Return inputline
End Function

Type UDT
Dim As Integer number
Dim As Integer tempo
Dim As Any Ptr pThread
Dim As ULongInt count
Static As Any Ptr pMutex
Static As Integer numberMax
Static As Integer quit
End Type
Dim As Any Ptr UDT.pMutex
Dim As Integer UDT.numberMax
Dim As Integer UDT.quit

Sub Counter (ByVal pt As UDT Ptr)
With *pt
Locate .number, .number, 0
Sleep 5, 1
.count += 1
Print .count;
End With
End Sub

Sub Thread (ByVal p As Any Ptr)
Dim As Integer quit
Dim As UDT Ptr pUDT = p
With *pUDT
Do
MutexLock(.pMutex)
Counter(pUDT)
quit = .quit
MutexUnlock(.pMutex)
Sleep .tempo, 1
Loop Until quit = 1
End With
End Sub

Screen 12
UDT.numberMax = 6

Dim As UDT u(0 To UDT.numberMax)
For I As Integer = 0 To UDT.numberMax
u(I).number = i
u(I).tempo = 100 + 15 * I - 95 * Sgn(I)
Next I
UDT.pMutex = MutexCreate

Dim As Single t = Timer
For I As Integer = 1 To UDT.numberMax
u(I).pThread = ThreadCreate(@Thread, @u(I))
Next I

Do
Loop Until LCase(threadInput(8, 1, """quit"" for exit", 10, 1, UDT.pMutex)) = "quit"

UDT.quit = 1

For I As Integer = 1 To UDT.numberMax
ThreadWait(u(I).pThread)
Next I
t = Timer - t

MutexDestroy(UDT.pMutex)
Dim As ULongInt c
For I As Integer = 1 To UDT.numberMax
c += u(I).count
Next I
Locate UDT.numberMax + 4, 1
Print CULngInt(c / t) & " increments per second"

Sleep

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- Screenlock...Scrennunlock blocks are not compatible with multi-threading (otherwise, the program hangs). This is why a mutex block must be used around each such block to ensure the exclusion.
- The input keywords (like for keyboard, mouse) cannot be safely run when the screen is locked, therefore a such keyword must be outside of any Screenlock...Screenunlock block, so outside any Screenlock...Screenunlock block in its own thread, and protected of all Screenlock...Screenunlock blocks of other threads by a mutex block. Therefore, Getkey and Input, the statements that wait for keypress or line input are unusable, but Inkey that does not wait can work.

By applying some rules scrupulously, one can use Screenlock/Screenunlock inside the threads.
Principle of coding for all threads including the main code (main thread):

Do
	' instructions without display (printing/drawing, ...) neither input (input/inkey/mouse getting, ...)
	MutexLock(m)
		Screenlock
			' instructions with only display (printing/drawing, ...)
		Screenunlock
		' instructions with only input without waiting (inkey/mouse getting, ...)
	MutexUnlock(m)
	Sleep tempo
Loop Until condition

For example, it is mandatory to use one Mutexlock...Mutexunlock block around each Screenlock...Screenunlock block, and one other around the Inkey instruction which itself must always be outside of any Screenlock...Screenunlock bloc:

Type ThreadUDT
Dim handle As Any Ptr
Static sync As Any Ptr
Static quit As Byte
End Type
Dim ThreadUDT.sync As Any Ptr
Dim ThreadUDT.quit As Byte

Function ClockTime () As String
Return Time
End Function

Function Counter () As Integer
Static C As Integer
C = (C + 1) Mod 1000000
Return C
End Function

Sub ProcedureThread (ByVal param As Any Ptr)
With *Cast(ThreadUDT Ptr, param)
Do
MutexLock(.sync)
ScreenLock
Line (544, 0)-(639, 49), 0, BF 'clear the print area
Sleep 100, 1
Locate 2, 71
Print ClockTime();
ScreenUnlock
MutexUnlock(.sync)
Sleep 100, 1
Loop Until .quit = 1
End With
End Sub

Screen 12
Locate 30, 2
Print "<q/Q> : quit";

Dim TTptr As ThreadUDT Ptr = New ThreadUDT
ThreadUDT.sync = MutexCreate
TTptr->handle = ThreadCreate(@ProcedureThread, TTptr)

Dim As String s
Do
MutexLock(ThreadUDT.sync)
ScreenLock
Line (296, 208)-(376, 256), 0, BF 'clear the print area
Sleep 100, 1
Locate 15,40
Print Using "######"; Counter();
ScreenUnlock
s = Inkey
MutexUnlock(ThreadUDT.sync)
Sleep 100, 1
Loop Until LCase(s) = "q"

ThreadUDT.quit = 1
ThreadWait(TTptr->handle)
MutexDestroy(ThreadUDT.sync)
Delete TTptr

Note: The Sleep keyword just after the 'clear the print area' lines is only here to highlight the flickering if no screen locking is used.

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Instead of "screen locking" (see the above paragraph), "video paging (double buffering or page flipping)" can more simply be used with multi-threading, but be careful that many states in the gfxlib2 are thread-dependent like Screenset (and also View settings, graphic cursor position, graphic colors, ...).
Therefore, the setting for the working page and the visible page must always be controlled in each thread code which want to work with a multi-video page configuration.

- Example for a double buffering method (at each step, each thread needs to update the working page and copy it to the visible page, from within an exclusion mutex code block):

Type ThreadUDT
Dim handle As Any Ptr
Static sync As Any Ptr
Static quit As Byte
End Type
Dim ThreadUDT.sync As Any Ptr
Dim ThreadUDT.quit As Byte

Function ClockTime () As String
Return Time
End Function

Function Counter () As Integer
Static C As Integer
C = (C + 1) Mod 1000000
Return C
End Function

Sub ProcedureThread (ByVal param As Any Ptr)
ScreenSet 1, 0 '' setting to define in each thread
With *Cast(ThreadUDT Ptr, param)
Do
MutexLock(.sync)
Line (544, 0)-(639, 49), 0, BF '' clear the print area
Sleep 100, 1
Locate 2, 71
Print ClockTime();
ScreenCopy
MutexUnlock(.sync)
Sleep 100, 1
Loop Until .quit = 1
End With
End Sub

Screen 12, , 2
ScreenSet 1, 0 '' setting to define in each thread
Locate 30, 2
Print "<q/Q> : quit";
ScreenCopy

Dim TTptr As ThreadUDT Ptr = New ThreadUDT
ThreadUDT.sync = MutexCreate
TTptr->handle = ThreadCreate(@ProcedureThread, TTptr)

Dim s As String
Do
MutexLock(ThreadUDT.sync)
Line (296, 208)-(376, 256), 0, BF '' clear the print area
Sleep 100, 1
Locate 15,40
Print Using "######"; Counter();
ScreenCopy
s = Inkey
MutexUnlock(ThreadUDT.sync)
Sleep 100, 1
Loop Until LCase(s) = "q"

ThreadUDT.quit = 1
ThreadWait(TTptr->handle)
MutexDestroy(ThreadUDT.sync)
Delete TTptr

Note: The Sleep keyword just after the 'clear the print area' lines is only here to highlight the flickering if no double buffering is used.

- Example for a two page flipping method (at each step, each thread needs to update and flip, from within the same exclusion mutex code block, the two screen pages):

Type ThreadUDT
Dim handle As Any Ptr
Static sync As Any Ptr
Static quit As Byte
End Type
Dim ThreadUDT.sync As Any Ptr
Dim ThreadUDT.quit As Byte

Function ClockTime () As String
Return Time
End Function

Function Counter () As Integer
Static C As Integer
C = (C + 1) Mod 1000000
Return C
End Function

Sub ProcedureThread (ByVal param As Any Ptr)
Dim p0 As Integer = 0
Dim p1 As Integer = 1
ScreenSet 1, 0 '' setting to define in each thread
With *Cast(ThreadUDT Ptr, param)
Do
MutexLock(.sync)
Dim s As String = ClockTime()
For I As Integer = 1 To 2 '' updating the two screen pages
Line (544, 0)-(639, 49), 0, BF '' clear the print area
Sleep 100, 1
Locate 2, 71
Print s;
ScreenSet p0, p1
Swap p0, p1
Next I
MutexUnlock(.sync)
Sleep 100, 1
Loop Until .quit = 1
End With
End Sub

Screen 12, , 2
Dim p0 As Integer = 0
Dim p1 As Integer = 1
ScreenSet 1, 0 '' setting to define in each thread
For I As Integer = 1 To 2 '' updating the two screen pages
Locate 30, 2
Print "<q/Q> : quit";
ScreenSet p0, p1
Swap p0, p1
Next I

Dim TTptr As ThreadUDT Ptr = New ThreadUDT
ThreadUDT.sync = MutexCreate
TTptr->handle = ThreadCreate(@ProcedureThread, TTptr)

Dim s As String
Do
MutexLock(ThreadUDT.sync)
Dim C As Integer = Counter()
For I As Integer = 1 To 2 '' updating the two screen pages
Line (296, 208)-(376, 256), 0, BF '' clear the print area
Sleep 100, 1
Locate 15,40
Print Using "######"; c;
ScreenSet p0, p1
Swap p0, p1
Next I
s = Inkey
MutexUnlock(ThreadUDT.sync)
Sleep 100, 1
Loop Until LCase(s) = "q"

ThreadUDT.quit = 1
ThreadWait(TTptr->handle)
MutexDestroy(ThreadUDT.sync)
Delete TTptr

Note: The Sleep keyword just after the 'clear the print area' lines is only here to highlight the flickering if no two page flipping is used.

Note: In these two examples, an exclusion mutex code block is mandatory in the two threads, not only because of using console statements + Inkey, but around also the graphics statements + Screencopy only because of using double buffering method (without anti-flickering process, the graphics statements could be outside the exclusion mutex code block).

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The source code of gfxlib2 uses TLS (Thread Local Storage) to store many states, so many things are thread-specific.
Since gfxlib2 is thread-safe, mutex exclusion between threads is not necessary for the graphics statements themselves (including Draw String).
In contrast, console statements such as Locate, Print, ... are not thread-safe as previously mentioned (for example, text cursor position is common to all threads).

- Simple example showing that graphic states (such as graphic cursor position, graphic colors) are thread-dependent:

Screen 12

Sub thread(ByVal p As Any Ptr)
Color 10
PSet(150, 10)
For I As Integer = 1 To 40
Line -Step(10, 10)
Sleep 150, 1
Next I
Draw String Step (-40, 10), "user thread"
End Sub

Dim As Any Ptr p = ThreadCreate(@thread)

Color 14
PSet(10, 100)
For I As Integer = 1 To 24
Line -Step(10, 10)
Sleep 250, 1
Next I
Draw String Step (-40, 10), "main thread"

ThreadWait(p)

Color 15
Locate 4, 2
Print "Any key for exit"

Sleep

- Example showing that graphics statements (such as Line and Draw String and Screencopy) in a thread can compete with console statements (such as Inkey) in another thread, without using any exclusion (by mutex):

#include "vbcompat.bi"

Screen 12, , 2
ScreenSet 1, 0
Color 0, 7
Cls

Dim Shared terminate As Integer = 0

Sub thread (ByVal param As Any Ptr)
ScreenSet 1, 0
Do
Line (16, 432)-Step(96, 32), 11, BF 'clear print area
Sleep 100, 1
Draw String (24, 432), Format(Now,"dd/mm/yyyy"), 0
Draw String (32, 448), Format(Now,"hh:mm:ss"), 0
ScreenCopy
Sleep 100, 1
Loop Until terminate = 1
End Sub

Dim As String reply
Locate 2, 2
Print "Enter ""q"" to quit"
ScreenCopy

Dim p As Any Ptr = ThreadCreate(@thread)

Do
reply = Inkey
Sleep 100, 1
Loop Until LCase(reply) = "q"

Print " Stop the thread"
ScreenCopy
terminate=1
ThreadWait (p)
Print " Thread terminated"
ScreenCopy

Sleep

Note: The Sleep keyword just after the 'clear the print area' line is only here to highlight the flickering if no double buffering is used.

- From the above example, if the date displaying and the time displaying are now two separate threads, an exclusion mutex code block between these two threads is mandatory, not due to the graphics statements themselves competing, but only due to the double buffering method used (against flickering) that puts competing these two threads:

#include "vbcompat.bi"

Screen 12, , 2
ScreenSet 1, 0
Color 0, 7
Cls

Dim Shared terminate As Integer = 0
Dim Shared mutex As Any Ptr

Sub thread1 (ByVal param As Any Ptr)
ScreenSet 1, 0
Do
MutexLock(mutex)
Line (16, 432)-Step(96, 16), 11, BF 'clear the print area
Sleep 200, 1
Draw String (24, 432), Format(Now,"dd/mm/yyyy"), 0
ScreenCopy
MutexUnlock(mutex)
Sleep 100, 1
Loop Until terminate = 1
End Sub

Sub thread2 (ByVal param As Any Ptr)
ScreenSet 1, 0
Do
MutexLock(mutex)
Line (16, 448)-Step(96, 16), 11, BF 'clear the print area
Sleep 100, 1
Draw String (32, 448), Format(Now,"hh:mm:ss"), 0
ScreenCopy
MutexUnlock(mutex)
Sleep 100, 1
Loop Until terminate = 1
End Sub

Dim As String reply
Locate 2, 2
Print "Enter ""q"" to quit"
ScreenCopy

mutex = MutexCreate
Dim p1 As Any Ptr = ThreadCreate(@thread1)
Dim p2 As Any Ptr = ThreadCreate(@thread2)

Do
reply = Inkey
Sleep 100, 1
Loop Until LCase(reply) = "q"

Print " Stop the threads"
ScreenCopy
terminate=1
ThreadWait (p1)
ThreadWait (p2)
MutexDestroy(mutex)
Print " Threads terminated"
ScreenCopy

Sleep

Note: The Sleep keyword just after the 'clear the print area' lines is only here to highlight the flickering if no double buffering is used, or if no mutex is used.

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Console statements (such as Locate, Print, Color, ...), as well as Locate and Print on Graphics window (but not Color on Graphics Window), and keyboard inputs (such as Inkey, Getkey, Input, ...) are not thread-safe:

- Thus when they are used in competing sections of different threads, mutual exclusion is mandatory by means of mutex locking blocks in which in addition code can restore states (such as text cursor position, console color, ...) at end of the block (after its own usage), as they were before (at begin of the block).
- But the Getkey or Input keyword cannot be enclosed inside a mutex locking block (as it can be do with the Inkey keyword), because as long as the keyboard input is not completed, the other threads in compete would be fully blocked (waiting for the mutex unlocking).

- Example showing that the keywords Locate and Print are not thread-safe both when applied on a console window or when applied on a graphics window (the text cursor states being not thread dependent in the two cases):

Sub Thread (ByVal p As Any Ptr)
Locate Cast(Integer, p), Cast(Integer, p)
For I As Integer = 1 To 50 - 2 * Cast(Integer, p)
Sleep 20 * Cast(Integer, p), 1
Print Str(Cast(Integer, p));
Next I
End Sub

Sub test ()
Dim As Any Ptr p(1 To 9)
For I As Integer = 1 To 9
p(I) = ThreadCreate(@Thread, Cast(Any Ptr, I))
Sleep 25, 1
Next I
For I As Integer = 1 To 9
ThreadWait(p(I))
Next I
End Sub

Screen 0
test()
Locate 15, 1
Print "Any key to continue"
Sleep

Screen 12
test()
Locate 15, 1
Print "Any key to quit"
Sleep

Note: One can see that each thread does not write on its own line corresponding to its thread number (id between 1 and 9), on the console window and on the graphics window.

- From the above example, the thread code has been completed in its competing sections by mutex locking blocks and by saving/restoring cursor states before/after its own cursor moving:

Dim Shared As Any Ptr mutex

Sub Thread (ByVal p As Any Ptr)
MutexLock(mutex)
Dim As Long l0 = Locate()
Locate Cast(Integer, p), Cast(Integer, p)
Dim As Long l = Locate()
Locate HiByte(LoWord(l0)), LoByte(LoWord(l0)), HiWord(l0)
MutexUnlock(mutex)
For I As Integer = 1 To 50 - 2 * Cast(Integer, p)
Sleep 20 * Cast(Integer, p), 1
MutexLock(mutex)
l0 = Locate()
Locate HiByte(LoWord(l)), LoByte(LoWord(l)), HiWord(l)
Print Str(Cast(Integer, p));
l = Locate()
Locate HiByte(LoWord(l0)), LoByte(LoWord(l0)), HiWord(l0)
MutexUnlock(mutex)
Next I
End Sub

Sub test ()
Dim As Any Ptr p(1 To 9)
For I As Integer = 1 To 9
p(I) = ThreadCreate(@Thread, Cast(Any Ptr, I))
Sleep 25, 1
Next I
For I As Integer = 1 To 9
ThreadWait(p(I))
Next I
End Sub

mutex = MutexCreate

Screen 0
test()
Locate 15, 1
Print "Any key to continue"
Sleep

Screen 12
test()
Locate 15, 1
Print "Any key to quit"
Sleep

MutexDestroy(mutex)

Note: One can see that each thread writes now on its own line corresponding to its thread number (id between 1 and 9), on the console window and on the graphics window.

- Example showing that the Color keyword is not thread-safe when applied on a console window, but is thread-safe when applied on a graphics window (the color states being thread dependent in that case):

Sub Thread (ByVal p As Any Ptr)
Color Cast(Integer, p) + 8, Cast(Integer, p)
For I As Integer = 1 To 50 - 2 * Cast(Integer, p)
Print " " & Cast(Integer, p) & " ";
Sleep 20 * Cast(Integer, p), 1
Next I
End Sub

Sub test ()
Dim As Any Ptr p(1 To 9)
Locate 1, 1
For I As Integer = 1 To 9
p(I) = ThreadCreate(@Thread, Cast(Any Ptr, I))
Sleep 25, 1
Next I
For I As Integer = 1 To 9
ThreadWait(p(I))
Next I
Locate 16, 1
End Sub

Screen 0
test()
Print "Any key to continue"
Sleep

Screen 12
test()
Print "Any key to quit"
Sleep

Note: One can see that the foreground/background colors are not specific to the thread number (id between 1 and 9) on the console window, but this works great on the graphics window.

- From the above example, the thread code has been completed in its competing sections by mutex locking blocks and by saving/restoring color states before/after its own color values usage:

Dim Shared As Any Ptr mutex

Sub Thread (ByVal p As Any Ptr)
MutexLock(mutex)
Dim As Ulong c0 = Color(Cast(Integer, p) + 8, Cast(Integer, p))
Dim As Ulong c = Color()
Color(LoWord(c0), HiWord(c0))
MutexUnlock(mutex)
For I As Integer = 1 To 50 - 2 * Cast(Integer, p)
MutexLock(mutex)
c0 = Color(LoWord(c), HiWord(c))
Print " " & Cast(Integer, p) & " ";
Color(LoWord(c0), HiWord(c0))
MutexUnlock(mutex)
Sleep 20 * Cast(Integer, p), 1
Next I
End Sub

Sub test ()
Dim As Any Ptr p(1 To 9)
Locate 1, 1
For I As Integer = 1 To 9
p(I) = ThreadCreate(@Thread, Cast(Any Ptr, I))
Sleep 25, 1
Next I
For I As Integer = 1 To 9
ThreadWait(p(I))
Next I
Locate 16, 1
End Sub

mutex = MutexCreate

Screen 0
test()
Print "Any key to continue"
Sleep

Screen 12
test()
Print "Any key to quit"
Sleep

MutexDestroy(mutex)

Note: One can see that the foreground/background colors are now specific to the thread number (id between 1 and 9) on the console window (obviously this always works on the graphics window).

Therefore, for using Getkey or Input in competing sections of threads:

- Only a single thread (for example, the main thread) can uses Getkey or Input in addition to console statements (such as Locate, Print, Color, ...) and also Inkey, in its competing sections.
- The other threads must not to use in their competing sections any console statement neither any keyboard input keyword, but can use by cons graphics statements (such as Pset, Line, Circle, Draw String, graphic Color, ...) which are themselves thread-safe (they can interlace graphically with the main thread without any problem).
- Input and Getkey also exclude the screen locking usage in competing sections of threads (double buffering is recommended as anti-flickering method).

- Example showing that graphics statements (such as Line and Draw String and Screencopy) in a thread (user thread here) can compete with console statements (such as Locate and Print and Input) in another thread (main thread here), without using any exclusion (by mutex):

#include "vbcompat.bi"

Screen 12, , 2
ScreenSet 1, 0
Color 0, 7
Cls

Dim Shared terminate As Integer = 0

Sub thread (ByVal param As Any Ptr)
ScreenSet 1, 0
Do
Line (16, 432)-Step(96, 32), 11, BF 'clear the print area
Sleep 100, 1
Draw String (24, 432), Format(Now,"dd/mm/yyyy"), 0
Draw String (32, 448), Format(Now,"hh:mm:ss"), 0
ScreenCopy
Sleep 100, 1
Loop Until terminate = 1
End Sub

Dim As String reply
Locate 2, 2
Print "Enter ""quit"" to quit"
ScreenCopy

Dim p As Any Ptr = ThreadCreate(@thread)

Do
Locate 3, 2
Print Space(Len(reply) + 2);
Locate 3, 2
Input reply
Loop Until LCase(reply) = "quit"

Print " Stop the thread"
ScreenCopy
terminate=1
ThreadWait (p)
Print " Thread terminated"
ScreenCopy

Sleep

Note: The Sleep keyword just after the 'clear the print area' line is only here to highlight the flickering if no double buffering is used (screen locking being forbidden by Input usage).

- From the above example, if the date displaying and the time displaying are now two separate user threads, an exclusion mutex code block between these two threads only is mandatory, not due to the graphics statements themselves competing, but only due to the double buffering method used (against flickering) that puts competing these two user threads only:

#include "vbcompat.bi"

Screen 12, , 2
ScreenSet 1, 0
Color 0, 7
Cls

Dim Shared terminate As Integer = 0
Dim Shared mutex As Any Ptr

Sub thread1 (ByVal param As Any Ptr)
ScreenSet 1, 0
Do
MutexLock(mutex)
Line (16, 432)-Step(96, 16), 11, BF 'clear the print area
Sleep 200, 1
Draw String (24, 432), Format(Now,"dd/mm/yyyy"), 0
ScreenCopy
MutexUnlock(mutex)
Sleep 100, 1
Loop Until terminate = 1
End Sub

Sub thread2 (ByVal param As Any Ptr)
ScreenSet 1, 0
Do
MutexLock(mutex)
Line (16, 448)-Step(96, 16), 11, BF 'clear the print area
Sleep 100, 1
Draw String (32, 448), Format(Now,"hh:mm:ss"), 0
ScreenCopy
MutexUnlock(mutex)
Sleep 100, 1
Loop Until terminate = 1
End Sub

Dim As String reply
Locate 2, 2
Print "Enter ""quit"" to quit"
ScreenCopy

mutex = MutexCreate
Dim p1 As Any Ptr = ThreadCreate(@thread1)
Dim p2 As Any Ptr = ThreadCreate(@thread2)

Do
Locate 3, 2
Print Space(Len(reply) + 2);
Locate 3, 2
Input reply
Loop Until LCase(reply) = "quit"

Print " Stop the threads"
ScreenCopy
terminate=1
ThreadWait (p1)
ThreadWait (p2)
MutexDestroy(mutex)
Print " Threads terminated"
ScreenCopy

Sleep

Note: The Sleep keyword just after the 'clear the print area' lines is only here to highlight the flickering if no double buffering is used (screen locking being forbidden by Input usage).

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