MutexCreate

Declare Function MutexCreate ( ) As Any Ptr

result = MutexCreate

The Any Ptr handle of the mutex created, or the null pointer (0) on failure.

Mutexes, short for "Mutually Exclusive", are a way of synchronizing shared data within threads. If there is a global variable used by multiple threads (or a local static variable used by a single thread called multiple times), it should be "locked" during its use with a mutex. This halts all threads using MutexLock with that mutex (including the main thread executing main program), until it is unlocked with MutexUnlock.

Mutexcreate creates a mutex, returning a handle which is to be referred to when locking, unlocking, or destroying the mutex. Mutexes created with Mutexcreate should be destroyed when no longer needed or before the end of the program with MutexDestroy.

A mutex is a lock that guarantees three things:
1. Atomicity - Locking a mutex is an atomic operation, meaning that the operating system (or threads library) assures you that if you locked a mutex, no other thread succeeded in locking this mutex at the same time.
2. Singularity - If a thread managed to lock a mutex, it is assured that no other thread will be able to lock the thread until the original thread releases the lock.
3. Non-Busy Wait - If a thread attempts to lock a thread that was locked by a second thread, the first thread will be suspended (and will not consume any CPU resources) until the lock is freed by the second thread. At this time, the first thread will wake up and continue execution, having the mutex locked by it.

See also the ThreadCreate examples.

'Visual example of mutual exclusion between 2 threads by using Mutex:
'the "user-defined thread" computes the points coordinates on a circle,
'and the "main thread" plots the points.
'
'Principle of mutual exclusion
'          Thread#A                XOR                  Thread#B
'.....                                         .....
'MutexLock(mut)                                MutexLock(mut)
'  Do_something#A_with_exclusion                 Do_something#B_with_exclusion
'MutexUnlock(mut)                              MutexUnlock(mut)
'.....                                         .....
'
'Behavior:
'- The first point must be pre-calculated.
'- 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).
'
'If you comment out the lines containing "MutexLock" and "MutexUnlock"
'(inside "user-defined thread" or/and "main thread"),
'there will be no longer mutual exclusion 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 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
    Do
        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;
        MutexUnlock(tp->sync)                        'Mutex (Unlock) for user thread
        Sleep 5
    Loop Until tp->quit = tp->true                   'Test for ending user thread
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
PointOnCircle(Pptr)                   ' Computation for a first point valid on the circle

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

Do
    Static As Integer I
    Sleep 5
    MutexLock(Tptr->sync)   'Mutex (Lock) for main thread
    PSet (Pptr->x, Pptr->y) 'Plotting one point
    I += 1
    Locate 30, 62
    Print I;
    MutexUnlock(Tptr->sync) 'Mutex (Unlock) for main thread
Loop Until Inkey <> ""
 
Tptr->quit = Tptr->true
ThreadWait(Tptr->handle)
MutexDestroy(Tptr->sync)
Delete Tptr
Delete Pptr

Sleep

See also the similar CondCreate example

• Threading is not allowed in the -lang qb dialect.
  
  

• The DOS version of FreeBASIC does not allow for threads, as the OS does not support them.
  
• In Linux the threads are always started in the order they are created, this can't be assumed in Win32. It's an OS, not a FreeBASIC issue.
  
  

• New to FreeBASIC
  
  

• MutexDestroy
  
• MutexLock
  
• MutexUnlock
  
• ThreadCreate
  
• ThreadWait