Revision [12357]

This is an old revision of KeyPgMutexCreate made by JeffMarshall on 2008-01-18 14:17:27.

 

MUTEXCREATE


Creates a mutex used for synchronizing the execution of threads

Syntax:
Usage:
result = Mutexcreate

Return Value:
The KeyPgAny any KeyPgPtr ptr handle of the mutex created, or the null pointer (0) on failure.

Description:
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 variable used by a single thread, called multiple times), it should be "locked" during its use with a mutex. This halts all threads using KeyPgMutexLock Mutexlock with that mutex, until it is unlocked with KeyPgMutexUnlock 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 KeyPgMutexDestroy 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.

Examples:
'' Threading syncronyzation using Mutexes
'' If you comment out the lines containing "MutexLock" and "MutexUnlock",
'' the threads will not be in sync and some of the data may be printed
'' out of place.

Const MAX_THREADS = 10

Declare Sub thread( ByVal id_ptr As Any Ptr )
Declare Sub teletype (ByVal text As String, ByVal x As Integer, ByVal y As Integer)

Dim Shared threadsync As Any Ptr

Dim i As Integer

Dim handleTb(0 To MAX_THREADS-1) As Any Ptr

'' Create a mutex to syncronize the threads
threadsync = MutexCreate

'' Create threads
For i = 0 To MAX_THREADS-1
    handleTb(i) = ThreadCreate(@thread, @i)
    If handleTb(i) = 0 Then
        Print "Error creating thread:"; i
        Exit For
    End If
Next

'' Wait until all threads are finished
For i = 0 To MAX_THREADS-1
    If( handleTb(i) <> 0 ) Then
        ThreadWait( handleTb(i) )
    End If
Next

teletype "Testing.................", 1, 1
teletype "Testing again...........", 10, 1

'' Discard the mutex when we are through using teletype
MutexDestroy threadsync

Sub thread( ByVal id_ptr As Any Ptr )
    Dim id As Integer
    id = *Cast( Integer Ptr, id_ptr )
    teletype "Thread (" & id & ").........", 1, 1+id
End Sub

'' Teletype unfurls some text across the screen at a given location
Sub teletype (ByVal text As String, ByVal x As Integer, ByVal y As Integer)
    Dim i As Integer

    For i = 0 To Len(text)-1
        '' MutexLock prevents the two simultaneously running
        '' threads from sharing "x", "y", and "a"
        MutexLock threadsync

        Locate y, x+i
        Print Chr(text[i])

        '' MutexUnlock releases these variables for other use
        MutexUnlock threadsync

        Sleep 25
   Next
End Sub


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