CONDWAIT


Stops execution of current thread until some condition becomes true

Syntax:
declare sub Condwait ( byval handle as any ptr, byval mutex as any ptr )

Usage:
Condwait ( handle, mutex )

Parameters:
handle
The handle of a conditional variable, or the null pointer (0) on failure.
mutex
The mutex associated with this conditional variable, which must be locked when testing the condition and calling Condwait.

Description:
Function that stops the thread where it is called until some other thread Condsignals or Condbroadcasts the handle.

Once the conditional variable is created with Condcreate and the threads are started, one of 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.

When calling Condwait, mutex should already be locked (using the same mutex as one used with Condsignal or Condbroadcast). An atomic unlock of the mutex and wait on the conditional variable will occur. The calling thread execution is suspended and does not consume any CPU time until the condition variable is signaled. When the condition variable becomes signaled, mutex will be locked again and then execution will return to the thread after the Condwait call, but with the mutex owned by the caller. The caller is then responsible for unlocking mutex in order to complete Condwait routine so that execution after the Condwait call can then resume.

Note: It is a good habit to use Condwait in a protected way against eventual spurious wakeups.
For that, Condwait is put within a loop for checking that a Boolean predicate is indeed true (predicate set true by another thread just before executing Condsignal or Condbroadcast) when the thread has finished waiting: [While predicate <> true : Condwait(handle, mutex) : Wend]. The loop can terminate only when the predicate is true.
On the other hand, if the predicate is already true before the thread reaches the loop, Condwait is downright skipped (allowing to take into account a case of Condsignal or Condbroadcast that would have been lost otherwise, because prematurely executed in a second thread before the first thread is really waiting for this).
See example below for detailed coding.

Examples:
See also Condcreate and Condsignal

' This simple example code demonstrates the use of several condition variable routines.
' The main routine creates three threads.
' Two of the threads update a "count" variable.
' The third thread waits until the count variable reaches a specified value.

#define numThread  3
#define countThreshold 6

Dim Shared As Integer count = 0
Dim Shared As Any Ptr countMutex
Dim Shared As Any Ptr countThresholdCV
Dim As Any Ptr threadID(0 To numThread-1)
Dim Shared As Integer ok = 0

Sub threadCount (ByVal p As Any Ptr)
    Print "Starting threadCount(): thread#" & p
    Do
        Print "threadCount(): thread#" & p & ", locking mutex"
        MutexLock(countMutex)
        count += 1
        ' Check the value of count and signal waiting thread when condition is reached.
        ' Note that this occurs while mutex is locked.
        If count >= countThreshold Then
            If count = countThreshold Then
                Print "threadCount(): thread#" & p & ", count = " & count & ", threshold reached, unlocking mutex"
                ok = 1
                CondSignal(countThresholdCV)
            Else
                Print "threadCount(): thread#" & p & ", count = " & count & ", threshold exceeded, unlocking mutex"
            End If
            MutexUnlock(countMutex)
            Exit Do
        End If
        Print "threadCount(): thread#" & p & ", count = " & count & ", unlocking mutex"
        MutexUnlock(countMutex)
        Sleep 100
    Loop
End Sub

Sub threadWatch (ByVal p As Any Ptr)
    Print "Starting threadWatch(): thread#" & p & ", locking mutex, waiting for conditional"
    MutexLock(countMutex)
    ' Note that the Condwait routine will automatically and atomically unlock mutex while it waits.
    While ok = 0
        CondWait(countThresholdCV, countMutex)
    Wend
    Print "threadWatch(): thread#" & p & ", condition signal received"
    Print "threadWatch(): thread#" & p & ", count now = " & count & ", unlocking mutex"
    MutexUnlock(countMutex)
End Sub

' Create mutex and condition variable
countMutex = MutexCreate
countThresholdCV = CondCreate
' Create threads
threadID(0) = ThreadCreate(@threadWatch, Cast(Any Ptr, 1))
threadID(1) = ThreadCreate(@threadCount, Cast(Any Ptr, 2))
threadID(2) = ThreadCreate(@threadCount, Cast(Any Ptr, 3))
' Wait for all threads to complete
For I As Integer = 0 To numThread-1
    ThreadWait(threadID(I))
    Print "Main(): Waited on thread#" & I+1 & " Done"
Next I
MutexDestroy(countMutex)
CondDestroy(countThresholdCV)


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