CondSignalsub

Restarts a thread suspended by a call to CondWait

Threading Support Functionssubdocumented

Syntax

Declare Sub CondSignal ( ByVal handle As Any Ptr )

Parameters

Name		Description
`handle`		The handle of a conditional variable.

Description

Once the conditional is created with CondCreate and the threads are started, one of more of them (including the implicit 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.

Condsignal restarts one thread waiting. It should be called after mutex is locked (using the same mutex as one used with CondWait). If no threads are waiting on the conditional, nothing happens (the signal is lost forever); if several are waiting, only one is restarted. The caller must then unlock mutex in order for CondWait subroutine to complete.

Remarks

Usage



CondSignal ( handle )

Dialect Differences

Threading is not allowed in -lang qb

Platform Differences

Condsignal is not available with the DOS version / target of FreeBASIC, because multithreading is not supported by DOS kernel nor the used extender.
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.

Differences from QB

New to FreeBASIC

Example

See also CondCreate and CondWait and CondBroadcast

' This very simple example code demonstrates the use of several condition variable routines.

' The main routine initializes a string and creates one thread.

' The main routine waits until receive the condition signal from the thread, then print the complemented string.

' The thread complements the string, then sends a condition signal.

'

'Principle of mutual exclusion + simple synchronization

'          Thread#A             XOR + ==>            Thread#B

'.....                                     .....

'MutexLock(mut)                            MutexLock(mut)

'  Do_something_with_exclusion               Do_something_with_exclusion

'  Thread_signal = true -------------------> While Thread_signal <> true

'  CondSignal(cond) -------------------------> CondWait(cond, mut)

'  Do_something_with_exclusion      .------> Wend

'MutexUnlock(mut) ------------------'        Thread_signal = false

'.....                                       Do_something_with_exclusion

'.....                                     MutexUnlock(mut)

'.....                                     .....





Dim Shared As Any Ptr mutex

Dim Shared As Any Ptr cond

Dim Shared As String txt

Dim As Any Ptr pt

Dim Shared As Integer ok = 0



Sub thread (ByVal p As Any Ptr)

    Print "thread is complementing the string"

    MutexLock(mutex)

    Sleep 400, 1

    txt &= " complemented by thread"

    ok = 1

    CondSignal(cond)

    MutexUnlock(mutex)

    Print "thread signals the processing completed"

End Sub



mutex = MutexCreate

cond = CondCreate



txt = "example of text"

Print "main() initializes a string = " & txt

Print "main creates one thread"

Print

pt = ThreadCreate(@thread)

MutexLock(mutex)

While ok <> 1

    CondWait(cond, mutex)

Wend

Print

Print "back in main(), the string = " & txt

ok = 0

MutexUnlock(mutex)



ThreadWait(pt)

MutexDestroy(mutex)

CondDestroy(cond)

Reference

Documented in KeyPgCondSignal.html