PureBasic Forums - English

The WaitOnAddress function can be used by a thread to wait for a particular value to change from some undesired value to any other value. WaitOnAddress is more efficient than using the Sleep function inside a while loop because WaitOnAddress does not interfere with the thread scheduler. WaitOnAddress is also simpler to use than an event object because it is not necessary to create and initialize an event and then make sure it is synchronized correctly with the value. WaitOnAddress is not affected by low-memory conditions, other than potentially waking the thread early as noted below.

Any thread within the same process that changes the value at the address on which threads are waiting should call WakeByAddressSingle to wake a single waiting thread or WakeByAddressAll to wake all waiting threads. If WakeByAddressSingle is called, other waiting threads continue to wait.

OpenLibrary(0, "api-ms-win-core-synch-l1-2-0.dll")
Prototype WaitOnAddress(*Address, *CompareAddress, AddressSize.l, dwMilliseconds.l)
Prototype WakeByAddressSingle(*Address)

Global WaitOnAddress.WaitOnAddress = GetFunction(0, "WaitOnAddress")
Global WakeByAddressSingle.WakeByAddressSingle = GetFunction(0, "WakeByAddressSingle")
Global g_TargetValue

Procedure Thread(val)
  
  Debug "Waiting for value at address " + @g_TargetValue + " to change"
  UndesiredValue = 0 ;<-- if this value matches g_TargetValue, thread will sleep and wait for change
  WaitOnAddress(@g_TargetValue, @UndesiredValue, 1, #INFINITE)
  Debug "Value at address " + @g_TargetValue + " changed"
  
EndProcedure

Thread = CreateThread(@Thread(), 0)

g_TargetValue = 0 ;<-- same as Undesired value so thread will sleep
WakeByAddressSingle(@g_TargetValue)

Delay(3000)

g_TargetValue = 1 ;<-- different to Undesired value so thread will resume
WakeByAddressSingle(@g_TargetValue)

WaitThread(Thread)

Caronte3D wrote: Tue Feb 14, 2023 12:16 pm Thi's correct?

Code: Select all

    CapturedValue = g_TargetValue
    UndesiredValue = g_TargetValue

Caronte3D wrote: Tue Feb 14, 2023 4:33 pm Ah! Ok, I understand better now
Just curious... What is the advantage of using this dll instead of simply check directly if the variable was changed?