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Coordinating threads
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Name | Description
Other Techniques for Sharing Memory | When using VCL objects, use the main thread to execute your code. Using the main thread ensures that the object does not indirectly access any memory that is also used by VCL objects in other threads. See Using the Main VCL Thread for more information on the main thread. If the global memory does not need to be shared by multiple threads, consider using thread-local variables instead of global variables. By using thread-local variables, your thread does not need to wait for or lock out any other threads. See Using Thread-local Variables for more information about thread-local variables.
Avoiding Simultaneous Access | To avoid clashing with other threads when accessing global objects or variables, you may need to block the execution of other threads until your thread code has finished an operation. Be careful not to block other execution threads unnecessarily. Doing so can cause performance to degrade seriously and negate most of the advantages of using multiple threads. Three techniques prevent other threads from accessing the same memory as your thread: Locking Objects Using Critical Sections Using a Multi-read Exclusive-write Synchronizer
Coordinating Threads | When writing the code that runs when your thread is executed, you must consider the behavior of other threads that may be executing simultaneously. In particular, care must be taken to avoid two threads trying to use the same global object or variable at the same time. In addition, the code in one thread can depend on the results of tasks performed by other threads. Whether using thread objects or generating threads using BeginThread, the following topics describe techniques for coordinating threads: Avoiding Simultaneous Access Waiting for Other Threads Using the Main VCL Thread When global memory does not... more
Locking Objects | Some objects have built-in locking that prevents the execution of other threads from using that object instance. For example, canvas objects (TCanvas and descendants) have a Lock method that prevents other threads from accessing the canvas until the Unlock method is called. VCL applications also include a thread-safe list object, TThreadList. Calling LockList returns the list object while also blocking other execution threads from using the list until the UnlockList method is called. Calls to TCanvas.Lock or TThreadList.LockList can be safely nested. The lock is not released until the last locking call is matched with a corresponding unlock call... more
Using Critical Sections | If objects do not provide built-in locking, you can use a critical section. Critical sections work like gates that allow only a single thread to enter at a time. To use a critical section, create a global instance of TCriticalSection. TCriticalSection has two methods, Acquire(which blocks other threads from executing the section) and Release(which removes the block). Each critical section is associated with the global memory you want to protect. Every thread that accesses that global memory should first use the Acquire method to ensure that no other thread is using it. When finished, threads call the Release... more
Using the Multi-read Exclusive-write Synchronizer | When you use critical sections to protect global memory, only one thread can use the memory at a time. This can be more protection than you need, especially if you have an object or variable that must be read often but to which you very seldom write. There is no danger in multiple threads reading the same memory simultaneously, as long as no thread is writing to it. When you have some global memory that is read often, but to which threads occasionally write, you can protect it using TMultiReadExclusiveWriteSynchronizer. This object acts like a critical section, but allows multiple... more
Waiting for a Task to Be Completed | Sometimes, you need to wait for a thread to finish some operation rather than waiting for a particular thread to complete execution. To do this, use an event object. Event objects (TEvent) should be created with global scope so that they can act like signals that are visible to all threads. When a thread completes an operation that other threads depend on, it calls TEvent.SetEvent. SetEvent turns on the signal, so any other thread that checks will know that the operation has completed. To turn off the signal, use the ResetEvent method. For example, consider a situation where you... more
Waiting for a Thread to Finish Executing | To wait for another thread to finish executing, use the WaitFor method of that other thread. WaitFor doesn't return until the other thread terminates, either by finishing its own Execute method or by terminating due to an exception. For example, the following code waits until another thread fills a thread list object before accessing the objects in the list:
Waiting for Other Threads | If your thread must wait for another thread to finish some task, you can tell your thread to temporarily suspend execution. You can either Wait for another thread to completely finish executing, or Wait for a task to be completed.

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