Programming C++Builder How to avoid simultaneous access in multi-threaded applications?
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How to avoid 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.

The VCL and CLX include support for three techniques that prevent other threads from accessing the same memory as your thread:

  1. Locking objects
  2. Using critical sections
  3. Using a multi-read exclusive-write synchronizer

How to lock the 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.

The VCL and CLX also both include a thread-safe list object, TThreadList. Calling TThreadList::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 in the same thread.

How to use the critical sessions?

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 method so that other threads can access the global memory by calling Acquire.

Warning: Critical sections only work if every thread uses them to access the associated global memory. Threads that ignore the critical section and access the global memory without calling Acquire can introduce problems of simultaneous access.

For example, consider an application that has a global critical section variable, pLockXY, that blocks access to global variables X and Y. Any thread that uses X or Y must surround that use with calls to the critical section such as the following:

pLockXY->Acquire(); // lock out other threads

try
{
  Y = sin(X);
}
__finally
{
  pLockXY->Release();
}

How to use 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 threads to read the memory it protects as long as no thread is writing to it. Threads must have exclusive access to write to memory protected by TMultiReadExclusiveWriteSynchronizer.

To use a multi-read exclusive-write synchronizer, create a global instance of TMultiReadExclusiveWriteSynchronizer that is associated with the global memory you want to protect. Every thread that reads from this memory must first call the BeginRead method. BeginRead ensures that no other thread is currently writing to the memory. When a thread finishes reading the protected memory, it calls the EndRead method. Any thread that writes to the protected memory must call BeginWrite first. BeginWrite ensures that no other thread is currently reading or writing to the memory. When a thread finishes writing to the protected memory, it calls the EndWrite method, so that threads waiting to read the memory can begin.

Warning: Like critical sections, the multi-read exclusive-write synchronizer only works if every thread uses it to access the associated global memory. Threads that ignore the synchronizer and access the global memory without calling BeginRead or BeginWrite introduce problems of simultaneous access.


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