Joined: 23 Aug 2006
|Posted: Mon Sep 04, 2006 4:26 am Post subject: Synchronizing threads in Windows
|Synchronizing threads in Windows
BY DR.WAEL ALBAYAYDH
A thread describes a path of execution within a process. Every time a process in initialized, the system creates a primary thread. This thread starts at the C runtime's start-up code, which in turn calls the WinMain function and continues executing until the WinMain function returns and the C runtime's start up code calls ExitProcess. For many applications, this primary thread is the only one the application requires. However, processes can create additional threads to help them do their work. The whole idea behind creating additional threads is to utilize as much of the CPU's power as possible.
As we know, to run threads, we need to schedule them. In order to run them effectively, they need to be synchronized. Suppose one thread creates a brush and then creates several threads that share the brush and draw with it. The first thread must not destroy the brush until the other threads finish drawing. This requires a means of coordinating the sequence of actions in several threads.
One way is to create a global Boolean variable that one thread uses to signal another. The writing thread will set this parameter to TRUE and the reading thread might loop until it sees the flag change. This will definitely work, but the looping thread wastes a lot of processor time.
Instead, Win32 supports a set of synchronization objects such as mutexes, semaphores, events and critical sections. These are the system objects created by the object manager. All of them will work in a similar way. A thread that wants to perform some coordinated action waits for a response from one of these objects and proceeds only after receiving it. The scheduler removes the waiting objects from the dispatch queue so that they will not consume processor time
It is important to keep in mind that:
• A mutex object works like a narrow gate for one thread to pass at a time.
• A semaphore object work like a multi-lane gate that a limited number of threads can pass through together.
• An event object broadcasts a public signal for any listening thread to hear.
• A critical section object works like a mutex but only within a single process.
Mutexes, semaphores and events can coordinate threads in different processes, but critical sections are only visible to threads in a single process.
Mutexex: These are very much like critical sections except that they can be used to synchronize data across multiple processes. To do this, a thread in each process must have its own process-relative handle to a single mutex object.
Semaphores: These objects are used for resource counting. They offer a thread the ability to query the number of resources available; if one or more resources are available, the count of available resources is decremented. Thus semaphores perform the test and set operations automatically, that is, when you request a resource from a semaphore, the operating system checks whether the resource is available and decrements the count of the available resources without letting another thread interfere. Only after the resource count has been decremented does the system allow another thread to request a resource.
For example, let us say that a computer has three serial ports. No more than three threads can use the serial ports at any given time; each port can be assigned to one thread. This situation provides a perfect opportunity to use a semaphore. To monitor serial port usage, you can create a semaphore with a count of three - one for each port. A semaphore is signaled when its resource count is greater than zero and is non-signaled when the count is zero.
Because several threads can affect a semaphore's resource count, a semaphore, unlike a critical section or mutex, is not considered to be owned by a thread. This means that it is possible to have one thread wait for the semaphore object and another thread release the object.
Events: Even objects are the most primitive form of synchronization objects and they are quite different from mutexes and semaphores. Mutexes and semaphores are usually used to control access to data, but events are used to signal that some operation has been completed.
There are two different types of event objects - manual reset events and auto reset events. A manual reset event is used to signal several threads simultaneously to say that an operation has finished, and an auto reset event is used to signal a single thread to say that an operation has been completed.
Events are most commonly used when one thread performs initialization work and, when it finishes, signals another thread to perform the remaining work. The initialization thread sets the event to the non-signaled state and begins to perform the initialization. Then, after the initialization has been completed, the thread sets the event to the signaled state. Waiting for the event, the worker thread wakes up and performs the rest of the work.
For example, a process might be running two threads. The first thread reads data from a file into a memory buffer. After the data has been read, the first thread signals the second thread that it can process the data. When the second thread finishes processing the data, it might need to signal the first thread again, so that the first thread can read the next block of data from the file.
Critical sections: A critical section is a small section of the code that required exclusive access to some shared data before the code can execute. Of all synchronization objects, critical sections are the simplest to use, but they can be used to synchronize threads only within a single process. Critical sections allow only one thread at a time to gain access to a region of data.
Dr.Wael AlBayaydh has a PhD degree in computer engineering. He has been working in information technology for several years, concentrating on areas such as operating system, networking, network security, electronic commerce, Internet services, LDAP and Web servers. AlBayaydh has authored a number of articles for trade publications, and he presents his own papers at industry conferences. He can be reached at wr_y@spam