Explanation of synchronous, asynchronous, blocking, and non-blocking

Source: Internet
Author: User

From: csdn http://blog.csdn.net/MSDA/article/details/5766917

During network programming, we often see four call Methods: synchronous, asynchronous, blocking, and non-blocking. These methods are not easy to understand. The following are my differences in these terms.


The so-called synchronization means that when a function call is sent, the call will not return until the result is not obtained. According to this definition, most functions are called synchronously (such as sin and isdigit ). But in general, when we talk about synchronization and Asynchronization, we are referring to the tasks that require the collaboration of other components or a certain amount of time. The most common example is sendmessage. This function sends a message to a window. This function does not return a message before the recipient finishes processing the message. After the other party completes processing, the function returns the lresult value returned by the message processing function to the caller.


Asynchronous concept and synchronization relative. When an asynchronous process is called, the caller cannot obtain the result immediately. After the call is completed, the caller is notified by status, notification, and callback. Take the casycsocket class as an example (note that csocket is derived from casyncsocket, but the function has been converted from Asynchronous to synchronous). When a client sends a connection request by calling the connect function, the caller's thread can run down immediately. After the connection is established, the socket underlying layer sends a message to notify the object.
The Execution Component and caller can return results in three ways: Status, notification, and callback. Which one can be used depends on the implementation of the execution part, unless the execution part provides multiple options, it is not controlled by the caller. If the execution part is notified by the status, the caller needs to check the status at intervals, which is very inefficient. (Some new programmers prefer to use a loop to check the value of a variable, this is actually a very serious error ). If you use the notification method, the efficiency is very high, because the execution of components almost do not need to do additional operations. As for the callback function, there is actually no much difference with the notification.


Blocking call means that the current thread will be suspended before the call result is returned. The function is returned only after the result is obtained.
Some people may equate blocking calls with synchronous calls. In fact, they are different. For synchronous calls, the current thread is still activated in many cases, but the current function does not return logically. For example, we call the receive function in csocket. If there is no data in the buffer zone, this function will wait until data is returned. At this time, the current thread will continue to process a variety of messages. If the main window and the calling function are in the same thread, the main interface should be refreshed unless you call the function in special interface operations.
Another function Recv used by socket to receive data is an example of blocking calls. When the socket works in blocking mode, if the function is called without data, the current thread will be suspended until there is data.


The concept of non-blocking corresponds to blocking, which means that the function will not block the current thread and return immediately before the result cannot be obtained immediately.

Object blocking mode and function calling

Whether the object is in blocking mode is highly correlated with whether the function is blocked or not, but it is not one-to-one. Blocking objects can have non-blocking calling methods. We can use certain APIs to poll the status and call the blocking function as appropriate to avoid blocking. For non-blocking objects, calling special functions can also be blocked. The select function is an example.

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