"Turn" IO-synchronous, asynchronous, blocking, non-blocking (mend)

The concept is important and must be mastered. Practice is based on their own yo

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http://blog.csdn.net/historyasamirror/article/details/5778378

Stevens compared five IO Model in the article:
Blocking IO
nonblocking IO
IO multiplexing
Signal Driven IO
Asynchronous IO
Since signal driven IO is not commonly used in practice, I only refer to the remaining four IO Model.

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First answer the simplest of this: blocking vs non-blocking. The difference between the two is clearly explained in the previous introduction. Calling blocking IO will block the corresponding process until the operation is complete, and non-blocking IO will return immediately when the kernel is ready for the data.

Before explaining the difference between synchronous IO and asynchronous IO, you need to give a definition of both. The definition given by Stevens (in fact, the definition of POSIX) is this:
A Synchronous I/O operation causes the requesting process to being blocked until that I/O operationcompletes;
An asynchronous I/O operation does not cause the requesting process to be blocked;
The difference is that synchronous IO will block the process when it does "IO operation". According to this definition, the blocking io,non-blocking Io,io Multiplexing described previously are synchronous IO. One might say that non-blocking io is not block. Here is a very "tricky" place, defined in the "IO operation" refers to the real IO operation, is the example of recvfrom this system call. Non-blocking IO does not block the process when it executes recvfrom this system call if the kernel data is not ready. However, when the data in the kernel is ready, recvfrom copies the data from the kernel to the user's memory, at which point the process is blocked, during which time the process is block. The asynchronous IO is not the same, and when the process initiates an IO operation, the direct return is ignored until the kernel sends a signal telling the process that IO is complete. Throughout this process, the process has not been blocked at all.

Comparison of each IO model:

As described above, the difference between non-blocking io and asynchronous io is obvious. In non-blocking io, although the process will not be blocked for most of the time, it still requires the process to go to the active check, and when the data is ready, it is also necessary for the process to proactively call Recvfrom to copy the data to the user's memory. and asynchronous Io is completely different. It's like a user process handing over an entire IO operation to someone else (kernel) and then sending a signal notification when someone finishes it. During this time, the user process does not need to check the status of the IO operation, nor does it need to actively copy the data.

Finally, a few more examples are not very appropriate to illustrate these four IO Model:
There are a,b,c,d of four people in fishing:
A used is the most old-fashioned fishing rod, so, have to keep guarding, wait until the fish hooked up again lever;
B's Fishing rod has a function, can show whether there are fish hooked, so, B and next to the MM chat, and then see if there are fish bait, some words on the rapid lever;
C with the fishing rod and B almost, but he thought of a good way, is to put several fishing rods at the same time, and then keep in the side, once the show said the fish hooked, it will be the corresponding rod pull up;
D is a rich man, simply hired a person to help him to fish, once the person caught the fish up, send a message to D.

"Turn" IO-synchronous, asynchronous, blocking, non-blocking (mend)