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Master's column retardedFor example, system calls, the application will be blocked and kernel context switch. Then triggered read operation, when the response returns (from our being read from the device to return), the data is moved to the user space buffer. Then the application is unblocked (read call returns).For a simple example, like you go to a bank counter to save money. First, you will save money list filled, then handed the teller. Here, you like that application, the call list iscall, the teller is the kernel. Submit a good list, you sat on the counter, etc., which is equivalent to begin to wait. After the teller will give you a receipt to run, which means to run, this isresponse. Then you can hold receipts do other things. Note that this time, if you go right after finishing audit, has been hit save money on your account. Later you will findIt is important to talk about the next synchronization nonblocking IO look at this chart,In linux, the application can set the file descriptor attribute O_NONBLOCK, I / O operations can return immediately, but does not guarantee I / O operation was successful. That is, when the application is set O_NONBLOCK, perform write operations, call the appropriate system call, the call will return immediately from the kernel. However, at this point in time the return data may not be written to the specified real place. That is, kernel just quickly return to thissystem call (so that the application will not be the IO operation blocking), but the call to perform specific things (write data) may not be complete. For applications, although this IO operation soon returned, but it does not know whether this really successful IO operation, if you wantKnow, you need to take the initiative to ask the kernel application this is not go to the bank to save money, but to bank transfer. Similarly, you also need to fill out the money order and then handed the teller, teller for aSome simple procedures can give you a receipt processing. But you get the receipt does not mean money has hit the other side of the account. In fact, the general remittance cycle is about 24 hours, if you want to save the model toRemittances, then means that you need in the bank, etc. 24 hours, which is clearly unrealistic. Therefore, the non-blocking IO synchronization in real life also has its meaning again to talk about blocking asynchronous IO in linux, often through select / poll mechanism to achieve this figure, for example, as before, an application to perform a read operation A call to a system call, the call is passed to the kernel. But in the application side, it calls call after the kernel does not wait to return response, it is not the same as the previous two mechanisms place. This is why it is called asynchronously causes. But why call blockingIt? This is because although the application is an asynchronous manner, but the select () function converts the application blocked and wait until the outcome of this call to return, and then notify the application. In other words, I / O, and then use the blocking select system call to determine an I / O