On 20/11/2014 14:16, lin li wrote:

Hi, guys. I have some problem with null_buffers in Asio. According to the document, 'A |null_buffers| operation doesn't return until the I/O object is "ready" to perform the operation'. However, when I tried to use it in a asynchronous read, it keeps returning after all data has been received and the other end has closed the socket! So here is my questions: 1. what is "ready" exactly?

I'm not 100% certain of this, but I believe it just means "becomes readable". It doesn't actually read anything though so you have to do a "real" read in your handler before doing a null_buffers read again, or it will just stay ready forever. Usually there's not a lot of point in doing a null_buffers + not-null_buffers read -- just combine them into one real read. The exception is when you're trying to wrap external code that does a synchronous read internally; null_buffers provides a way to make it asynchronous.

2. If we use null_buffers with async_read_some, will it detect some errors like disconnection and pass the error code to the completion handler?

With sockets at least it is not guaranteed that you will *ever* get a disconnection event from a read operation. You have to send something to detect ungraceful disconnects (such as remote device abruptly lost network connection or power cycled).