Re: [Boost-users] [Asio] null_buffers problem

Subject: Re: [Boost-users] [Asio] null_buffers problem
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2014-11-24 01:57:46

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).