Re: [Boost-users] [asio] Socket closed notification?

Jonathan Franklin writes:

On Mon, Dec 14, 2009 at 9:17 PM, Scott Gifford wrote:

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Andrew Maclean writes:

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On Tue, Dec 15, 2009 at 2:50 PM, Scott Gifford wrote:

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Jonathan Franklin writes: Sure, just use any of the read functions and see if they return an error.  In my application, I use read_until(), but any function that reads should do.

If the remote app closes unexpectedly, you read will return EOF.

From when I tested this a while back, my notes indicate I got one of these errors for an unclean shutdown (timeout or a TCP RST packet):

boost::system::errc::no_such_file_or_directory boost::asio::error::shut_down but it's been awhile and I don't rememberly clearly under exactly what circumstances.

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Unfortunately, write isn't reliable since it will always succeed until you fill up your send buffer.

Well, OK, maybe a write and a flush.  Certainly there are othe buffers, but typically a TCP implementation will time out at some point if it has some pending data which is not acknowledged, and typically once data is flushed to the TCP layer it will try to write it and begin waiting for its timeout.

Yeah, it's not reliable, and can take *many* writes.

The TCP stack may have to do many writes, but the app should only have to write once and do a flush to detect an unresponsive other side. At the very least, this is a technique I have consistently seen recommended, and has always worked for me. ----Scott.