Re: [Boost-users] Boost.Asio -- combining sync read with async read (Igor R)

Subject: Re: [Boost-users] Boost.Asio -- combining sync read with async read (Igor R)
From: Igor R (boost.lists_at_[hidden])
Date: 2009-10-14 12:25:39

 > how do I know if the previous read is complete? I figured when
async_read_until() calls the handler, the read was complete and everything
read was sitting in the buffer?

Correct. But do not call another asio::async_read_until() before the first
one's handler is invoked (for the same socket).

> Do you mean to say that if the client receives two lines, then
async_read_until() will read both lines but then put only the first line in
the buffer?

No, it will read anything available in the socket and put it in the buffer.
It can be 1 line, 2 lines or 3.25 lines. What I mean is that you have to be
aware of the fact that the buffer might contain more data than you
"requested" from read_until.

> Then I can't call async_read_until() again because there is still another
pending call to the handler that will process the second line? So how do I
know at the end of the handler whether or not to call async_read_until() ?

Every time you get the read hanlder called, you have to call async_read_XXX
again - until your application decides that it doesn't want data from the
server anymore.

But imagine the following scenario (though I don't know if it's relevant to
your application): you expect 3 lines from the server, and want to process
them line by line.
//...
asio::async_read_until(socket_, buffer_, "\r\n", ...);
//...

void handle(...)
{
  process(buffer_);
   asio::async_read_until(socket_, buffer_, "\r\n", ...);
}

If the server may send 3 lines at once, and process() function assumes that
the buffer contains 1 line, it's a bug: the 2 latter lines in the buffer_
will never be processed, and the second async_read_until() will never
complete, because the server already sent all it wanted.