Very Computer

Question - how do you determine if a C++ ifstream has buffered any
           unread data?  (without a blocking call like peek() or get())

I have some socket-based communication code where messages are sent
through C++ streams (ifstream/ofstream) bound to stream-type sockets.
The sockets are blocking, but I don't want the input-parsing code
to block on reads.  So I am using the select() call on the ifstream's
file descriptor to determine whether there is any unread data on the
ifstream.  But select() isn't accurate -- sometimes it claims that
there is no unread data when the ifstream still hasn't parsed all
the messages from the socket.

I think that this is a C++ stream buffering issue.  It may be that
calls to ifstream.operator>>() (for parsing input) buffer all available
data from the socket into a private buffer.  Then select() really sees
no unread data on the socket's file descriptor, even when my code
has not parsed all the socket messages from the ifstream.

So the question is - how do I tell when the ifstream has buffered
data available?  (using non-blocking calls)  Is there some member
function that tells you?  What do gcount() and gtell() do?

  thanks,

Eylon Caspi
Naval Research Lab, D.C.
Tel. (202) 767-3107

  ...

Quote:

> So the question is - how do I tell when the ifstream has buffered
> data available?  (using non-blocking calls)  Is there some member
> function that tells you?  What do gcount() and gtell() do?

        my_ifstream.rdbuf()->in_avail();

Now the question is whether the streambuf class interface is
standardized?  i.e. does streambuf::in_avail() exist in all
C++ compilers?  I have found it in g++, Centerline, and Sun
SPARCworks C++ compilers (all on UNIX), but each implementation
has a slightly different meaning.  The Centerline C++ version
returns the number of unread buffered characters, but the g++
2.7.0 version returns only 1 or 0.

--
Eylon Caspi
Naval Research Lab, D.C.
Tel. (202) 767-3107

Hi,
: Question - how do you determine if a C++ ifstream has buffered any
:            unread data?  (without a blocking call like peek() or get())

You might try 'instream.rdbuf()->in_avail()' (if it is already
supported by your IOStreams library; at least it is part of the
standard C++ library). I think for 'ifstream' it should do what you
want (i.e. 'showmanyc()' will always return '0' such that 'in_avail()'
always return the number of characters).

: So the question is - how do I tell when the ifstream has buffered
: data available?  (using non-blocking calls)  Is there some member
: function that tells you?  What do gcount() and gtell() do?

'gtell()' is not a standard function and 'gcount()' returns the number
of characters extracted by the last call to an unformatted input
function for the object.
--

<http://www.informatik.uni-konstanz.de/~kuehl/>
I am a realistic optimist - that's why I appear to be slightly pessimistic