ReadLine from a network stream hangs if it is empty

[Scott]

Hi guys,

If I try to call read(), readline(), readtoend() and
there is nothing to read (from a never ending loop for
example) the program seems to continue but it exits the
loop for no apparent reason.

We also can't check the stream for the length, as the
network stream doesn't support seek operations

MSDN reckons that the functions should return null if
there is nothing to read but it doesn't.

Can anybody post some example code that shows how to read
the entire contents of a network stream and handling if
there isn't anything in it?

Thanks in advance,


Scott

[Jon Skeet [C# MVP]]

Re: ReadLine from a network stream hangs if it is empty

Scott <scott@discussi ons.microsoft.c om> wrote:[color=blue]
> If I try to call read(), readline(), readtoend() and
> there is nothing to read (from a never ending loop for
> example) the program seems to continue but it exits the
> loop for no apparent reason.
>
> We also can't check the stream for the length, as the
> network stream doesn't support seek operations
>
> MSDN reckons that the functions should return null if
> there is nothing to read but it doesn't.[/color]

No, ReadLine should return null if the stream has been *closed*. It
should block if there's just no data ready.
[color=blue]
> Can anybody post some example code that shows how to read
> the entire contents of a network stream and handling if
> there isn't anything in it?[/color]

Your protocol should indicate when all the data has been read, either
by stating in advance how much there is to read, or by indicating the
"end of data". Alternatively, if the client isn't expected to do
anything else afterwards, the server can close the socket.

--
Jon Skeet - <skeet@pobox.co m>

http://www.pobox.com/~skeet

If replying to the group, please do not mail me too

[Scott]

Re: ReadLine from a network stream hangs if it is empty

Hi Jon,

Thanks for the info, I guess my real problem is that I
don't know what the correct method is to check if there
is any data in the stream without causing it to screw up
(if there is no data ready)

Is there a way to check if readline has anything in it
before trying to get data from it? because currently if
we try and get the contents of readline() into a variable
and readline is empty, it exits our 'never ending' loop
and processing stops completely, but we dont get any
errors thrown at us

Cheers,

Scott

[color=blue]
>-----Original Message-----
>Scott <scott@discussi ons.microsoft.c om> wrote:[color=green]
>> If I try to call read(), readline(), readtoend() and
>> there is nothing to read (from a never ending loop for
>> example) the program seems to continue but it exits[/color][/color]
the[color=blue][color=green]
>> loop for no apparent reason.
>>
>> We also can't check the stream for the length, as the
>> network stream doesn't support seek operations
>>
>> MSDN reckons that the functions should return null if
>> there is nothing to read but it doesn't.[/color]
>
>No, ReadLine should return null if the stream has been[/color]
*closed*. It[color=blue]
>should block if there's just no data ready.
>[color=green]
>> Can anybody post some example code that shows how to[/color][/color]
read[color=blue][color=green]
>> the entire contents of a network stream and handling[/color][/color]
if[color=blue][color=green]
>> there isn't anything in it?[/color]
>
>Your protocol should indicate when all the data has been[/color]
read, either[color=blue]
>by stating in advance how much there is to read, or by[/color]
indicating the[color=blue]
>"end of data". Alternatively, if the client isn't[/color]
expected to do[color=blue]
>anything else afterwards, the server can close the[/color]
socket.[color=blue]
>
>--
>Jon Skeet - <skeet@pobox.co m>
>http://www.pobox.com/~skeet
>If replying to the group, please do not mail me too
>.
>[/color]

[Kevin Aubuchon]

Re: ReadLine from a network stream hangs if it is empty

Use the DataAvailable property on NetworkStream.

kevin aubuchon


"Scott" <Scott@discussi ons.microsoft.c om> wrote in message
news:3d4401c472 97$419d1be0$a50 1280a@phx.gbl.. .[color=blue]
> Hi Jon,
>
> Thanks for the info, I guess my real problem is that I
> don't know what the correct method is to check if there
> is any data in the stream without causing it to screw up
> (if there is no data ready)
>
> Is there a way to check if readline has anything in it
> before trying to get data from it? because currently if
> we try and get the contents of readline() into a variable
> and readline is empty, it exits our 'never ending' loop
> and processing stops completely, but we dont get any
> errors thrown at us
>
> Cheers,
>
> Scott
>
>[color=green]
> >-----Original Message-----
> >Scott <scott@discussi ons.microsoft.c om> wrote:[color=darkred]
> >> If I try to call read(), readline(), readtoend() and
> >> there is nothing to read (from a never ending loop for
> >> example) the program seems to continue but it exits[/color][/color]
> the[color=green][color=darkred]
> >> loop for no apparent reason.
> >>
> >> We also can't check the stream for the length, as the
> >> network stream doesn't support seek operations
> >>
> >> MSDN reckons that the functions should return null if
> >> there is nothing to read but it doesn't.[/color]
> >
> >No, ReadLine should return null if the stream has been[/color]
> *closed*. It[color=green]
> >should block if there's just no data ready.
> >[color=darkred]
> >> Can anybody post some example code that shows how to[/color][/color]
> read[color=green][color=darkred]
> >> the entire contents of a network stream and handling[/color][/color]
> if[color=green][color=darkred]
> >> there isn't anything in it?[/color]
> >
> >Your protocol should indicate when all the data has been[/color]
> read, either[color=green]
> >by stating in advance how much there is to read, or by[/color]
> indicating the[color=green]
> >"end of data". Alternatively, if the client isn't[/color]
> expected to do[color=green]
> >anything else afterwards, the server can close the[/color]
> socket.[color=green]
> >
> >--
> >Jon Skeet - <skeet@pobox.co m>
> >http://www.pobox.com/~skeet
> >If replying to the group, please do not mail me too
> >.
> >[/color][/color]

[Jon Skeet [C# MVP]]

Re: ReadLine from a network stream hangs if it is empty

Scott <Scott@discussi ons.microsoft.c om> wrote:[color=blue]
> Thanks for the info, I guess my real problem is that I
> don't know what the correct method is to check if there
> is any data in the stream without causing it to screw up
> (if there is no data ready)
>
> Is there a way to check if readline has anything in it
> before trying to get data from it? because currently if
> we try and get the contents of readline() into a variable
> and readline is empty, it exits our 'never ending' loop
> and processing stops completely, but we dont get any
> errors thrown at us[/color]

As Kevin says, you can use DataAvailable - but then you can easily read
to the point where that will return false even though there's more data
which is *going* to be sent. As I say, the *protocol* should indicate
whether or not there should be any data in the stream.

--
Jon Skeet - <skeet@pobox.co m>

http://www.pobox.com/~skeet

If replying to the group, please do not mail me too

[Scrubbing Bubbles]

Re: ReadLine from a network stream hangs if it is empty



while(true)
{

int bytes = cSocket.Receive (buffer, buffer.Length, 0);
mes += ASCII.GetString (buffer, 0, bytes);

if(bytes < buffer.Length)
{
break;
}
}


Scott wrote:
[color=blue]
> Hi Jon,
>
> Thanks for the info, I guess my real problem is that I
> don't know what the correct method is to check if there
> is any data in the stream without causing it to screw up
> (if there is no data ready)
>
> Is there a way to check if readline has anything in it
> before trying to get data from it? because currently if
> we try and get the contents of readline() into a variable
> and readline is empty, it exits our 'never ending' loop
> and processing stops completely, but we dont get any
> errors thrown at us
>
> Cheers,
>
> Scott
>
>[color=green]
>>-----Original Message-----
>>Scott <scott@discussi ons.microsoft.c om> wrote:[color=darkred]
>>> If I try to call read(), readline(), readtoend() and
>>> there is nothing to read (from a never ending loop for
>>> example) the program seems to continue but it exits[/color][/color]
> the[color=green][color=darkred]
>>> loop for no apparent reason.
>>>
>>> We also can't check the stream for the length, as the
>>> network stream doesn't support seek operations
>>>
>>> MSDN reckons that the functions should return null if
>>> there is nothing to read but it doesn't.[/color]
>>
>>No, ReadLine should return null if the stream has been[/color]
> *closed*. It[color=green]
>>should block if there's just no data ready.
>>[color=darkred]
>>> Can anybody post some example code that shows how to[/color][/color]
> read[color=green][color=darkred]
>>> the entire contents of a network stream and handling[/color][/color]
> if[color=green][color=darkred]
>>> there isn't anything in it?[/color]
>>
>>Your protocol should indicate when all the data has been[/color]
> read, either[color=green]
>>by stating in advance how much there is to read, or by[/color]
> indicating the[color=green]
>>"end of data". Alternatively, if the client isn't[/color]
> expected to do[color=green]
>>anything else afterwards, the server can close the[/color]
> socket.[color=green]
>>
>>--
>>Jon Skeet - <skeet@pobox.co m>
>>http://www.pobox.com/~skeet
>>If replying to the group, please do not mail me too
>>.
>>[/color][/color]

[Jon Skeet [C# MVP]]

Re: ReadLine from a network stream hangs if it is empty

Scrubbing Bubbles <scrubbling.bub bles@scrubbing. bubbles> wrote:[color=blue]
> while(true)
> {
>
> int bytes = cSocket.Receive (buffer, buffer.Length, 0);
> mes += ASCII.GetString (buffer, 0, bytes);
>
> if(bytes < buffer.Length)
> {
> break;
> }
> }[/color]

No, that's no good - there's no guarantee that a network stream will
return data filling the whole buffer even if there's then going to be
data available in a moment.

--
Jon Skeet - <skeet@pobox.co m>

http://www.pobox.com/~skeet

If replying to the group, please do not mail me too

[Jell-O Biafra]

Re: ReadLine from a network stream hangs if it is empty


How about this?

// Address of URL
string URL = textBox1.Text;
try
{
// Get HTML data
WebClient client = new WebClient();
Stream data = client.OpenRead (URL);
StreamReader reader = new StreamReader(da ta);
string str = "";
str = reader.ReadLine ();


while( str != null)
{
Console.WriteLi ne(str);
str = reader.ReadLine ();
}
data.Close();
}
catch(WebExcept ion exp)
{
MessageBox.Show (exp.Message, "Exception" );
}


Jon Skeet [C# MVP] wrote:
[color=blue]
> Scrubbing Bubbles <scrubbling.bub bles@scrubbing. bubbles> wrote:[color=green]
>> while(true)
>> {
>>
>> int bytes = cSocket.Receive (buffer, buffer.Length, 0);
>> mes += ASCII.GetString (buffer, 0, bytes);
>>
>> if(bytes < buffer.Length)
>> {
>> break;
>> }
>> }[/color]
>
> No, that's no good - there's no guarantee that a network stream will
> return data filling the whole buffer even if there's then going to be
> data available in a moment.
>[/color]

[Jon Skeet [C# MVP]]

Re: ReadLine from a network stream hangs if it is empty

Jell-O Biafra <jbiafra@dead.k ennedies> wrote:[color=blue]
> How about this?
>
> // Address of URL
> string URL = textBox1.Text;
> try
> {
> // Get HTML data
> WebClient client = new WebClient();
> Stream data = client.OpenRead (URL);
> StreamReader reader = new StreamReader(da ta);
> string str = "";
> str = reader.ReadLine ();
>
>
> while( str != null)
> {
> Console.WriteLi ne(str);
> str = reader.ReadLine ();
> }
> data.Close();
> }
> catch(WebExcept ion exp)
> {
> MessageBox.Show (exp.Message, "Exception" );
> }[/color]

Well, I'd use:

string str;

while ( (str=reader.Rea dLine()) != null)
{
...
}

myself, but yes, that's the gist of it. (I'd also use using statements
to close the reader/stream, btw.)

However, that then gets back to the OP's problem of ReadLine blocking
when there's no more data available but the stream hasn't been closed
by the other end.

--
Jon Skeet - <skeet@pobox.co m>

http://www.pobox.com/~skeet

If replying to the group, please do not mail me too