Tcl server receiving empty string from python client

[Shaun Kulesa]

Hello, I was testing sockets in tcl and I got a tcl server and tcl client to communicate correctly.

I thought I would try using a python client to send data to my tcl server.

The python client will send data but the tcl server will output it as a empty string when I get it from the channel.

I was wondering if this was the tcl servers fault or the python clients fault?

Thanks.

server:

proc accept {chan addr port} {
fconfigure $chan -blocking 0 -buffering line
fileevent $chan readable [list receive $chan]

puts "$addr joined"
}

proc receive {channel} {
puts [chan gets $channel]

close $channel
}

socket -server accept 5000
vwait forever

client:

import socket

def client_program():
host = "shaun.rubenportier.be" # as both code is running on same pc
port = 5000 # socket server port number

client_socket = socket.socket() # instantiate
client_socket.connect((host, port)) # connect to the server

message = input(" -> ") # take input


client_socket.send(message.encode()) # send message


client_socket.close() # close the connection

if __name__ == '__main__':
client_program()

[saitology9]

On 4/10/2023 1:34 PM, Shaun Kulesa wrote:
> Hello, I was testing sockets in tcl and I got a tcl server and tcl client to communicate correctly.
>
> I thought I would try using a python client to send data to my tcl server.
>
> The python client will send data but the tcl server will output it as a empty string when I get it from the channel.
>
> I was wondering if this was the tcl servers fault or the python clients fault?
>

Not user fault then :-)

Add a newline character yo your messages when sending them from Python,
within this line or as a new subsequent message, and see if it improves
things:

> client_socket.send(message.encode()) # send message

[Shaun Kulesa]

I've tried a mix of combinations using the new line character but the output is still an empty string.

message = input(" -> ") # take input

client_socket.send("\n".encode())
client_socket.send(("\n" + message).encode())
client_socket.send("\n".encode())

[saitology9]

Interesting... I think newlines should not be encoded - they will cease
to become newlines.

[Shaun Kulesa]

Unfortunately the argument has to be in bytes.

I changed the tcl server receive function to do 3 outputs to match the inputs given by the python client but none of them had the input message from the client as they were all empty strings.

proc receive {channel} {
puts [chan gets $channel]

flush $channel
puts [chan gets $channel]
flush $channel
puts [chan gets $channel]
flush $channel

close $channel
}

[saitology9]

On 4/10/2023 3:57 PM, Shaun Kulesa wrote:
> Unfortunately the argument has to be in bytes.
>
> I changed the tcl server receive function to do 3 outputs to match the inputs given by the python client but none of them had the input message from the client as they were all empty strings.
>

Try removing the buffering from this line on the tcl side:

> fconfigure $chan -blocking 0 -buffering line

to this:

fconfigure $chan -blocking 0

[Christian Gollwitzer]

Am 10.04.23 um 19:34 schrieb Shaun Kulesa:

> server:
>
> proc accept {chan addr port} {
> fconfigure $chan -blocking 0 -buffering line
> fileevent $chan readable [list receive $chan]
>
> puts "$addr joined"
> }
>
> proc receive {channel} {
> puts [chan gets $channel]
>
> close $channel
> }

I don't know how well non-blocking I/O and gets work together. It may
well be that the first read receives nothing and the second read would
deliber the data. Try changing the receiver like this:

proc receive {channel} {
puts "Got: [read $channel]"
}

Does it change anything? The server won't stop then after 1 line,
obviously, but maybe it spits out an empty string and then the
transmitted data.

Christian

[Shaun Kulesa]

Thanks Christian Gollwitzer, I tested that with two inputs on one connection and it receives both of them. I'm trying to close the connection after the client disconnects as it creates an infinite loop of printing "Got: ".

I got it to work as it closes the socket connection but the receive function continues to run causing an error as the channel no longer exists.

python input:

-> hello
-> hello again

tcl output:

87.115.149.231 joined
Got: hello
Got: hello again
Got:
Client has disconnected.
can not find channel named "sock5573b3ce5050"
while executing
"read $channel"
(procedure "receive" line 8)
invoked from within
"receive sock5573b3ce5050"

receive function:

proc receive {channel} {
if {[eof $channel]} {
close $channel
puts "Client has disconnected."
}

puts "Got: [read $channel]"
flush $channel
}

[Rich]

Reread the Tcl 'gets' man page, specifically the part about
'non-blocking' IO.

If channelId is in non-blocking mode and there is not a full
line of input available, the command returns an empty string and
does not consume any input.

In 'non-blocking' mode, returning "empty string" is Tcl's way of
indicating "not a full line available".

If you use the alternate gets call, with a varname into which to store
the string, then you'll receive -1 as the length from gets when it
returns the "not enough data for a full line" indication.

[Rich]

Move your 'eof' to after the read. The [eof] signal only arrives
*after* you attempt to perform a read from the channel.

This is also documented in the [eof] manpage:

Returns 1 if an end of file condition occurred during the most
recent input operation on channelId (such as gets), 0 otherwise.

[Shaun Kulesa]

Thank you for your explanations, it now works correctly.

Also thank you to saitology9 for spending your time trying to help me in the beginning.

87.115.149.231 joined
Got: hello
Got: hello2
Got:
Client has disconnected.