C++/Qt: QTcpSocket won't write after reading -

i creating network client application sends requests server using qtcpsocket , expects responses in return. no higher protocol involved (http, etc.), exchange simple custom strings.

in order test, have created tcp server in python listens on socket , logs strings receives , sends back.

i can send first request ok , expected response. however, when send second request, not seem written network.

i have attached debug slots qtcpsocket's notification signals, such byteswritten(...), connected(), error(), statechanged(...), etc. , see connection being established, first request sent, first response processed, number of bytes written - adds up...

only second request never seems sent :-(

after attempting send it, socket sends error(remotehostclosederror) signal followed closingstate , unconnectedstate state change signals.

before go deeper this, couple of (probably basic) questions:

1. do need "clear" underlying socket in way after reading ?
2. is possible / probable not reading data server has sent me prevents me writing ?
3. why server close connection ? or sign not right ? tried setting lowdelay , keepalive socket options, didn't change anything. i've checked socket's state() , isvalid() , they're - although latter returns true when unconnected...
4. in earlier version of application, closed , re-opened connection before sending request. worked ok. prefer keeping connection open though. not reasonable approach ? 'canonical' way to implement tcp network communication ? read/write or re-open every time ?
5. does way read socket have impact on how can write ? sample code uses readall(...) available data; read piece piece need , << qtextstream when writing...

could possibly bug in qt event loop ? have observed output in qt creator console created qdebug() << ... gets cut short, i.e. stops. more output printed when shut down application.

this latest qt 5.4.1 on mac os x 10.8, issue occurs on windows 7.

update after first answer , comments:

the test server dead simple , taken official python socketserver.tcpserver example:

import socketserver  class mytcphandler(socketserver.streamrequesthandler):      def handle(self):         request  = self.rfile.readline().strip()         print "rx [%s]: %s" % (self.client_address[0], request)          response = self.processrequest(request)          print "tx [%s]: %s" % (self.client_address[0], response)         self.wfile.write(response)      def processrequest(self, message):         if   message == 'request type 01':             return 'response type 01'         elif message == 'request type 02':             return 'response type 02'  if __name__ == "__main__":     server = socketserver.tcpserver(('localhost', 12345), mytcphandler)     server.serve_forever() 

the output is

rx [127.0.0.1]: request type 01 tx [127.0.0.1]: response type 01 

also, nothing happens when re-send message after - not surprising socket closed. guess i'll have figure out why closed...

next update:

i've captured network traffic using wireshark , while network stuff doesn't tell me lot, see first request , response. right after client [ack]nowledges response, server sends connection finish (fin). don't see second request anywhere.

last update:

i have posted follow-up question @ python: socketserver closes tcp connection unexpectedly.

only second request never seems sent :-(

i highly recommend running program wireshark , seeing packets getting sent , received across network. (as is, can't know sure whether bug on client side or in server, , first thing need figure out)

do need "clear" underlying socket in way after reading ?

no.

is possible / probable not reading data server has sent me prevents me writing ?

no.

why server close connection ?

it's impossible without looking @ server's code.

does or sign not right ?

again, depend on how server written.

this worked ok. prefer keeping connection open though. not reasonable approach ?

keeping connection open reasonable approach.

what 'canonical' way to implement tcp network communication ? read/write or re-open every time ?

neither canonical; depends on attempting accomplish.

does way read socket have impact on how can write ?

no.

could possibly bug in qt event loop ?

that's extremely unlikely. qt code has been used years tens of thousands of programs, bug serious have been found , fixed long ago. it's more either there bug in client, or bug in server, or mismatch between how expect api call behave , how behaves.