[zeromq-dev] Need advice on ZMQ_STREAM socket, zmq_recv truncating data

Michal Vyskocil michal.vyskocil at gmail.com
Thu Aug 2 10:44:52 CEST 2018


Hi Joe,

would you mind to use czmq? This hides all the details of using zeromq API
and simplify developer's work a lot!

#include <czmq.h>

zsock_t *socket = zsock_stream_new (endpoint);

while (!zsys_interrupted) {
     zmsg_t *msg = zmsg_recv (socket);
    // ... process the msg
     zmsg_destroy (&msg);
}

zsock_destroy (&socket);



On Thu, Aug 2, 2018 at 8:45 AM, Joe Lin via zeromq-dev <
zeromq-dev at lists.zeromq.org> wrote:

> I’m using ZMQ_STREAM socket talking to a TCP server. A simplified workflow
> is that the server will stream large amount of data upon client connection.
> The stream is composed of long sequence of various sized chunks.
>
> The client code that reads the stream (omit the error checking):
>
> char buf[BUF_SIZE];
> int64_t more;
> size_t more_size = sizeof(more);
>
> while (true) {
>   // first read the socket id
>   int nbytes = zmq_recv(client_socket, buf, BUF_SIZE, 0);
>   more = 1;
>
>   // read the data
>   while ( more ) {
>     // read the data chunk, but data get truncated from time to time ??
>     nbytes = zmq_recv(client_socket, buf, BUF_SIZE, 0);
>
>     //… do something with the received data in the buffer
>
>     // more always return 0 ?? The second while loop is of no use
>     zmq_getsockopt(client_socket, ZMQ_RCVMORE, &more, &more_size)
>   }
> }
>
> I have two questions. First is that I find the data that I received from
> zmq_recv can get truncated. I know this is the intend behavior of zmq_recv.
> But I cannot think of a good way to avoid this unless I allocate a very
> large buffer to begin with. What is a good practice for this problem?
>
> Second is that I found that zmq_getsockop of ZMQ_RCVMORE always return
> more=0.  Why is that? I wonder how zmq_recv buffers data internally for
> ZMQ_STREAM socket. I would expect when dealing with a TCP stream, zmq_recv
> should fill the supplied buffer as much as possible (as opposed to
> truncating) and if there are still some more data to be read then
> zmq_getsockop of ZMQ_RCVMORE would return more=1.
>
> Thanks for any help.
>
> _______________________________________________
> zeromq-dev mailing list
> zeromq-dev at lists.zeromq.org
> https://lists.zeromq.org/mailman/listinfo/zeromq-dev
>



-- 
best regards
     Michal Vyskocil
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