We always define messages using zmq_msg. When we want to send multi-part messages, again, we need to use zmq_msg. For example, if the data package is divided into 10 parts, you need to create 10 zmq_msg sockets. The client either receives all the message parts or nothing at all. In order to send multi-part messages, the ZMQ_SNDMORE flag must be set during the zmq_send call.
int zmq_send(void* socket, void* buf, size_t len, int flags);
The following is the request-reply example that we used in Chapter 1, Getting Started, but this time we will send the message in multiple parts. First, let's have a look at the server code:
/*
Request - Reply
Send "world" in multiple-parts.
server.c
*/
#include <string.h>
#include <stdio.h>
#include <unistd.h>
#include "zmq.h"
int main (int argc, char const *argv[]) {
void* context = zmq_ctx_new();
void* respond = zmq_socket(context, ZMQ_REP);
zmq_bind(respond, "tcp://*:4040");
printf("Starting...
");
for(;;) {
zmq_msg_t request;
zmq_msg_init(&request);
zmq_msg_recv(&request, respond, 0);
printf("Received: hello
");
zmq_msg_close(&request);
sleep(1);
zmq_msg_t msg1, msg2, msg3, msg4, msg5;
zmq_msg_init_size(&msg1, 2);
zmq_msg_init_size(&msg2, 2);
zmq_msg_init_size(&msg3, 2);
zmq_msg_init_size(&msg4, 2);
zmq_msg_init_size(&msg5, 2);
memcpy(zmq_msg_data(&msg1), "w", 2);
zmq_msg_send(&msg1, respond, ZMQ_SNDMORE);
memcpy(zmq_msg_data(&msg2), "o", 2);
zmq_msg_send(&msg2, respond, ZMQ_SNDMORE);
memcpy(zmq_msg_data(&msg3), "r", 2);
zmq_msg_send(&msg3, respond, ZMQ_SNDMORE);
memcpy(zmq_msg_data(&msg4), "l", 2);
zmq_msg_send(&msg4, respond, ZMQ_SNDMORE);
memcpy(zmq_msg_data(&msg5), "d", 2);
zmq_msg_send(&msg5, respond, 0);
zmq_msg_close(&msg1);
zmq_msg_close(&msg2);
zmq_msg_close(&msg3);
zmq_msg_close(&msg4);
zmq_msg_close(&msg5);
}
zmq_close(respond);
zmq_ctx_destroy(context);
return 0;
}And the following is the client code:
/*
Request - Reply
Receive "world" in multi-parts.
client.c
*/
#include <string.h>
#include <stdio.h>
#include <stdint.h>
#include <unistd.h>
#include "zmq.h"
int main (int argc, char const *argv[]) {
void* context = zmq_ctx_new();
printf("Client Starting....
");
void* request = zmq_socket(context, ZMQ_REQ);
zmq_connect(request, "tcp://localhost:4040");
for(;;) {
zmq_msg_t reply;
zmq_msg_init(&reply);
zmq_msg_recv(&reply, request, 0);
printf("Received: %s
", (char *) zmq_msg_data(&reply));
zmq_msg_close(&reply);
uint64_t more_part;
size_t more_size = sizeof(more_part);
zmq_getsockopt(request, ZMQ_RCVMORE, &more_part, &more_size);
if (!more_part)
break;
}
zmq_close(request);
zmq_ctx_destroy(context);
return 0;
}We should note that the last part of the message flag that you send should always be set to 0. If you set it to ZMQ_SNDMORE
, then the client will not receive any messages.
zmq_msg_t msg1, msg2, msg3, msg4, msg5; zmq_msg_init_size(&msg1, 2); zmq_msg_init_size(&msg2, 2); zmq_msg_init_size(&msg3, 2); zmq_msg_init_size(&msg4, 2); zmq_msg_init_size(&msg5, 2); zmq_msg_send(&msg1, respond, ZMQ_SNDMORE); zmq_msg_send(&msg2, respond, ZMQ_SNDMORE); zmq_msg_send(&msg3, respond, ZMQ_SNDMORE); zmq_msg_send(&msg4, respond, ZMQ_SNDMORE); zmq_msg_send(&msg5, respond, ZMQ_SNDMORE);
The previous code snippet will not work. You could change the relevant parts of server code with it and experience it yourself.