NNG pattern guide

pair communication topology

Pair (Two Way Radio)

The pair pattern is used when there a one-to-one peer relationship. Only one peer may be connected to another peer at a time, but both may speak freely.

pair.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include <nng/nng.h>

static int
nng_recv_alloc_(nng_socket sock, void **buf, size_t *size)
{
        nng_msg *msg = NULL;
        int rv;
        if ((rv = nng_recvmsg(sock, &msg, 0)) != 0) return rv;
        *size = nng_msg_len(msg);
        if ((*buf = nng_alloc(*size)) == NULL) { nng_msg_free(msg); return NNG_ENOMEM; }
        memcpy(*buf, nng_msg_body(msg), *size);
        nng_msg_free(msg);
        return 0;
}
#define nng_recv_alloc(sock, buf, size) nng_recv_alloc_((sock), (void **)(buf), (size))

#define NODE0 "node0"
#define NODE1 "node1"

void
fatal(const char *func, int rv)
{
        fprintf(stderr, "%s: %s\n", func, nng_strerror(rv));
        exit(1);
}

int
send_name(nng_socket sock, char *name)
{
	int rv;
        printf("%s: SENDING \"%s\"\n", name, name);
	if ((rv = nng_send(sock, name, strlen(name) + 1, 0)) != 0) {
		fatal("nng_send", rv);
	}
	return (rv);
}

int
recv_name(nng_socket sock, char *name)
{
        char *buf = NULL;
	int rv;
	size_t sz;
        if ((rv = nng_recv_alloc(sock, &buf, &sz)) == 0) {
		printf("%s: RECEIVED \"%s\"\n", name, buf); (1)
		nng_free(buf, sz);
	}
        return (rv);
}

int
send_recv(nng_socket sock, char *name)
{
	int rv;
	if ((rv = nng_socket_set_ms(sock, NNG_OPT_RECVTIMEO, 100)) != 0) {
		fatal("nng_socket_set_ms", rv);
	}
        for (;;) {
                recv_name(sock, name);
                sleep(1);
                send_name(sock, name);
        }
}

int
node0(const char *url)
{
        nng_socket sock;
	int rv;
        if ((rv = nng_pair0_open(&sock)) != 0) {
                fatal("nng_pair0_open", rv);
        }
         if ((rv = nng_listen(sock, url, NULL, 0)) !=0) {
                fatal("nng_listen", rv);
        }
        return (send_recv(sock, NODE0));
}

int
node1(const char *url)
{
        nng_socket sock;
	int rv;
	sleep(1);
        if ((rv = nng_pair0_open(&sock)) != 0) {
                fatal("nng_pair0_open", rv);
        }
        if ((rv = nng_dial(sock, url, NULL, 0)) != 0) {
                fatal("nng_dial", rv);
        }
        return (send_recv(sock, NODE1));
}

int
main(int argc, char **argv)
{
        if ((argc > 1) && (strcmp(NODE0, argv[1]) == 0))
                return (node0(argv[2]));

        if ((argc > 1) && (strcmp(NODE1, argv[1]) == 0))
                return (node1(argv[2]));

        fprintf(stderr, "Usage: pair %s|%s <URL> <ARG> ...\n", NODE0, NODE1);
        return 1;
}
1 Blithely assumes message is ASCIIZ string. Real code should check it.
Compilation
gcc pair.c -lnng -o pair
Execution
./pair node0 ipc:///tmp/pair.ipc & node0=$!
./pair node1 ipc:///tmp/pair.ipc & node1=$!
sleep 3
kill $node0 $node1
Output
node0: SENDING "node0"
node1: RECEIVED "node0"
node1: SENDING "node1"
node0: SENDING "node0"
node0: RECEIVED "node1"
node1: RECEIVED "node0"

This migrated example has been checked against NNG main. It uses the current socket API and receives messages through nng_recvmsg; the original NNG_FLAG_ALLOC form is no longer available.