Problem: Unable to bind UDP to wildcard port

[StevePER]

Issue description

With TCP it's possible to bind to any available port by specifying a port of 0 or *. I would like the ability to do the same with UDP.

It seems trivial to add this capability to udp_address_t::resolve using the same logic as tcp_address_t::resolve, as follows:

//  Allow 0 specifically, to detect invalid port error in atoi if not
uint16_t port;
if (port_str == "*" || port_str == "0")
    //  Resolve wildcard to 0 to allow autoselection of port
    port = 0;
else {
    //  Parse the port number (0 is not a valid port).
    port = (uint16_t) atoi (port_str.c_str ());
    if (port == 0) {
        errno = EINVAL;
        return -1;
    }
}

But I can't see a clean way to implement getting the bound port back out again. (Caveat: I am new to zmq). For TCP, the last_address is set in socket_base_t::bind, but as far as I can see the UDP socket isn't bound to a port at this point.

Environment

• libzmq version (commit hash if unreleased): 4.2.3
• OS: Red Hat Linux 7.4

[ljluestc]

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

int main() {
    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd == -1) {
        perror("socket");
        exit(1);
    }

    struct sockaddr_in addr;
    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_port = htons(0);
    addr.sin_addr.s_addr = htonl(INADDR_ANY);

    if (bind(sockfd, (struct sockaddr*)&addr, sizeof(addr)) == -1) {
        perror("bind");
        close(sockfd);
        exit(1);
    }

    struct sockaddr_in localAddr;
    socklen_t len = sizeof(localAddr);
    if (getsockname(sockfd, (struct sockaddr*)&localAddr, &len) == -1) {
        perror("getsockname");
        close(sockfd);
        exit(1);
    }

    printf("Bound to port: %d\n", ntohs(localAddr.sin_port));

    close(sockfd);
    return 0;
}