IPv4 and IPv6 sockets on Linux.

listem.c

//  listen.c
//
//  This is small tool that helps to clarify how to listen IPv6 socket and it
//  is nessesary or not listen IPv4 socket at the same time.
//
//  Compile `gcc -std=c11 -o listen listen.c`
//
//  Run binary with option `-4` or `-6` and connect to port(default is 5489)
//  with netcat in the following way.
//      1. `nc -4 127.0.0.1 5489`
//      2. `nc -6 ::1 5489`
//
//  On linux hosts one should listen at least IPv6 socket.

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#include <errno.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <arpa/inet.h>

#define MAGIC 5489

void echo(void) {
    int ret;
    int sock = socket(AF_INET, SOCK_STREAM, 0);
    struct sockaddr_in addr = {
        AF_INET,
        htons(MAGIC),
        inet_addr("127.0.0.1"),
    };

    if ((ret = bind(sock, (const struct sockaddr *)&addr, sizeof(addr))) < 0) {
        fprintf(stderr, "listen: %d\n", ret);
        return;
    }

    if ((ret = listen(sock, MAGIC)) < 0) {
        fprintf(stderr, "listen: %d\n", ret);
        return;
    }

    while (1) {
        struct sockaddr_in client_addr;
        socklen_t socklen = sizeof(client_addr);
        int client_sock = accept(sock,
                                 (struct sockaddr *)&client_addr,
                                 &socklen);
        printf("accept client from %s:%d\n",
               inet_ntoa(addr.sin_addr),
               htons(addr.sin_port));

        close(client_sock);
    }
}

void echo6(void) {
    int ret;
    int sock = socket(AF_INET6, SOCK_STREAM, 0);
    struct sockaddr_in6 addr = {
        AF_INET6,
        htons(MAGIC),
        0,
        {0},
        0,
    };

    inet_pton(AF_INET, "::1", &addr.sin6_addr);

    if ((ret = bind(sock, (const struct sockaddr *)&addr, sizeof(addr))) < 0) {
        fprintf(stderr, "listen: %d\n", ret);
        printf("%d\n", errno);
        return;
    }

    if ((ret = listen(sock, MAGIC)) < 0) {
        fprintf(stderr, "listen: %d\n", ret);
        return;
    }

    while (1) {
        static char hostname[255];
        struct sockaddr_in6 client_addr;
        socklen_t socklen = sizeof(client_addr);
        int client_sock = accept(sock,
                                 (struct sockaddr *)&client_addr,
                                 &socklen);

        inet_ntop(AF_INET,
                  &client_addr.sin6_addr,
                  hostname,
                  sizeof(client_addr));

        printf("accept client from %s:%d\n",
               hostname,
               htons(addr.sin6_port));

        close(client_sock);
    }
}

int main(int argc, char *argv[]) {
    if (argc != 2) {
        printf("%s: [-4|-6]\n", argv[0]);
        return 0;
    }

    if (!strcmp(argv[1], "-4")) {
        echo();
    }
    else if (!strcmp(argv[1], "-6")) {
        echo6();
    }
    else {
        printf("unknown argument: '%s'\n", argv[1]);
    }
}