#include <sys/types.h>
#include <sys/socket.h>
#include <sys/wait.h>

#include <arpa/inet.h>
#include <netinet/in.h>

#include <err.h>
#include <errno.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

static void
usage(const char *cmd)
{
        fprintf(stderr, "%s -p port\n", cmd);
        exit(1);
}

static void
connect_client(int port)
{
        struct sockaddr_in in;
        int s;

        s = socket(AF_INET, SOCK_STREAM, 0);
        if (s < 0)
                err(1, "client socket failed");

        memset(&in, 0, sizeof(in));
        in.sin_family = AF_INET;
        in.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
        in.sin_port = htons(port);
        if (connect(s, (const struct sockaddr *)&in, sizeof(in)) < 0)
                err(1, "connect failed");

        pause();
        exit(0);
}

int
main(int argc, char *argv[])
{
        struct sockaddr_in in;
        int serv_s, s;
        int opt, port, dummy, n, status, panic, ex;
        pid_t pid;

        ex = 1;
        panic = 0;

        port = 0;
        while ((opt = getopt(argc, argv, "p:")) != -1) {
                char *endptr;

                switch (opt) {
                case 'p':
                        port = strtol(optarg, &endptr, 0);
                        if (*endptr != '\0')
                                errx(1, "invalid -p");
                        break;

                default:
                        usage(argv[0]);
                }
        }
        if (port <= 0)
                usage(argv[0]);

        serv_s = socket(AF_INET, SOCK_STREAM, 0);
        if (serv_s < 0)
                err(1, "socket failed");

        memset(&in, 0, sizeof(in));
        in.sin_family = AF_INET;
        in.sin_port = htons(port);
        if (bind(serv_s, (const struct sockaddr *)&in, sizeof(in)) < 0)
                err(1, "bind failed");

        if (listen(serv_s, 0) < 0)
                err(1, "listen failed");

        pid = fork();
        if (pid < 0) {
                err(1, "fork failed");
        } else if (pid == 0) {
                connect_client(port);
                abort();
        }

        s = accept4(serv_s, NULL, NULL, SOCK_NONBLOCK);
        if (s < 0) {
                warn("accept4 failed");
                goto done;
        }
        panic = 1;

        n = read(s, &dummy, sizeof(dummy));
        if (n < 0) {
                int error = errno;

                if (error != EAGAIN) {
                        warnx("invaid errno %d", error);
                        goto done;
                }
        } else {
                warnx("read works");
                goto done;
        }

        ex = 0;
        panic = 0;
        fprintf(stderr, "passed\n");
done:
        kill(pid, SIGKILL);
        waitpid(pid, &status, 0);
        if (panic)
                abort();
        exit(ex);
}