setsockopt REUSEPORT

sock.c

#include <netinet/ip.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>

int main() {
  int res;
  int value = 1;
  int fd = socket(AF_INET, SOCK_STREAM, 0);
  struct sockaddr_in addr = {AF_INET, 0, INADDR_ANY};
  res = setsockopt(fd, SOL_SOCKET, SO_REUSEPORT, &value, sizeof(value));  // Without this setsockopt
  if (res == -1) {
    perror("setsockopt1");
    exit(-1);
  }
  res = bind(fd, (struct sockaddr *) &addr, sizeof(addr));
  if (res == -1) {
    perror("bind1");
    exit(-1);
  }
  socklen_t addrlen = sizeof(addr);
  res = getsockname(fd, (struct sockaddr *) &addr, &addrlen);
  if (res == -1) {
    perror("getsockname");
    exit(-1);
  }
  in_port_t port = addr.sin_port;
  listen(fd, 0);

  int fd2 = socket(AF_INET, SOCK_STREAM, 0);
  res = connect(fd2, (struct sockaddr *) &addr, sizeof(addr));
  if (res == -1) {
    perror("connect");
    exit(-1);
  }
  int cl = accept(fd, NULL, NULL);
  if (cl == -1) {
    perror("accept");
    exit(-1);
  }

  close(fd);

  fd = socket(AF_INET, SOCK_STREAM, 0);
  struct sockaddr_in addr2 = {AF_INET, port, INADDR_ANY};
  res = setsockopt(fd, SOL_SOCKET, SO_REUSEPORT, &value, sizeof(value));
  if (res == -1) {
    perror("setsockopt2");
    exit(-1);
  }
  res = bind(fd, (struct sockaddr *) &addr2, sizeof(addr2));    // This bind would fail
  if (res == -1) {
    perror("bind2");
    exit(-1);
  }
}