Single socket, multiple threads

accept.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <pthread.h>
#include <netdb.h>
#define PORT 2001
#define BACKLOG 128
#define THREAD_COUNT 3

typedef struct {
  int listen_fd;
  int id;
} thread_arg_t;

static void *accept_loop(void *argp) {
  thread_arg_t *arg = (thread_arg_t*)argp;
  int listen_fd = arg->listen_fd;
  int tid = arg->id;

  for (;;) {
    struct sockaddr_storage ss;
    socklen_t slen = sizeof ss;
    int client_fd = accept(listen_fd, (struct sockaddr*)&ss, &slen);
    if (client_fd < 0) {
      if (errno == EINTR) continue;
      perror("accept");
      sleep(1);
      continue;
    }

    char host[NI_MAXHOST], serv[NI_MAXSERV];
    if (getnameinfo((struct sockaddr*)&ss, slen, host, sizeof host, serv, sizeof serv,
                    NI_NUMERICHOST | NI_NUMERICSERV) == 0) {
      printf("Thread %d: accepted connection from %s:%s\n", tid, host, serv);
    } else {
      printf("Thread %d: accepted connection (fd=%d)\n", tid, client_fd);
    }

    close(client_fd); /* drop connection and accept next */
    break;
  }
  return NULL;
}

int main(void) {
  int listen_fd = socket(AF_INET6, SOCK_STREAM, 0);
  if (listen_fd < 0) {
    perror("socket");
    return 1;
  }

  /* Allow both IPv4 and IPv6 (v6->v4 mapping) */
  if (setsockopt(listen_fd, IPPROTO_IPV6, IPV6_V6ONLY, &(int){0}, sizeof(int)) < 0) {
    /* non-fatal on some systems */
  }

  struct sockaddr_in6 addr;
  memset(&addr, 0, sizeof addr);
  addr.sin6_family = AF_INET6;
  addr.sin6_addr = in6addr_any;
  addr.sin6_port = htons(PORT);

  if (bind(listen_fd, (struct sockaddr*)&addr, sizeof addr) < 0) {
    perror("bind (need root for ports <1024?)");
    close(listen_fd);
    return 1;
  }

  if (listen(listen_fd, BACKLOG) < 0) {
    perror("listen");
    close(listen_fd);
    return 1;
  }

  pthread_t threads[THREAD_COUNT];
  thread_arg_t args[THREAD_COUNT];

  for (int i = 0; i < THREAD_COUNT; ++i) {
    args[i].listen_fd = listen_fd;
    args[i].id = i;
    if (pthread_create(&threads[i], NULL, accept_loop, &args[i]) != 0) {
      perror("pthread_create");
      close(listen_fd);
      return 1;
    }
  }

  /* Main thread can wait or do other work; join threads to keep process alive */
  for (int i = 0; i < THREAD_COUNT; ++i) {
    pthread_join(threads[i], NULL);
  }

  close(listen_fd);
  return 0;
}