two processes sharing a listening socket

sharing.c

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>

/* Author: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
 
   Example listening daemon where two processes share a single socket,
   letting the kernel decide which one gets each incomingn connection

   This example uses AF_UNIX with SOCK_STREAM but you could use other
   families and types as well.

   Once it's running, connect to it and read the pid of the process
   that answers your connection.  Try this multiple times:

    socat STDIO UNIX-CONNECT:testing

 */

int main(int argc, char* argv[])
{
  struct sockaddr_un addr = {
    .sun_family = AF_UNIX,
    .sun_path = "testing",
  };
  int sock = socket(AF_UNIX, SOCK_STREAM, 0);
  if (sock == -1) {
    fprintf(stderr, "bad socket() %d (%s)\n", errno, strerror(errno));
    return 1;
  }
  
  if (bind(sock, (struct sockaddr*)(&addr), sizeof(addr))) {
    fprintf(stderr, "bad bind() %d (%s)\n", errno, strerror(errno));
    return 1;
  }
  if (listen(sock, 10)) {
    fprintf(stderr, "bad listen() %d (%s)\n", errno, strerror(errno));
    return 1;
  }

  if (fork() == -1) {
    fprintf(stderr, "bad fork() %d (%s)\n", errno, strerror(errno));
    return 1;
  }
  pid_t p = getpid();
  char view[256];
  sprintf(view, "pid %d\n", p);
  size_t viewsz = strlen(view);
  /* at this point there should be two processes, with different pids
     with a listening socket */
  int h = accept(sock, NULL, 0);
  while (h != -1) {
    ssize_t wrote = write(h, view, viewsz);
    if (wrote != viewsz) {
      fprintf(stderr, "bad write() -- wanted %ld, got %lu.  %d (%s)",
              viewsz, wrote, errno, strerror(errno));
        return 1;
    }
    if (close(h)) {
      fprintf(stderr, "bad close() %d (%s)\n", errno, strerror(errno));
      return 1;
    }
    h = accept(sock, NULL, 0);
  }
  fprintf(stderr, "bad accept() %d (%s)\n", errno, strerror(errno));
  return 1;
}