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int establish(void) {
  struct sockaddr_in sa; /* listening socket's address */
  int s; /* listening socket */

  /*  Fill up the structure with address and port number  */

  sa.sin_port = htons(portnum);

  /*  Other system calls like socket()  */

  if (bind(s, (struct sockaddr *)&sa,
        sizeof(struct sockaddr_in)) < 0) {
    /* Perform cleanup */
  }

  /* Return */
}

int main(void) {
   int s = establish();

  /*  Block with accept() until a client connects  */

   switch (fork()) {
      case -1 :  /* Error, clean up and quit */
      case  0 :  /* This is the child, handle the client */
      default :  /* This is the parent, continue blocking */
  }
  }return 0;
}

/*  Code with elevated privileges  */

int establish(void) {
  struct sockaddr_in sa; /* listening socket's address */
  int s; /* listening socket */

  /* Fill up the structure with address and port number */

  sa.sin_port = htons(portnum);

  /* Other system calls like socket() */

  if (bind(s, (struct sockaddr *)&sa,
        sizeof(struct sockaddr_in)) < 0) {
    /* Perform cleanup */
  }

  /* Return */
}

int main(void) {
  int s = establish();

  /* Drop privileges permanently */
  if (setuid(getuid()) == -1) {
     /*  Handle the error  */
  }

  if (setuid(0) != -1) {
    /* Privileges can be restored, handle error */
  }

  /* Block with accept() until a client connects */

  switch (fork()) {
     case -1: /* Error, clean up and quit */
     case  0: /* Close all open file descriptors
               * This is the child, handle the client
               */
     default: /* This is the parent, continue blocking */
  }
  return 0;
}