/* compile: gcc -o resolve-by-fd  -lsystemd  resolve-by-fd.c */
/* run: systemd-socket-activate -l @abstract -l /tmp/ss.sock -l 3333 */
/* activate: nc localhost 3333 */

#include <netdb.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <string.h>
#include <stddef.h>
#include <stdio.h>

#include <systemd/sd-daemon.h>
int main()
{
  char **names = NULL;
  int sd_sockets;

  sd_sockets= sd_listen_fds_with_names(0, &names);
  while (sd_sockets--)
  {
    int stype= 0, accepting= 0, getnameinfo_err;
    socklen_t l;
    union
    {
          struct sockaddr sa;
          struct sockaddr_storage storage;
          struct sockaddr_in in;
          struct sockaddr_in6 in6;
          struct sockaddr_un un;
    } addr;
    socklen_t addrlen= sizeof(addr);
    char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];

    int fd= SD_LISTEN_FDS_START + sd_sockets;
    getsockname(fd, &addr.sa, &addrlen);
    getnameinfo(&addr.sa, addrlen, hbuf, sizeof(hbuf), sbuf,
                sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV);
    if (sbuf[0] == '\0')
        sbuf[0] = '@';

    printf("Using systemd activated socket %s port %-*s\n",
           hbuf, addrlen - offsetof(struct sockaddr_un, sun_path), sbuf);

  }
  return 0;
}