/*
FreeBSD i386 12.0-RELEASE
controllen: 100
recvd: 3 bytes controllen is 100
0 len: 16 level: 0 type: 7 msg offsets: [0,16]
1 len: 13 level: 0 type: 65 msg offsets: [16,29]
2 len: 68 level: 0 type: 20 msg offsets: [32,100]

FreeBSD AMD64 COMPAT_FREEBSD32 12.0-RELEASE-p3
controllen: 100
recvd: 3 bytes controllen is 97 <-- should have been 100
0 len: 16 level: 0 type: 7 msg offsets: [0,16]
1 len: 13 level: 0 type: 65 msg offsets: [16,29]
2 len: 68 level: 0 type: 20 msg offsets: [32,100]
 */

#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <net/if_dl.h>

static struct sockaddr_in loc_addr;
static int s;

static void alarm_handler(int signo) {
  char buf[] = {0x1, 0x2, 0x3};
  sendto(s, buf, sizeof buf, 0, (struct sockaddr*)&loc_addr, sizeof loc_addr);
}

int main(int argc, char *argv[]) {
  memset(&loc_addr, 0, sizeof loc_addr);
  loc_addr.sin_family = AF_INET;
  loc_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
  loc_addr.sin_port = htons(7777);
  s = socket(AF_INET, SOCK_DGRAM, 0);
  bind(s, (struct sockaddr *)&loc_addr, sizeof loc_addr);

  int enable = 1;
  setsockopt(s, IPPROTO_IP, IP_RECVDSTADDR, &enable, sizeof enable);
  setsockopt(s, IPPROTO_IP, IP_RECVTTL, &enable, sizeof enable);
  setsockopt(s, IPPROTO_IP, IP_RECVIF, &enable, sizeof enable);

  struct msghdr h;
  char buf[16];
  struct iovec in[] = {{buf, sizeof buf}};
  memset(&h, 0, sizeof h);
  h.msg_iov = in;
  h.msg_iovlen = 1;
  h.msg_controllen = CMSG_SPACE(sizeof(struct in_addr)) /*IP_RECVDSTADDR*/ 
    + CMSG_SPACE(sizeof(u_char)) /*IP_RECVTTL*/
    + CMSG_SPACE(sizeof(struct sockaddr_dl)); /*IP_RECVIF*/
  h.msg_control = calloc(0, h.msg_controllen);

  printf("controllen: %d\n", h.msg_controllen);
  alarm(1);
  struct sigaction sig = {};
  sig.sa_handler = alarm_handler;
  sig.sa_flags = SA_RESTART;
  sigaction(SIGALRM, &sig, NULL);
  ssize_t recvd = recvmsg(s, &h, 0);
  printf("recvd: %zd bytes controllen is %d\n", recvd, h.msg_controllen);
  if (recvd < 0) {
    perror("recvmsg failed:");
    exit(-1);
  }

  struct cmsghdr* cmsg = NULL;
  int i = 0;
  while((cmsg = CMSG_NXTHDR(&h, cmsg)) != NULL) {
    if (cmsg->cmsg_len == 0) {
      break;
    }
    int first_byte = (void*)cmsg - h.msg_control;
    int last_byte = first_byte + cmsg->cmsg_len;
    printf("%d len: %d level: %d type: %d msg offsets: [%d,%d]\n",
           i++, cmsg->cmsg_len, cmsg->cmsg_level, cmsg->cmsg_type, first_byte, last_byte);
  }

  return 0;
}