Check padding of ulog_packet_msg_t

sizeof-ulog_packet_msg_t.c

#include <stddef.h>
#include <stdint.h>
#include <stdio.h>

#define IFNAMSIZ 16
#define ULOG_PREFIX_LEN 32
#define ULOG_MAC_LEN 80


typedef struct ulog_packet_msg {
  unsigned long mark;
  long timestamp_sec;
  long timestamp_usec;
  unsigned int hook;
  char indev_name[IFNAMSIZ];
  char outdev_name[IFNAMSIZ];
  size_t data_len;
  char prefix[ULOG_PREFIX_LEN];
  unsigned char mac_len;
  unsigned char mac[ULOG_MAC_LEN];
  unsigned char payload[0];
} ulog_packet_msg_t;

int main() {
  printf("sizeof(ulog_packet_msg_t) == %zu\n",
         sizeof(ulog_packet_msg_t));
  printf("Extra padding at end: %zu\n",
         sizeof(ulog_packet_msg_t) - offsetof(ulog_packet_msg_t, payload));
  return 0;
}