Why is the structure being converted incorrectly?

Question

C++ / C#

Why is it that when I convert from struct my_struct_in to struct my_struct and output it, it doesn’t work out as it should, why there aren’t eight 0s and what a garbage value. What is it for? I know it's bullshit, but I just want to get the gist of it.

#include <stdio.h>
#include <string.h>

#define AF_INET 1
#define __STRUCTADDR_COMON(sa_prefix) unsigned short int sa_prefix##family

struct my_struct 
{
  __STRUCTADDR_COMON(sa_);
  char data[14];
};

#define my_structContent_size (sizeof(struct my_struct) - sizeof(unsigned short int) - sizeof(unsigned short int) - sizeof(unsigned int))

struct my_struct_in
{
  __STRUCTADDR_COMON(sa_);
  unsigned short int port;
  unsigned long int addr;
  unsigned char sin_zero[my_structContent_size];
};
//base^exp
int power(int base, unsigned int exp)
{
    int i, result = 1;
    for (i = 0; i < exp; i++)
        result *= base;
    return result;
}

void print_byte(unsigned int *value, int col_byte)
{
  unsigned int byte, mask = 0xff*power(256, col_byte-1), shift =  power(256, col_byte-1), byte_itr, bit_itr;
               //mask *= power(256, col_byte-1);
               //  shift = power(256, col_byte-1);
  for(byte_itr = 0; byte_itr < col_byte; byte_itr++)
    {
      byte = ((*value) & mask) / shift;
      for(bit_itr = 0; bit_itr < 8; bit_itr++)
  {
    if(byte & 0x80)
      printf("1");
    else
      printf("0");
    byte*=2;
  }
      printf(" ");
      shift /= 256;
      mask /=256;
    }
  printf("\n");
}

int main(int argc, char **argv)
{
  struct my_struct_in host_addr;

  host_addr.sa_family = AF_INET;
  host_addr.port = 80;
  host_addr.addr = 1282342;
  memset(&(host_addr.sin_zero), '\0', my_structContent_size);

  struct my_struct *host;
  host = (struct my_struct *)&host_addr;
  unsigned int data;
  for(int i = 0; i < 14; i++)
    {
      data = (host->data[i]);
      print_byte(&data, 1);
    }
  return 0;
}

Answer 1

What is it for?

Because of the alignment of the data fields in the structure, and because long is 8 bytes. In the my_struct_in structure, you have short, short, and long in a row. The first two shorts are aligned, and to align the long, the compiler inserted a 4-byte hole in front of it into the structure. You can print offsetof(struct my_struct_in, addr)to verify this.
What to do? Use fixed-width types ( uint32_t addr, if you meant an IP address). Pack structures (if alignment is not possible), or arrange fields according to their natural alignment.