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Date: Thu, 16 Jan 2014 10:45:00 -0000
Message-ID: <CAKhyrx_DW_QNoARLLtBi_YPQ7EvcMB0j9sY-dBJ4b=iqVhho7g@mail.gmail.com>
Subject: RE: structure packing
From: vijay nag <vijunag@gmail.com>
To: "gcc-help@gcc.gnu.org" <gcc-help@gcc.gnu.org>
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Dear gcc,

Why is the size of below struct UnPackedStruct 6 ? Is there any switch
similar to -E in gcc to check how data alignment is performed by GCC
for structs ?


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

struct tlv_header_t {
    unsigned char tlv_length_upper : 3;
    unsigned char reserved : 5;
    unsigned char tlv_length_lower;
    uint16_t tlv_type;
};

typedef struct {
  struct tlv_header_t  sess;
  uint8_t p;
} UnPackedStruct;

typedef struct {
   int a;
   char b;
} PaddedStruct;

int main()
{
  UnPackedStruct a;
  PaddedStruct b;

  printf("size of a = %u\n", sizeof(a));
  printf("size of b = %u\n", sizeof(b));

  return 1;
}

./padding
size of a = 6
size of b = 8