From: Alex Vinokur <alexvn AT bigfoot DOT com>
Newsgroups: alt.comp.lang.learn.c-c++,comp.lang.c++,comp.os.msdos.djgpp
Subject: Re: Holes in structure
Date: Tue, 06 Nov 2001 15:57:05 +0200
Organization: Scopus Network Technologies
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Message-ID: <3BE7EC30.545427FD@bigfoot.com>
References: <3BE7D280 DOT C1732E58 AT bigfoot DOT com> <9s8n3t$t2b$1 AT antares DOT lu DOT erisoft DOT se>
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Martin Stromberg wrote:

> Alex Vinokur (alexvn AT bigfoot DOT com) wrote:

[snip]

>
> : : Struct type1 :
> :  sizeof   = 52
> :  All items sum  = 43
> :  Diff (52 - 43)  = 9
> :  All holes sum  = 6
>
> :  2. struct type1 :
> :            Why is not 'Diff' (between 'sizeof' and 'All items sum')
> :   equal 'All holes sum' ?
> :      Note. 'Diff' = 9, 'All holes sum' = 6.
> :     Where has 3 bytes ('Diff' - 'All holes sum') gone?
>
> It's padding after item15 I guess.
>
> Right,
>
>                                                 MartinS

You are right. Thanks.

Here is updated program and run log file.


#########################################################
################### C++ code : BEGIN ####################
// File main.c

#include <string>
#include <iostream>
#include <typeinfo>
#include <iomanip.h>

//===========================================
//================= DEFINES =================
//===========================================
#define ITEM_INTERNAL_ADDRESS(s, item) &(((s*)0)->item)
#define ITEM_ITSELF(s, item)  ((s*)0)->item

#define SHOW_ITEM_INFO(s, item)    \
 show_item_info (    \
  #s,     \
  #item,     \
  gstr_builtin_typename(ITEM_ITSELF(s, item)), \
  (size_t)ITEM_INTERNAL_ADDRESS(s, item), \
  sizeof (ITEM_ITSELF(s, item))  \
  )

#define SHOW_ENDOF_INFO(s)  \
 show_endof_info (  \
  #s,   \
  sizeof(s)  \
  )

#define SHOW_STRUCT_INFO(s)  \
 show_struct_info (  \
  #s,   \
  sizeof(s)  \
  )


//===========================================
//============= STATIC VARIABLES ============
//===========================================
static size_t prev_item_size_s = 0;
static size_t prev_item_address_s = 0;
static size_t size_of_all_items_s = 0;
static size_t sum_of_all_holes_s = 0;


//===========================================
//=========== AUXILIARY FUNCTIONS ===========
//===========================================
//--------------------------------
void show_item_info (
 const string& struct_name_i,
 const string& item_name_i,
 const string& item_type_name_i,
 const size_t item_address_i,
 const size_t item_sizeof_i
 )
//--------------------------------
{
size_t hole_value = 0;

  hole_value = item_address_i - (prev_item_address_s + prev_item_size_s);

  cout.setf (ios::left, ios::adjustfield);
  cout << struct_name_i
   << "."
   << setw(6)
 << item_name_i.c_str()
 << setw(10)
 << string (" (" + item_type_name_i + ")").c_str()
 << "-> Address = "
 << setw(3)
 << item_address_i
 << " : sizeof = "
 << item_sizeof_i
 << " : ";

  if (hole_value)
  {
    cout << "Hole = " << hole_value << " (before item)" ;
  }
  else
  {
    cout << "No hole";
  }
  cout << endl;

  //--------------
  prev_item_size_s    = item_sizeof_i;
  prev_item_address_s = item_address_i;

  size_of_all_items_s += item_sizeof_i;
  sum_of_all_holes_s  += hole_value;
  //--------------

} // show_item_info


//--------------------------------
void show_endof_info (
 const string& struct_name_i,
 const size_t struct_sizeof_i
 )
//--------------------------------
{
size_t hole_value = 0;

  hole_value = struct_sizeof_i - (prev_item_address_s + prev_item_size_s);

  cout.setf (ios::left, ios::adjustfield);
  cout << struct_name_i
   << "."
 << setw(45)
   << ""
 << " : ";

  if (hole_value)
  {
    cout << "Hole = " << hole_value << " (before end)" ;
  }
  else
  {
    cout << "No hole";
  }
  cout << endl;

  //--------------
  sum_of_all_holes_s  += hole_value;
  //--------------

} // show_endof_info


//--------------------------------
void show_struct_info (
 const string& struct_type_name_i,
 const size_t struct_sizeof_i
 )
//--------------------------------
{
  cout  << "Struct "
 << struct_type_name_i
 << " : "
 << endl
 << "\t"
 << "sizeof\t\t = "
 << struct_sizeof_i
 << endl
 << "\t"
 << "All items sum\t = "
 << size_of_all_items_s
 << endl
 << "\t"
 << "Diff ("
 << struct_sizeof_i
 << " - "
 << size_of_all_items_s
 << ")\t = "
 << (struct_sizeof_i - size_of_all_items_s)
 << endl
 << "\t"
 << "All holes sum\t = "
 << sum_of_all_holes_s
 << endl;

} // show_struct_info


//--------------------------------
void init_counters ()
//--------------------------------
{
  prev_item_size_s    = 0;
  prev_item_address_s = 0;
  size_of_all_items_s = 0;
  sum_of_all_holes_s  = 0;
} // init_counters


//--------------------------------
#define IF_TYPE(x) if (typeid(T) == typeid(x)) return #x
template <typename T>
string gstr_builtin_typename(const T&)
//--------------------------------
{
  IF_TYPE(char);
  IF_TYPE(short);
  IF_TYPE(int);
  IF_TYPE(long);
  IF_TYPE(float);
  IF_TYPE(double);
  return "Undefined typename";
}  // gstr_builtin_typenameconst


//===========================================
//============= TESTED STRUCTURES ===========
//===========================================
//-----------
struct type0
{
};
//-----------
struct type1
{
  long   item1;
  char   item2;
  short  item3;
  char   item4;
  long   item5;
  long   item6;
  int    item7;
  short  item8;
  short  item9;
  char   item10;
  char   item11;
  long   item12;
  float  item13;
  double item14;
  char   item15;
};


//===========================================
//============== MAIN FUNCTION ==============
//===========================================
int main ()
{

  //------ struct type1 ------

  cout << "\t### Test for struct type1 ###" << endl;

  init_counters ();

  SHOW_ITEM_INFO (type1, item1);
  SHOW_ITEM_INFO (type1, item2);
  SHOW_ITEM_INFO (type1, item3);
  SHOW_ITEM_INFO (type1, item4);
  SHOW_ITEM_INFO (type1, item5);
  SHOW_ITEM_INFO (type1, item6);
  SHOW_ITEM_INFO (type1, item7);
  SHOW_ITEM_INFO (type1, item8);
  SHOW_ITEM_INFO (type1, item9);
  SHOW_ITEM_INFO (type1, item10);
  SHOW_ITEM_INFO (type1, item11);
  SHOW_ITEM_INFO (type1, item12);
  SHOW_ITEM_INFO (type1, item13);
  SHOW_ITEM_INFO (type1, item14);
  SHOW_ITEM_INFO (type1, item15);
  SHOW_ENDOF_INFO(type1);

  cout << endl;
  SHOW_STRUCT_INFO (type1);

  //------ struct type0 ------

  cout << endl;
  cout << endl;

  cout << "\t### Test for struct type0 ###" << endl;

  init_counters ();

  SHOW_ENDOF_INFO(type0);    \
  SHOW_STRUCT_INFO (type0);

  return 0;
}



#################### C++ code : END #####################



#########################################################
############### Running (log file) : BEGIN ##############

%a.exe


 ### Test for struct type1 ###
type1.item1  (long)   -> Address = 0   : sizeof = 4 : No hole
type1.item2  (char)   -> Address = 4   : sizeof = 1 : No hole
type1.item3  (short)  -> Address = 6   : sizeof = 2 : Hole = 1 (before item)

type1.item4  (char)   -> Address = 8   : sizeof = 1 : No hole
type1.item5  (long)   -> Address = 12  : sizeof = 4 : Hole = 3 (before item)

type1.item6  (long)   -> Address = 16  : sizeof = 4 : No hole
type1.item7  (int)    -> Address = 20  : sizeof = 4 : No hole
type1.item8  (short)  -> Address = 24  : sizeof = 2 : No hole
type1.item9  (short)  -> Address = 26  : sizeof = 2 : No hole
type1.item10 (char)   -> Address = 28  : sizeof = 1 : No hole
type1.item11 (char)   -> Address = 29  : sizeof = 1 : No hole
type1.item12 (long)   -> Address = 32  : sizeof = 4 : Hole = 2 (before item)

type1.item13 (float)  -> Address = 36  : sizeof = 4 : No hole
type1.item14 (double) -> Address = 40  : sizeof = 8 : No hole
type1.item15 (char)   -> Address = 48  : sizeof = 1 : No hole
type1.                                              : Hole = 3 (before end)

Struct type1 :
 sizeof   = 52
 All items sum  = 43
 Diff (52 - 43)  = 9
 All holes sum  = 9


 ### Test for struct type0 ###
type0.                                              : Hole = 1 (before end)
Struct type0 :
 sizeof   = 1
 All items sum  = 0
 Diff (1 - 0)  = 1
 All holes sum  = 1


################ Running (log file) : END ###############


  ===========================
  Alex Vinokur
    mailto:alexvn AT bigfoot DOT com
    mailto:alexvn AT dr DOT com
    http://up.to/alexvn
    http://go.to/alexv_math
  ===========================