They remain empty. You cannot use them. The purpose for which they are left
will be defeated then.
On Sat, Jul 30, 2011 at 1:03 PM, Puneet Gautam <puneet.nsi...@gmail.com>wrote:
> @everyone: What happens to those padded byte addresses.. do they
> remain empty or what..?
> Can we utilize those padded bytes in any way ..?
>
>
> On 7/30/11, tech rascal <techrascal...@gmail.com> wrote:
> > can anyone explain in detail .....how structure padding is
> advantageous???
> >
> > On Fri, Jul 29, 2011 at 10:28 PM, Rohit Srivastava
> > <access2ro...@gmail.com>wrote:
> >
> >> padding based on the bit interleaving(low order) which is basically
> >> hardware dependent.
> >>
> >>
> >> On Fri, Jul 29, 2011 at 10:10 PM, Puneet Gautam
> >> <puneet.nsi...@gmail.com>wrote:
> >>
> >>> Thanks guys...so much...!!
> >>>
> >>> On 7/29/11, nullpointer <nullpointer...@gmail.com> wrote:
> >>> > #include<stdio.h>
> >>> > #include<conio.h>
> >>> > struc MyStructA {
> >>> >
> >>> > char a;
> >>> > char b;
> >>> > int c;
> >>> > };
> >>> >
> >>> > struct MyStructB {
> >>> > char a;
> >>> > int c;
> >>> > char b;
> >>> > };
> >>> >
> >>> > int main(void) {
> >>> >
> >>> > struct MyStructA A;
> >>> > struct MyStructB B;
> >>> >
> >>> > int sizeA = sizeof(struct MyStructA);
> >>> > int sizeB = sizeof(struct MyStructB);
> >>> > return 0;
> >>> >
> >>> > }
> >>> >
> >>> > OUTPUT IS
> >>> > A = 8
> >>> > B = 12
> >>> > Structure padding is done to try and make sure that variables start
> in
> >>> > memory at addresses that are a multiple of their size.
> >>> > This is more efficient at hardware level (needs less cpu ticks to
> read
> >>> > or write variables) and in some platforms this is mandatory, though
> >>> > not on i386. There are CPU's that can only handle double precision
> >>> > floats if they are aligned on addresses that are a multiople of 8.
> >>> >
> >>> >
> >>> > In this struct:
> >>> > struct MyStructA {
> >>> >
> >>> > char a;
> >>> > char b;
> >>> > int c;
> >>> > };
> >>> > the beginning of the struct is to be assumed at 0 (I'l explain later)
> >>> > a is 1 byte so it needs no padding in front of it.
> >>> > the same goes for b.
> >>> > but c is 4 bytes. it should be placed at an address boundary that is
> a
> >>> > multiple of 4, so the compiler adds 2 dummy bytes in front of it.
> >>> > These 2 bytes change the size from 6 to 8.
> >>> > Now in this struct:
> >>> > struct MyStructB {
> >>> > char a;
> >>> > int c;
> >>> > char b;
> >>> > };
> >>> >
> >>> > a starts on 0, so no need for padding.
> >>> > c needs 3 bytes in front of it, and b needs no padding.
> >>> > this would bring the struct size to 9. HOWEVER, suppose that you
> place
> >>> > 2 of those structs in an array, the address rules for the second
> >>> > struct in the array are the same as for the first struct.
> >>> > If that second struct would start at byte 10, this would not be true
> >>> > so the compiler also inserts some padding at the end of the structure
> >>> > so that the next struct after it starts at a multiple of the size of
> >>> > its largest member.
> >>> >
> >>> >
> >>> >
> >>> >
> >>> > On Jul 29, 3:36 pm, Arun Vishwanathan <aaron.nar...@gmail.com>
> wrote:
> >>> >> @puneet : no , in this case since 4 bytes will be used for int a and
> >>> int c
> >>> >> and then 1 byte for char b with 3 padded bytes next..it wud be the
> >>> >> same
> >>> >> here
> >>> >>
> >>> >> On Fri, Jul 29, 2011 at 12:11 PM, Puneet Gautam
> >>> >> <puneet.nsi...@gmail.com>wrote:
> >>> >>
> >>> >>
> >>> >>
> >>> >> > @nikhil: If i declare "Char b" after "int c".., would there be any
> >>> >> > difference...?
> >>> >>
> >>> >> > On 7/28/11, Nikhil Gupta <nikhilgupta2...@gmail.com> wrote:
> >>> >> > > Here's another example.
> >>> >>
> >>> >> > > struct example
> >>> >> > > {
> >>> >> > > int a;
> >>> >> > > char b;
> >>> >> > > int c;
> >>> >> > > }
> >>> >>
> >>> >> > > Now if a variable of type example is declared then
> >>> >> > > (considering base address as 2000)
> >>> >> > > a gets : 2000 to 2003
> >>> >> > > b gets : 2004
> >>> >> > > c gets : 2005 to 2008 ? NO
> >>> >>
> >>> >> > > It gets 2008 to 2011. The bytes from 2005 to 2007 (3 bytes) are
> >>> padded
> >>> >> > > in
> >>> >> > > this case.
> >>> >>
> >>> >> > > On Thu, Jul 28, 2011 at 12:18 AM, Aman Goyal <
> >>> aman.goya...@gmail.com>
> >>> >> > wrote:
> >>> >>
> >>> >> > >> yes this will be the case.
> >>> >>
> >>> >> > >> On Wed, Jul 27, 2011 at 11:35 PM, Puneet Gautam
> >>> >> > >> <puneet.nsi...@gmail.com>wrote:
> >>> >>
> >>> >> > >>> @nikhil:So what u mean is that if i have:
> >>> >>
> >>> >> > >>> struct{
> >>> >> > >>> int a;
> >>> >> > >>> char b[5];
> >>> >> > >>> };
> >>> >>
> >>> >> > >>> the size of this struct's node will be 12 not 9.., to make it
> a
> >>> >> > multiple
> >>> >> > >>> of 4??
> >>> >>
> >>> >> > >>> On 7/26/11, Nikhil Gupta <nikhilgupta2...@gmail.com> wrote:
> >>> >> > >>> > Padding is not a topic of self referential structure.
> >>> >>
> >>> >> > >>> > Padding means that extra spaces of memory are used by the
> >>> compiler
> >>> >> > >>> > to
> >>> >> > >>> > allocate memory. This is done to have the memory address as
> a
> >>> >> > multiple
> >>> >> > >>> of
> >>> >> > >>> > the size of the variable. This speeds up the processing of
> >>> these
> >>> >> > >>> variables
> >>> >> > >>> > by the compiler.
> >>> >>
> >>> >> > >>> > On Tue, Jul 26, 2011 at 8:09 PM, Puneet Gautam
> >>> >> > >>> > <puneet.nsi...@gmail.com>wrote:
> >>> >>
> >>> >> > >>> >> what is meant by padding in self_referenced structure?
> >>> >> > >>> >> Is it always necessary?
> >>> >>
> >>> >> > >>> >> --
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> >>> >>
> >>> >> > >>> > --
> >>> >> > >>> > Nikhil Gupta
> >>> >> > >>> > Senior Co-ordinator, Publicity
> >>> >> > >>> > CSI, NSIT Students' Branch
> >>> >> > >>> > NSIT, New Delhi, India
> >>> >>
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> >>> >> > > Senior Co-ordinator, Publicity
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> >>> >> --
> >>> >> Arun Vish
> >>> >> Graduate Student
> >>> >> Department of Computer Science
> >>> >> University of Southern California
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