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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>> >> > >>> > --
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>> >> > >>> > Senior Co-ordinator, Publicity
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>> >> > > --
>> >> > > Nikhil Gupta
>> >> > > Senior Co-ordinator, Publicity
>> >> > > CSI, NSIT Students' Branch
>> >> > > NSIT, New Delhi, India
>> >>
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>> >>
>> >> --
>> >> Arun Vish
>> >> Graduate Student
>> >> Department of Computer Science
>> >> University of Southern California
>> >
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