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[ Doubt ] Bitfields
Joined: 04 Jan 2005
Posts: 2
[ Doubt ] Bitfields
Hi All,
struct st { int i : 4; int j : 4; int k : 4; }st;
main()
{
printf( " size = %d ", sizeof(st));
}
The output of the program is 4 The size of structure is sum of size of its member. Hence i,j,k are each 4 bits and totally 12 bits(nearly 2 bytes). But the output is 4 bytes. How this is possible ?? I am using Microsoft C++ compiler (integer size is 4 bytes)
Thanks in Advance,
R.Rajesh.
Joined: 19 Jan 2005
Posts: 1
[ Doubt ] Bitfields
--- In Programmers-Town@yahoogroups.com, "R.Rajesh"
<contactrrajesh@h...> wrote:
> Hi All,
>
> struct st
> {
> int i : 4;
> int j : 4;
> int k : 4;
> }st;
>
> main()
> {
> printf( " size = %d ", sizeof(st));
> }
>
> The output of the program is 4 . The size of structure is sum of
size of its member. Hence i,j,k are each 4 bits and totally 12 bits
(nearly 2 bytes). But the output is 4 bytes. How this is
possible ?? I am using Microsoft C++ compiler (integer size is 4
bytes)
>
> Thanks in Advance,
> R.Rajesh.
That is because, the unused 2 bytes also belong to the same
structure as it wouldn`t be able to allocate this memory location to
some other variable, try adding three more members with each 4 bits,
even then the size of the structure would remain 4 bytes.
Joined: 26 Jan 2005
Posts: 3
[ Doubt ] Bitfields
hi,
the structure is aligned to the boundary of the
types used in the structure
int type boundary
i.e 4
--- "R.Rajesh" <contactrrajesh@...> wrote:
> Hi All,
>
> struct st
> {
> int i : 4;
> int j : 4;
> int k : 4;
> }st;
>
> main()
> {
> printf( " size = %d ", sizeof(st));
> }
>
> The output of the program is 4 . The size of
> structure is sum of size of its member. Hence
> i,j,k are each 4 bits and totally 12 bits(nearly 2
> bytes). But the output is 4 bytes. How this is
> possible ?? I am using Microsoft C++ compiler
> (integer size is 4 bytes)
>
> Thanks in Advance,
> R.Rajesh.
>
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Joined: 02 Jan 2005
Posts: 9
[ Doubt ] Bitfields
--- "R.Rajesh" <contactrrajesh@...> wrote:
> Hi All,
>
> struct st
> {
> int i : 4;
> int j : 4;
> int k : 4;
> }st;
>
> main()
> {
> printf( " size = %d ", sizeof(st));
> }
>
> The output of the program is 4 . The size of
> structure is sum of size of its member. Hence
> i,j,k are each 4 bits and totally 12 bits(nearly 2
> bytes). But the output is 4 bytes. How this is
> possible ?? I am using Microsoft C++ compiler
> (integer size is 4 bytes)
>
> Thanks in Advance,
> R.Rajesh.
>
Hi Rajesh,
The size of an int is 4 bytes which makes it 32 bits.
What you require in the structure is 12 bits which can
be easily accomodated in 32 bits.
Thats why size is 4.
Regards
Manik
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Joined: 29 Dec 2004
Posts: 31
[ Doubt ] Bitfields
From: R.Rajesh [mailto:contactrrajesh@...]
>Hi All,
>
> struct st
> {
> int i : 4;
> int j : 4;
> int k : 4;
> }st;
>
>main()
>{
> printf( " size = %d ", sizeof(st));
>}
>
>The output of the program is 4 The size of structure is sum
>of size of its member. Hence i,j,k are each 4 bits and
>totally 12 bits(nearly 2 bytes). But the output is 4 bytes.
>How this is possible ?? I am using Microsoft C++ compiler
>(integer size is 4 bytes)
It`s called padding. Each instance of `struct st` should start on
(apparently in this case) a 4 byte boundary. If you have an array of
`struct st`, then a padding of 2 bytes and 4 bits would provide this.
`Padding` bits/bytes are inaccessable, and cannot (reliably) be used.
There are compiler options in MSVC which will allow you to change the
default padding. Look in the help files. At a guess, you wil be able to
get sizeof(struct st) down to either 2 or 3 bytes (unsure without
looking whether each member must start on a byte boundary) with the
correct options.
--
PJH
bug, n: An elusive creature living in a program that makes it incorrect.
The activity of "debugging", or removing bugs from a program, ends when
people get tired of doing it, not when the bugs are removed. -
"Datamation", January 15, 1984
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Joined: 21 Jan 2005
Posts: 2
[ Doubt ] Bitfields
There is a concept called padding or byte alignment.
This has been done by most of the compilers for easy &
fast accessing. In ur query, the size of actual
structure is 12 bits(not 12bytes). 12 bits has been
aligned to nearest minimum size of 4bytes
RGDS
Alex Pally
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