sizeof

[nappaji]

struct TEST{
char a;
char b;
int c;
short d;
int *p;
};

when i do a sizeof(struct TEST), the value returned is 16.

But when i do a
sizeof (char) = 1
sizeof(int) =4;
sizeof(short) = 2;
sizeof(int *) =4;

so, should'nt the total size be , 1+1+4+2+4 = 12????

Please help.

 

[greatkingrat]

Because some padded bytes are inserted.
To access memory efficiently ( and to avoid
bus error for serveral systems ), a structure
is aligned by the most significant size.

The most significant size of your structure is 4.
So, actual memory block of your structure is


[a][ ][ ] [c][c][c][c] [d][d][ ][ ] [p][p][p][p]

[ ] means 1 byte and if there is no character, it
is padded byte to be aligned at the 4 bytes boundary.
Count them, you can see it is 16 bytes!

Another consideration to reduce you structure size :
If your strcuture is defined like this,
struct TEST {
char a;
char b;
short d;
int c;
int *p;
};

Then it is 12 bytes because,

[a][d][d] [c][c][c][c] [p][p][p][p]

I hope it is helpful to you. Thanks
Hee S. Chung
heesc@netian.com

http://www.netian.com/~heesc

 

[greatkingrat]

Sorry, the last memory block was mistyped...
[a][d][d] [c][c][c][c] [p][p][p][p]

I missed .
Hee S. Chung
heesc@netian.com

http://www.netian.com/~heesc

 

[greatkingrat]

hmm, maybe hypertext cannot print [ b ]...

The first memory block is

(a)(b)( )( ) (c)(c)(c)(c) (d)(d)( )( ) (p)(p)(p)(p)

The second is

(a)(b)(d)(d) (c)(c)(c)(c) (p)(p)(p)(p)

I previewed them, so they are exact.
I'm sorry but this server doesn' accept [ b ].
:-( Hee S. Chung
heesc@netian.com

http://www.netian.com/~heesc

 

[Zyrenthian]

Yes... it is a padding issue and on a side note:

ignore in brackets [ignore] will allow the or any other tag to be ignored. i constantly have this problem with when demonstrating for loops and ending up with italics[/ignore]

Matt

 

[SwapSawe]

Why not deselect Process TGML checkbox when using such things, it is in the bottom left of the textArea in which you type your Replies.
Regards,
SwapSawe.

 

[greatkingrat]

o.k. Thanks Hee S. Chung
heesc@netian.com

http://www.netian.com/~heesc