From: Robert Jaycocks <R DOT Jaycocks1 AT lut DOT ac DOT uk>
Subject: Re: Sizeof structure different in DJGPP?
To: MWODRICH AT eleceng DOT uct DOT ac DOT za (WODRICH M M WDRMAR002)
Date: Fri, 24 Mar 95 11:54:04 GMT
Cc: djgpp AT sun DOT soe DOT clarkson DOT edu

> 
> Hello
> 
> I seem to come up with a new question every day almost, but here goes!
> I have a structure as follows in my program :
> 
> struct something
> {
>    char ch;
>    unsigned short int number;
> };
> 
> In Turbo-C the sizeof this struct is, as expected, 3 bytes. In DJGPP 
> it is 4 bytes ???! The sizeof (char) and sizeof (unsigned short int) 
> is 1 & 2 respectively in both compilers... Does DJGPP Word-align data,
> promoting the 3-byte strucure to 4 bytes? The reason I am asking is 
> that I want to store many (60,000) such structs in an array, and 
> wasting 60Kb+ in DJGPP seems crazy, even though memory is no problem. 
> I want to avoid paging on small systems unless absolutely 
> neccessary (sp?), so the extra 1 byte is irritating me...
> 
> Thanks for an informative mailing list!
> Mark.--
> 
> Mark Wodrich                Email : mwodrich AT eleceng DOT uct DOT ac DOT za
> 13 Letchworth Road
> Pinelands                   " Excuse me, but I think your karma has
> 7405                          just run over my dogma ..."
> 
You can use the variable attribute packed, as in :

struct something
{
   char ch _attribute_ ((packed));
   u_short number _attribute_ ((packed));
};

Doing a sizeof(struct something) should now return 3.

Refer to the GNU GCC Info page on attribute variables.

Hope this helps

Robert.