Why GCC do this???

[Claudio Bley]

Hi.

On Fri, May 14, 2004 at 02:46:22PM +0530, Jyotirmoy Das wrote:
> Hi Andrea,
>   Basic reason behind this for the following fact:
>  Size of a structure may not be always equal to size of its data member
> due to memory alignment. Therefore, that is why in the first case, you
> are not getting the desired result. However, when you did the read from
> file using the individual field, it works.

... and you may use the attribute "packed" to a struct type in order to specify 
that the minimum required memory be used to represent the type. (see gcc's info pages,
Node: Type Attributes)

E.g.:

  typedef struct BMP_H {
	word ID;
	dword size;
	dword res;
  } __attribute__ ((packed)) BMP_H; 

should work for your program.

PS: Please, don't top-post. See http://www.faqs.org/docs/jargon/T/top-post.html

> -----Original Message-----
> From: gcc-help-owner@gcc.gnu.org [mailto:gcc-help-owner@gcc.gnu.org] On
> Behalf Of Andrea Pretto
> Sent: Friday, May 14, 2004 2:38 PM
> To: gcc-help@gcc.gnu.org
> Subject: Why GCC do this???
> 
> Hello.
> 
> I'm writing a chunk of software that must read a
> bitmap file.
> The first field of bitmap header is the ID ( 16 bit ),
> and then the size of file ( 32 bit ).
> ( after these , thre is a reserved field ( 32 bit ))
> 
> In example.
> 
> 42 4d c6 4a 43 00 00 00 00 00
> 
> ID is   42 4d   ,  that is BM
> Size is   00 43 4a c6, that is 4410045 ( 4.4 mb)
> Reserved is 00 00 00 00 ...... nothing.
> 
> I have defined this structure.
> 
> typedef unsigned short word;
> typedef unsigned long dword;
> 
> typdef struct BMP_H {
>    word ID;
>    dword size;
>    dword res;
> } BMP_H; //this structure is 10 byte
> 
> then...
> 
> fread( (void *)&my, 12 ,1,file );
> 
> ..don't read the header correct.
> It read
> 
> Id: BM ( correct )
> size: 67 ( no )
> res: 3538944 ( no )
> 
> Looking at file above, I understand that gcc read 4
> byte for Id, in fact 67 ( in hex is 43 )....
> 
> WHY?????????
> Why it read 4 byte instead of 2???
> 
> In addition sizeof return me 12 instead of 10.
> 
> but, in this way...
> 
> fread( (void *)&my.ID, 2 ,1,file );
> fread( (void *)&my.size, 10 -2, 1, file);
> 
> ..the result is correct....
> 
> why with the first way don't run correctly??
> 
> ( i've tried with  old TURBO C for msdos, and the
> resulty is correct in either ways..)
> 
> Who explane me this, please???
> 
> 
> 	
> 
> 	
> 		
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-- 
Claudio