Why GCC do this???

[Jyotirmoy Das]

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.

Regards,
Jyoti
-----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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