From: "Alberto Chessa" <fiargbgm AT tin DOT it>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Struct with array. HELP
Date: 22 Apr 1998 14:50:26 GMT
Organization: FIAR S.p.A.
Lines: 38
Message-ID: <01bd6dfe$1a58f440$92c809c0@CHESSA>
References: <6hi61a$avd1 AT willy DOT cra DOT enel DOT it>
Reply-To: "\"Alberto Chessa\" <Alberto Chessa" <fiargbgm AT tin DOT it>
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To: djgpp AT delorie DOT com
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massarin AT alpha3 DOT cram DOT enel DOT it wrote in article
<6hi61a$avd1 AT willy DOT cra DOT enel DOT it>...
> My problem is that when I declare a struct like:
> 
> struct a {
> 
>   char b[n];
>   int  c;
>   .      }
> 
> if n (number of char in array) is not a multiple of 4
> the array is enlarged to fit a 4' multiple, 
> 
> Does anyone know how to make the struct, and the array, of
> the proper size ,apart from doing the struct as an array ?
> Is it a bug of compiler ?
> 
> Thanks.
> 
> ps. Please answer by newsgroup .
> 
> 
It's not a bug, it's an optimization! The processor always read memory at
word aligned addresses. Thus, to improve performance, a datum should never
begin on odd address. On 486 and Pentium processor, the data should be
aligned at 8 byte and procedure at 16 byte (the cache fetch at 0-mod-16
addresses).
Two way to pack structures:
 1. Use compiler switch "-fpack-struct": A bad solution since You'll pack
everithing      (degraded performance)
 2. Use __attribute__((paked)) (Info gcc.info, node "Variable Attributes"):
	struct a {
	  char b[n];
	  int  c		__attribute__((packed));
	}