Date: Thu, 4 May 1995 11:35:13 +0900
From: Stephen Turnbull <turnbull AT shako DOT sk DOT tsukuba DOT ac DOT jp>
To: eheft AT valhalla DOT cs DOT wright DOT edu
Cc: djgpp AT sun DOT soe DOT clarkson DOT edu
Subject: [Q] Structs and spacing of data

   Hi,
      I've got a stuct like 

   typedef struct {
      short id;
      char  fc[3];
      short pos;
      ...
   } trREC,*tpREC;

   when I go to read the struct from a binary file the struct
   appears to have padded a space between then end of fc and
   start of pos. Is there a pragma to make sure the struct uses
   the minimum amout of space?

I think this is a FAQ, see FAQxxx.ZIP at your SimTel mirror.  Use
__attribute__ ((packed)).  According to Info gcc, item variable
attributes:

   typedef struct {
      short id __attribute__ ((packed)); /* this is a no-op */
      char  fc[3] __attribute__ ((packed)); /* no padding after id */
      short pos __attribute__ ((packed)); /* no padding after fc */
      ...
   } trREC,*tpREC;


   How much of a speed penalty will
   I pay in the rest of the code (hav'ing non-word boundries)?
   Would it be better skip the pragma and just to read the
   fields one at a time?

IMHO, no.  This use of __attribute__ documents the file structure in
the code.

-- 
Stephen Turnbull  /  Yaseppochi-gumi  /  <turnbull AT shako DOT sk DOT tsukuba DOT ac DOT jp>
http://turnbull.sk.tsukuba.ac.jp/      anon FTP: turnbull.sk.tsukuba.ac.jp
Check out Kansai-WWW, too ------------> http://pclsp2.kuicr.kyoto-u.ac.jp/