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From: Holcomb, Michael J
Date: Mon, May 17, 1993 7:58 AM
Subject: Packed Structures
To: GNU GCC; GNU
I am using the GNU C compiler running on the SUN for a target MC68040 
processor.  I would like to know if there is a way to specify that structures 
be packed on 1-byte boundaries.  I am working with the following structures:

typedef struct
  {
  unsigned Start_Byte   : 8;
  unsigned Message_Type : 8;
  } Message_Header_Type;

typedef struct
  {
  unsigned Checksum_0_6  : 8;
  unsigned Checksum_7_13 : 8;
  unsigned End_Byte      : 8;
  } Message_Trailer_Type;

typedef struct
  {
  unsigned Bute_1 : 8;
  unsigned Byte_2 : 8;
  unsigned Byte_3 : 8;
  } Message_Body_Type;

typedef struct
  {
  Message_Header_Type  Message_Header;
  Message_Body_Type    Message_Body;
  Message_Trailer_Type Message_Trailer;
  } Output_Message_Type;

Using the 'sizeof' operator on each of the above structures gives me the 
follow results: 
         sizeof (Message_Header_Type)  = 2
         sizeof (Message_Body_Type)    = 4
         sizeof (Message_Trailer_Type) = 4
         sizeof (Output_Message_Type)  = 10

It seems that any structure with an odd number of bytes get padded with an 
extra byte so that it stays aligned on a word boundary.  This extra padding 
byte on the two Message_Boby and Message_Trailer structures is causing 
problems in my coding application.

Most compilers have a 'pack' or 'align' specifier to cause structure members 
to be packed into memory on 1-byte, 2-byte, or 4-byte boundaries.  If your 
complier has any switches or a #pragma preprocessor directive to force 1-byte 
packing please let me know.  Your assistance to my problem would be 
appreciated.

Thank You,

  Mike Holcomb
  Hughes Aircraft Company
  P.O Box 3310 M/S 604/E222
  Fullerton, CA 92634 

  Internet 0012310@msgate.emis.hac.com