HOW TO PACK VARIABLES IN GNU C/C++
		  ==================================

LAST EDIT: 10 Iyar 5761, 2001/05/03, Haim Roman

gcc version egcs-2.91.66 19990314/Linux (egcs-1.1.2 release)

The following is an excerpt from GNU C's documentation (emacs info
format) explaining how to "pack" variables (i.e., prevent the compiler
from inserting "holes", or unused bytes, between variables).

NOTE: this is one of GNU's *extensions* to standard C/C++.  
      Other C/C++ compilers will probably use different methods
      (if they have one)

=============================================================

Specifying Attributes of Variables
==================================

   The keyword `__attribute__' allows you to specify special attributes
of variables or structure fields.  This keyword is followed by an
attribute specification inside double parentheses.  Eight attributes
are currently defined for variables: `aligned', `mode', `nocommon',
`packed', `section', `transparent_union', `unused', and `weak'.  Other
attributes are available for functions (*note Function Attributes::) and
for types (*note Type Attributes::).

   You may also specify attributes with `__' preceding and following
each keyword.  This allows you to use them in header files without
being concerned about a possible macro of the same name.  For example,
you may use `__aligned__' instead of `aligned'.

`packed'
     The `packed' attribute specifies that a variable or structure field
     should have the smallest possible alignment--one byte for a
     variable, and one bit for a field, unless you specify a larger
     value with the `aligned' attribute.

     Here is a structure in which the field `x' is packed, so that it
     immediately follows `a':

          struct foo
          {
            char a;
            int x[2] __attribute__ ((packed));
          };

====================================================================

Here are some examples from /usr/include/net/ethernet.h (Red Hat 6.2 Linux):

/*-------------------------------------------------------
  This is a name for the 48 bit ethernet address available on many systems.  
  -------------------------------------------------------*/
struct ether_addr
{
      u_int8_t ether_addr_octet[ETH_ALEN];
} __attribute__ ((__packed__));

/*-------------------------------------------------------
  10Mb/s ethernet header   
  -------------------------------------------------------*/
struct ether_header
{
      u_int8_t    ether_dhost[ETH_ALEN];      /* destination eth addr */
      u_int8_t    ether_shost[ETH_ALEN];      /* source ether addr    */
      u_int16_t   ether_type;                 /* packet type ID field */
} __attribute__ ((__packed__));