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Date: Tue, 10 Sep 2002 10:29:00 -0000
From: John Kwan <jkwan@mvista.com>
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Subject: The pack attribute
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Consider the following program:

// ***************  test.c
#include <stdio.h>

// case 1
typedef  struct {
    unsigned char byte1;
    int i;
    unsigned char byte2;
 } test_struct_1 __attribute__((packed));

// case 2
struct test_struct_2 {
    unsigned char byte1;
    int i;
    unsigned char byte2;test_struct_1 var1;
} __attribute__((packed));

test_struct_1              var1;
struct test_struct_2 var2;

main()
{
  printf("SIZE of var1 = %d\n",sizeof(var1));  // prints 12 bytes
  printf("SIZE of var2 = %d\n",sizeof(var2));  // prints   6 bytes
}
//****************   end of program

Intuitively I would expect case 1 and case 2 to product the same result
but they do not.  The 3.1.1 gcc compiler prints 12 for case 1 and 6 for
case 2.

The gcc on line manual states:
   "specifying this attribute for struct and union types is equivalent
     to specifying the pack attribute on each of the structure or union
members"
This seems to imply that case 1 should also be 6 bytes in size.

However, 2lines later, the manual further states:
    "You may only specify this attribute after the closing curly brace
on an enum
       definition not in a typedef declaration ..."
This seems to imply that __attribute__((packed)) does not work on the
typedef
of an enum. No mention is made about structure/union typedefs.

My question is: Should case 1 and case 2 work differently? Or is this
feature
badly implemented.

Thanks.

JOhn W Kwan
MontaVista Software