[lug] OFF: sizeof structs in gcc?

[Tkil]

>>>>> "Scott" == Scott A Herod <herod at interact-tv.com> writes:

Scott> Is it true that gcc allocates space for objects/stucts in 4
Scott> byte words?  

it usually prefers the fastest allocation for the given platform,
which is usually at least 4 bytes (e.g. doubles might be faster on
8-byte boundaries)

Scott> Is there a way to avoid this?

you want to look at the "packed" attribute.  from:

   info '(gcc.info)Type Attributes'

we have:

| `packed'
|      This attribute, attached to an `enum', `struct', or `union'
|      type definition, specified that the minimum required memory be
|      used to represent the type.
| 
|      Specifying this attribute for `struct' and `union' types is
|      equivalent to specifying the `packed' attribute on each of the
|      structure or union members.  Specifying the `-fshort-enums'
|      flag on the line is equivalent to specifying the `packed'
|      attribute on all `enum' definitions.
| 
|      You may only specify this attribute after a closing curly brace
|      on an `enum' definition, not in a `typedef' declaration, unless
|      that declaration also contains the definition of the `enum'.

so, you might do:

| #include <stdio.h>
| #include <stddef.h> /* for offsetof() macro */
| 
| struct unpacked_struct { short a; long b; };
| struct packed_struct   { short a; long b; } __attribute__ ((packed));
| 
| int main(int argc, char * argv [])
| {
|   printf("unpacked: sizeof=%d, offset(a)=%d, offset(b)=%d\n",
| 	 sizeof  (struct unpacked_struct), 
| 	 offsetof(struct unpacked_struct, a),
| 	 offsetof(struct unpacked_struct, b) );
|   printf("packed:   sizeof=%d, offset(a)=%d, offset(b)=%d\n",
| 	 sizeof  (struct packed_struct), 
| 	 offsetof(struct packed_struct, a),
| 	 offsetof(struct packed_struct, b) );
|   return 0;
| }

which, on kevin's athlon, yields:

| $ ./pack
| unpacked: sizeof=8, offset(a)=0, offset(b)=4
| packed:   sizeof=6, offset(a)=0, offset(b)=2

t.