>
>
> Hi there - over on the gtk-app-devel list, Tor Lillqvist and I were
> discussing the compiler flag -mms-bitfields. When building a
> gtk-win32 app,
> this is needed to ensure that structs will be aligned to the
> traditional
> Microsoft (8-byte) alignment standard.
>
No, 8-byte structure packing is default on mingw32
-mms-bitfields is much more specific
It is explained in gcc.info.
<quote>
MSVC interprets zero-length bitfields in the following ways:
1. If a zero-length bitfield is inserted between two bitfields
that would normally be coalesced, the bitfields will not be
coalesced.
For example:
struct
{
unsigned long bf_1 : 12;
unsigned long : 0;
unsigned long bf_2 : 12;
} t1;
The size of `t1' would be 8 bytes with the zero-length
bitfield. If the zero-length bitfield were removed, `t1''s
size would be 4 bytes.
2. If a zero-length bitfield is inserted after a bitfield,
`foo', and the alignment of the zero-length bitfield is
greater than the member that follows it, `bar', `bar' will be
aligned as the type of the zero-length bitfield.
For example:
struct
{
char foo : 4;
short : 0;
char bar;
} t2;
struct
{
char foo : 4;
short : 0;
double bar;
} t3;
For `t2', `bar' will be placed at offset 2, rather than
offset 1. Accordingly, the size of `t2' will be 4. For
`t3', the zero-length bitfield will not affect the alignment
of `bar' or, as a result, the size of the structure.
Taking this into account, it is important to note the
following:
1. If a zero-length bitfield follows a normal bitfield, the
type of the zero-length bitfield may affect the
alignment of the structure as whole. For example, `t2'
has a size of 4 bytes, since the zero-length bitfield
follows a normal bitfield, and is of type short.
2. Even if a zero-length bitfield is not followed by a
normal bitfield, it may still affect the alignment of
the structure:
struct
{
char foo : 6;
long : 0;
} t4;
Here, `t4' will take up 4 bytes.
3. Zero-length bitfields following non-bitfield members are
ignored:
struct
{
char foo;
long : 0;
char bar;
} t5;
Here, `t5' will take up 2 bytes.
</quote>
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