On 07/28/2017 04:51 AM, Geza Herman wrote:
There's an option in GCC "-mms-bitfields". The doc about it begins with:
"If packed is used on a structure, or if bit-fields are used, it may be
that the Microsoft ABI lays out the structure differently than the way
GCC normally does. Particularly when moving packed data between
functions compiled with GCC and the native Microsoft compiler (either
via function call or as data in a file), it may be necessary to access
either format."
I'm particularly interested in packed structs, bit-fields are not a
concern now. Does this doc mean, that a packed struct layout may differ
between GCC and MSVC? The doc doesn't give an example of this, it just
talks about bit-fields. If the packed layout can differ, in which way
does it? Previously I thought that both compilers put members into the
struct without any padding, so the layout must match.
Different ABIs handle bit-fields differently, and as a result, different
ABIs handle packed structures with bit-fields differently.
There are testcases for this in the gcc testsuite. In the gcc sources,
look at gcc/testsuite/gcc.dg/bf-ms-layout.c.
Otherwise, for a packed structure without bit-fields, and without
sub-structures, it is probably handled the same across most ABIs.
Plus, __attribute__((packed)) documentation have changed:
https://gcc.gnu.org/onlinedocs/gcc-7.1.0/gcc/Common-Variable-Attributes.html,
here the text is:
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.
https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html, here the
text is:
"This attribute, attached to struct or union type definition, specifies
that each member (other than zero-width bit-fields) of the structure or
union is placed to minimize the memory required"
What does "minimize" mean here? Does it give the same guarantees as the
previous definition? Could it mean that there's still padding in the
struct?
Structure layout is complicated, and it isn't possible to explain all
details in a single sentence. There may be cases where a packed
structure still contains padding. It depends on the ABI, and how
bit-fields are handled, etc. You can see some examples with the
-ms-bitfields examples given above, where some packed structures are
larger with gcc than msvc, and some are larger with msvc than gcc.
If in doubt, the best solution is to write some testcases to check, or
add some consistency checking code, e.g. you can add some code to verify
that type sizes are what you expect at compile-time.
Jim
