On Sun, Nov 10, 2024 at 11:04 PM Brian Bi <bbi5291_at_[hidden]> wrote:
> Unless I've misunderstood, the C23 draft appears to say that as soon as
> you *write* to any member of a struct, all padding bits are reset to
> unspecified values.
> 6.2.6.1/6: *When a value is stored in an object of structure or union
> type, including in a member object, the bytes of the object representation
> that correspond to any padding bytes take unspecified values* [...]
> That seems to prevent the use of `memcmp` for comparing the values of
> struct objects in most cases.
> Or is the C standard only talking about writes of struct type? (Which, as
> you probably know, don't exist in C++: an assignment operator, possibly a
> trivial one, is always called.)
>
> --
> *Brian Bi*


The number of cases for memcmp would be not as wide as one hoped, but still
good enough. The way I read the C standard, it is also about member wise
modifications as well, basically anything that changes the value, but not
clear if they bless changing padding bits in the code - though for
something like *__builtin_clear_padding* it has to be possible.

-Mykola


>