On Mon, Sep 19, 2016 at 2:08 PM, Ali Demir <dem...@gmail.com> wrote:
> Packing option is explicitly specified for VC++:
> https://msdn.microsoft.com/en-us/library/xh3e3fd0.aspx
> The user needs to make sure whatever they pick (or default) in their project
> matches what Go compiler used. So it may help to specify what go compiler is
> using.

There is no answer to that question.  The Go compiler is not running
the C compiler.

In general the Go compiler expects fields to be aligned to their size.
That is, a four byte field is aligned to a four byte boundary, and an
eight byte field is aligned to an eight byte boundary.  As I said
earlier, the compiler emits padding fields so that everything will
work if the C compiler uses a smaller alignment.  However, things will
fail if the C compiler uses a larger alignment.  I'm not aware of any
such C compiler, but I guess it wouldn't shock me.


> On Mon, Sep 19, 2016 at 10:00 AM, Ian Lance Taylor <i...@golang.org> wrote:
>> On Mon, Sep 19, 2016 at 8:50 AM, Ali Demir <dem...@gmail.com> wrote:
>> > Header file won't have padding / alignment info. That would be a
>> > compiler
>> > setting on the C side that is using the header. So the programmer needs
>> > to
>> > setup C compiler in a way that is compatible with the go compiler. How
>> > would
>> > he know how to set it up? Do we just assume defaults are the same
>> > between Go
>> > and C?
>> The cgo tool will generate padding fields as needed to ensure that the
>> fields are aligned as expected.
>> It's true that the cgo tool does assume that it knows the alignment
>> requirements of the basic types.  These requirements are not normally
>> changed by compiler options.  The alignment requirements of unusual
>> types, such as processor-specific vector types, are not relevant,
>> since Go can't represent them and they will therefore never appear in
>> the Go/C interface.
>> Do you have a specific concern, or is this purely abstract?
>> Ian

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