Hi!

I'm in the process of translating some C headers (still ffmpef -> libavcode and libavformat) to D and there are some really ugly structs in the C headers which I'm trying to translate.

- C -

typedef struct xy
{
    int a:1;
    int b:2;
    int c;
}

- D -

struct xy
{
    align (1)
    {
        int a;
    }
    align (2)
    {
        int b;
    }
    int c;
}

Are those definitions equivalent?

-Mike


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On Wed, Feb 4, 2009 at 2:55 PM, Mike <vertex@gmx.at> wrote:
> Hi!
>
> I'm in the process of translating some C headers (still ffmpef -> libavcode and libavformat) to D and there are some really ugly structs in the C headers which I'm trying to translate.
>
> - C -
>
> typedef struct xy
> {
>    int a:1;
>    int b:2;
>    int c;
> }
>
> - D -
>
> struct xy
> {
>    align (1)
>    {
>        int a;
>    }
>    align (2)
>    {
>        int b;
>    }
>    int c;
> }
>
> Are those definitions equivalent?

Nope.  The C struct is defining a bitfield; a and b will actually be contained within a single 4-byte field.  Your D version defines three integers.

Unfortunately the C specification does not specify any required ordering for bitfields, padding, ordering etc.  Fortunately, most compilers just put them in order, starting from the lowest bits.

What you'll have to do, then, is put a single int field that corresponds to the C struct's bitfields.  Then, have methods which shift and mask the bits to get and set the individual bitfields.  htod will do this for you, if you're on windows and just want to run an .h file containing that struct through it.

On Wed, 04 Feb 2009 21:04:16 +0100, Jarrett Billingsley <jarrett.billingsley@gmail.com> wrote:

> Nope.  The C struct is defining a bitfield; a and b will actually be
> contained within a single 4-byte field.  Your D version defines three
> integers.
>
> Unfortunately the C specification does not specify any required
> ordering for bitfields, padding, ordering etc.  Fortunately, most
> compilers just put them in order, starting from the lowest bits.
>
> What you'll have to do, then, is put a single int field that
> corresponds to the C struct's bitfields.  Then, have methods which
> shift and mask the bits to get and set the individual bitfields.  htod
> will do this for you, if you're on windows and just want to run an .h
> file containing that struct through it.

Thanks. I tried htod with Wine, and although it runs it can't open include files, so I thought I'd translate the headers manually. However - I found more of those bitfields; I'll better find a machine with Windows then :)

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Mike wrote:
> On Wed, 04 Feb 2009 21:04:16 +0100, Jarrett Billingsley <jarrett.billingsley@gmail.com> wrote:
> 
>> Nope.  The C struct is defining a bitfield; a and b will actually be
>> contained within a single 4-byte field.  Your D version defines three
>> integers.
>>
>> Unfortunately the C specification does not specify any required
>> ordering for bitfields, padding, ordering etc.  Fortunately, most
>> compilers just put them in order, starting from the lowest bits.
>>
>> What you'll have to do, then, is put a single int field that
>> corresponds to the C struct's bitfields.  Then, have methods which
>> shift and mask the bits to get and set the individual bitfields.  htod
>> will do this for you, if you're on windows and just want to run an .h
>> file containing that struct through it.
> 
> Thanks. I tried htod with Wine, and although it runs it can't open include files, so I thought I'd translate the headers manually. However - I found more of those bitfields; I'll better find a machine with Windows then :)
> 

If you end up doing it manually, look in Phobos2, typecons.d has functions for generating bitfields. It'll work on D1 with trivial modifications.