[ocaml-ctypes] Bit field encoding.

[Jeremy Yallop]

Dear Philippe,

On 16/05/2014, Philippe Veber <philippe.veber at gmail.com> wrote:
> Has ctypes some support for bit fields in C struct?

Not yet, I'm afraid.

> If not is it correct to
> encode:
>
> typedef struct {
>    int32_t pos;
>    uint32_t bin:16, qual:8, l_qname:8;
>    ...
> } bam1_core_t
>
> as:
>
>    type bam1_core_t
>    let bam1_core_t : bam1_core_t structure typ = structure "bam1_core_t"
>    let bam1_core_t_pos_f = field bam1_core_t "pos" int32_t
>    let bam1_core_t_props_f = field bam1_core_t "props" int32_t
>    ...
>    let () = seal bam1_core_t
>
> and then play with the fake "props" field directly?

It's quite likely to work but it's not guaranteed, since the C
standard gives compilers a fair amount of freedom in laying out
bit-fields.  The good news is that the standard also requires
compilers to document how bit-fields are laid out, so your compiler
documentation may have helpful details.

Kind regards,

Jeremy.