How is a C-style structure with a bitfield represented in a Rust # [repr (C)] structure?

Question

I have a C structure defined as:

struct my_c_s {
    u_char          *ptr;
    unsigned        flag_a:1;
    unsigned        flag_b:1;
    int             some_num;
}

How would flag_a

u be presented flag_b

?

#[repr(C)]
pub struct my_rust_s {
    pub ptr: *const u_char,
    //pub flag_a: ?,
    //pub flag_b: ?,
    pub some_num: ::libc::c_int,
}

Can I declare them as bool

s? Or should it all be some kind of set of bits with one field and then I cross them out?

eg. pub flag_bits: ::libc::c_uint,

+3

marathon 21 jul. 17 at 4:48

1 answer

Masaki hara · Accepted Answer · 2017-07-21T05:19:41+0000

No, you cannot.

There is an open issue regarding bit-field support that doesn't seem to be active. In this issue @ retep998 explains how bit fields are handled inwinapi

. This can be useful if you need to handle bit fields in the C interface.

The OP seems to be targeting C interoperability, but if you just want the bitpod functionality, there are several solutions.

You should always consider a simple redundant solution: avoid bit-fields and let the fields align naturally.
bitfield

, as per the comment - I didn 't know this, but it seems to be the bit equivalent of C.
bitflags

... This seems appropriate for bit flags, which are commonly referred to as enum

in C.
#[repr(packed)]

if you just want to pack the margins to some extent, ignoring the alignment. The margins will still be byte-aligned.
bit-vec

if you want homogeneous bit arrays.