As you know bit's fields in C are not portable when we're talking about external world (network/different architecture/file transmission): order of the fields is unspecified, also as real wrapper struct...
Often bitwise operations are used instead of bitfield struct syntax but result code may be less readable opposite to original idea: bitfields. But there is simple solution to workaround weird syntax. Here you are:
#define __BITSTRUCT_MASK(LEN) ((1<<(LEN))-1)
#ifndef BITSTRUCT_GETBITS_IMPL
# define BITSTRUCT_GETBITS_IMPL(BS, OFFSET, LEN) \
(((BS)>>(OFFSET)) & __BITSTRUCT_MASK(LEN))
#endif
#ifndef BITSTRUCT_SETBITS_IMPL
# define BITSTRUCT_SETBITS_IMPL(BS, OFFSET, LEN, VALUE) \
((BS) & ~(__BITSTRUCT_MASK(LEN)<<(OFFSET)) | \
(((VALUE) & __BITSTRUCT_MASK(LEN))<<(OFFSET)))
#endif
#define BITSTRUCT enum
#define BITFIELD(NAME, OFFSET, LEN) NAME##_OFF = (OFFSET), NAME##_LEN=(LEN)
#define BITFIELD_MASK(NAME) __BITSTRUCT_MASK(NAME##_LEN)
#define BITFIELD_GET(NAME, BS) \
BITSTRUCT_GETBITS_IMPL(BS, NAME##_OFF, NAME##_LEN)
#define BITFIELD_SET(NAME, BS, VALUE) \
BITSTRUCT_SETBITS_IMPL(BS, NAME##_OFF, NAME##_LEN, VALUE)
So now it's possible to write something like this to describe register X:
|15.....8|7....4|3.....0|
| ENABLE | FLAG | COUNT |
BITSTRUCT RegisterX {
// BITFIELD(NAME, OFFSET, LENGTH)
BITFIELD(RX_COUNT, 0, 4),
BITFIELD(RX_FLAG, 4, 4),
BITFIELD(RX_ENABLE, 8, 8)
};
int main(void)
{
int x = 0x1234; // raw value of register
// example of get/set field of bits' struct
x = BITFIELD_SET(RX_ENABLE, x, 0x98);
printf("%0X", BITFIELD_GET(RX_ENABLE, x));
return (0);
}
As you can see code is readable as in usual bitfield's struct, but instead of
struct BITSTRUCT should be used,
and BITFIELD() descriptor instead of
type NAME:len form. To set or to get field value, use BITFIELD_SET/GET() macros. BITFIELD_MASK() returns mask of field but from zero bit.
But note the *_IMPL macros, using they, you can
customize behavior of SET/GET implementation, for example to use some
DSP core what can improve program performance dramatically! What you need is only to define own GET/SET helpers-implementators (BITSTRUCT_GETBITS_IMPL/BITSTRUCT_SETBITS_IMPL).
