Bitfield assignment is broken

[xlab]

Bitfield assignment is broken

[cznic]

@pwaller & @xlab

I haven't really checked but it's quite possible the bitfield handling is broken in cc. I've run into several such issues and somehow bypassed them in ccir, for example. Once I will get [more] confident about the correctness of the bypass, I gues it's due to backport ccir's hack to cc.

However, please feel free to report any cc issue directly at its issue tracker. No promises about when the fixes will come, but I'm currently becoming more and more dependent on [at least some] correctness of cc, so bug reports are definitely welcome.

[pwaller]

@cznic as far as I can tell, the interpretation of the bitfield is correct from cc's side. The problem is a difference between what c-for-go and cgo expect. From a careful inspection of the error output (above); you can see that for some reason, even though the bitfield's type is an int, if the bitfield with is 8 - even though the type is an int, cgo expects you to assign a uint8 to it! Not a C.uint8_t, but a go uint8. I think this is a surprising feature of cgo and I can't find it documented anywhere.

---

Separate thought:

I didn't think to check non-multiples of 8, but in that case I get:

typedef struct {
        int i : 1;
        int j : 7;
        int k : 15;
        int l : 31;
} Foo;

bitfield/cgo_helpers.go:159: x.refb43e23c1.i undefined (type *C.struct___0 has no field or method i)
bitfield/cgo_helpers.go:160: x.refb43e23c1.j undefined (type *C.struct___0 has no field or method j)
bitfield/cgo_helpers.go:161: x.refb43e23c1.k undefined (type *C.struct___0 has no field or method k)
bitfield/cgo_helpers.go:162: x.refb43e23c1.l undefined (type *C.struct___0 has no field or method l)

... which seems to suggest that CGo doesn't support odd-sized bitfields? Am I missing something here - is the above struct definition valid C?

[cznic]

Am I missing something here - is the above struct definition valid C?

Seems valid. I'm not aware of any requirement of the bitfileds to be of width a multiply of something.

[CtrlZvi]

I don't think it's just odd bitfield sizes. I think it might be any bitfields that are not powers of 2? From the cgo command documentation: "C struct fields that cannot be expressed in Go, such as bit fields or misaligned data, are omitted in the Go struct, replaced by appropriate padding to reach the next field or the end of the struct."