Re: [Tinycc-devel] bitfield handling

[Michael Matz]

Hi,

On Sun, 30 Apr 2017, Vicente Bergas wrote:

> > > struct{short x:12; char y:1;}s; s.y=1;
> >
> > Note that the above is tricky: char can be signed and unsigned if not 
> > explicitely specified, and if it's signed then a 1-bit bitfield of char only 
> > contains values -1 and 0, so storing a 1 into it actually makes the value be 
> > -1. So depending on what you tested that might actually have been the problem. 
> > (I.e. always specify signedness of char when using it to store numeric values 
> > and not characters).
>
> There is no problem with signedness. To factor out the sign, you can
> also test this:
> struct{unsigned short x:12; unsigned char y:2;}s; s.y=1;
> which also fails. Both fail even after applying your patch.

Indeed, I thought I tested the above variant before writing the mail but 
obviously I fat-fingered something, it still failed.  Fixed in mob.  I 
chose to adjust field layout instead of field type; the latter can cause 
invalid mem accesses with packed structs/fields: consider the above struct 
being packed (hence having size 2) and placed at the last two bytes of a 
page with the page after that not being accessible.  Reading a whole int 
will then segfault.



Ciao,
Michael.