Re: use bit field or kernel *_bit()

[Ming Zhang <mingz@xxxxxxxxxxx>]

On Wed, 2006-03-01 at 16:11 +0100, Arjan van de Ven wrote:
> On Wed, 2006-03-01 at 09:57 -0500, Ming Zhang wrote:
> > On Wed, 2006-03-01 at 15:48 +0100, Arjan van de Ven wrote:
> > > On Wed, 2006-03-01 at 09:38 -0500, Ming Zhang wrote:
> > > > On Wed, 2006-03-01 at 15:29 +0100, Arjan van de Ven wrote:
> > > > > > 
> > > > > > one more question, for bit field, if we use long xyz:n, will that be an
> > > > > > issue in 32/64bit arch? i feel no, just to make sure.
> > > > > 
> > > > > first of all, you almost always want to make the bitfield an "unsigned"
> > > > > type; either "unsigned int" or "unsigned long". (there are exceptions to
> > > > > this, but if you need them you should ask yourself if you really really
> > > > > want bitfields in that case)
> > > > 
> > > > why unsigned int will have difference from signed int? i always access
> > > > it by bits right?
> > > 
> > > NO! it will make the BITFIELD signed. Which means if you have a 3 bit
> > > bitfield, the top bit of the three will be the sign bit; which is most
> > > likely not what you want!
> > 
> > ok, so my understanding is wrong, i assumed if i have
> > 
> > int foo1:3;
> > int foo2:3;
> > 
> > compile will use a int (32bit) to hold foo1 and foo2, and each use 3bits
> > (MSB or LSB not sure. depends on arch). so this is not the case?
> 
> yup that's not the case

so the case is that it will use int for each bit field?


> > 
> > so the tricky question is what if i only have 1 bit? will that bit be
> > sign bit and no data bit at all?
> 
> yes well but remember that "-1" and "1" have the same representation in
> "signed 1 bit" binary in 2's complement systems. So you've been pure
> lucky ;)

o, yes. so here 1 bit or 2 bits will get same data range. :P i am
lucky. :P


> 
> 
> > i DO have a code here that use int and many bit field are foo:1. code
> > runs ok.
> 
> I'd suggest you change that to unsigned int... the tricky part is when
> you start using < and > and such
> 

agree, must be crazy when debug bug like that!

thanks!



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