Re: [RFC] Splitting out kernel<=>userspace ABI headers

[Kyle Moffett]

On Sep 14, 2005, at 09:46:04, Bill Davidsen wrote:
> H. Peter Anvin wrote:
> > Followup to:  <20050902235833.GA28238@xxxxxxxxxxxx>
> > By author:    Erik Andersen <andersen@xxxxxxxxxxxx>
> > In newsgroup: linux.dev.kernel
> > > <uClibc maintainer hat on>
> > > That would be wonderful.
> > > </off>
> > >
> > > It would be especially nice if everything targeting user space  
> > > were to use only all the nice standard ISO C99 types as defined  
> > > in include/stdint.h such as uint32_t and friends...
> > Absolutely not.  This would be a POSIX namespace violation; they  
> > *must* use double-underscore types.
>
> Could you explain why you think it would be a violation to use  
> POSIX types instead of defining our own? That's what the types are  
> for, to avoid having everyone define some slightly conflicting types.
>
> The kernel predates C99, sort of, and it would be a massive but  
> valuable  task to figure out where a type is really, for instance,  
> 32 bits rather than "size of default int" in length, etc, and use  
> POSIX types where they are correct. Fewer things to maintain, and  
> would make it clear when something is 32 bits by default and when  
> it really must be 32 bits.

Argh, it seems I'm going to be giving this example forever!  Here's  
why this won't work.  We want to have sys/stat.h do something like  
the following:

# define __kabi_stat64 stat
# include <kabi/stat.h>
/* Now we expect struct stat to be defined with correct types */

Since struct stat in that case uses fixed-bit-size types, the header  
fine <kabi/stat.h> needs to include a file providing those types.  If  
it used stdint.h types, such as uint32_t, then it would need to  
#include <stdint.h> or provide the stdint.h types itself.  In order  
to remain POSIX compliant, sys/stat.h must not include stdint.h or  
assume that stdint.h is included or that those types were defined by  
the user program.  Therefore, kabi/*.h cannot use the stdint.h types  
at all!  The solution is a separate file, kabi/types.h, which  
properly defines __kabi_[su]{8,16,32,64} which are safe to include  
and reuse anywhere.  Then sys/types.h would look like this:

# include <kabi/types.h>
typedef __kabi_u8 u_int8;
typedef __kabi_s8 int8;
typedef __kabi_u16 u_int16;
typedef __kabi_s16 int16;
[...]

Cheers,
Kyle Moffett

--
Premature optimization is the root of all evil in programming
  -- C.A.R. Hoare



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