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

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On Sep 14, 2005, at 09:46:04, Bill Davidsen wrote:
H. Peter Anvin wrote:
Followup to:  <[email protected]>
By author:    Erik Andersen <[email protected]>
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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