Re: kernel header compatibility

[Rich Felker <dalias@libc.org>]

On Tue, Nov 08, 2016 at 08:11:45PM -0500, Felix Janda wrote:
> The recent commit 04983f2272382af92eb8f8838964ff944fbb8258 (make
> netinet/in.h suppress clashing definitions from kernel headers)
> intends to address some of the conflicts between the kernel and musl
> libc headers. Namely it tries to allow the inclusion of kernel headers
> after libc header by defining __UAP_DEF_* macros. However this doesn't
> work because the relevant linux headers include <linux/libc-compat.h>,
> which unconditionally redefines the constants. For example

Oh, how awful. I missed the whole bogus #else part after the #if
defined(__GLIBC__) block.
> 
> #include <netinet/in.h>
> #include <linux/in6.h>
> 
> leads to
> 
> #define __UAPI_DEF_IN_ADDR 0 // from <netinet/in.h>
> #define __UAPI_DEF_IN_ADDR 1 // from <linux/libc-compat.h>

Conflicting defines should be an error already.

> So we still get two conflicting definitions of struct in6_addr.
> 
> 
> By adding the hack "#define _LIBC_COMPAT_H" to <netinet/in.h>, this
> particular example compiles.
> 
> 
> Maybe the kernel people can be convinced to add #ifdef guards around
> all of the (non glibc) __UAPI_* definitions in <linux/libc-compat.h>.

I think they should, but I don't mind just suppressing the whole
header by defining _UAPI_LIBC_COMPAT_H if that works for all kernel
versions. It seems to; see:

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/include/uapi/linux/libc-compat.h?id=cfd280c91253cc28e4919e349fa7a813b63e71e8

where the file was introduced.

Rich