[RFC] time: Create a endian_t.h headerfile

[Joseph Myers]

On Thu, 19 Sep 2019, Alistair Francis wrote:

> To allow struct __timespec64 and struct timespec to be the same on a
> 32-bit architecture with a 64-bit time_t we need to add padding to the
> struct timespec. This padding requires knowledge of the machines
> endianess, which requires us exporting the endianness in
> time/bits/types/struct_timespec.h.
> 
> To reduce the number of macros that we are exporting let's split out an
> endian_t.h header file that can be used.

An installed header has to be appropriately listed in "headers" in some 
directory's Makefile to cause it to be installed; otherwise you get code 
that builds file in the glibc source tree but breaks with an installed 
compiler and headers.

The bits/types/ namespace is *only* for headers defining a single type 
(that is, bits/types/endian_t.h would be a header that defines the 
endian_t typedef and nothing else).  So it's not appropriate for this 
header.

You can't use "#include <time/...>" in an installed header because that 
would also break when using an installed compiler and headers.  #includes 
in installed headers need to use the <bits/*> path the included header 
would have when installed.  Then you add an include/bits/ wrapper in the 
glibc source tree to allow the uninstalled header to be found when 
building glibc.  (And time/ is not the right location for a header about 
endianness.  Either string/, or use the top-level bits/ directory and so 
avoid the need for any wrapper at all.)

If anything, I think the cleanest naming would be to have <bits/endian.h> 
be the architecture-independent header that defines these three macros and 
includes <bits/endian-arch.h>, where the existing <bits/endian.h> headers 
that define __BYTE_ORDER all get renamed to <bits/endian-arch.h>.  So 
you'd need to add bits/endian-arch.h to the "headers" setting in 
string/Makefile.

-- 
Joseph S. Myers
joseph@codesourcery.com