[PATCH] copy_file_range: New function to copy file data

[Joseph Myers]

On Mon, 18 Dec 2017, Adhemerval Zanella wrote:

> Will again return EFBIG.  We have some options as 1. handle EFBIG
> as an expected retuned error, 2. do not declare copy_file_range for
> !__USE_FILE_OFFSET64, 3. add a dummy implementation for non-LFS
> (which return ENOSYS).

Since the default on all platforms, even those with 64-bit off_t 
unconditionally, is !__USE_FILE_OFFSET64, I don't think 2. is a good 
option; it would gratuitously result in the interface being undeclared on 
64-bit platforms when _FILE_OFFSET_BITS is not defined.

(Even on 64-bit platforms, _FILE_OFFSET_BITS=64 results in some changes to 
C++ name mangling, as discussed in bug 15766, and breaks namespace rules, 
bug 14106; on mips64, it also changes struct stat layout.  So, while I 
think we should move to _FILE_OFFSET_BITS=64 by default on all platforms, 
bug 13047, it's not a completely API-compatible change anywhere, and 
fixing namespace issues would be desirable before making such a change.)

-- 
Joseph S. Myers
joseph@codesourcery.com