[PATCH v3] ungetc: Guarantee single char pushback

[Maciej W. Rozycki]

On Tue, 10 Dec 2024, Andreas Schwab wrote:

> >  I don't thing an app is allowed to poke at FILE in the first place, it's 
> > supposed to be an opaque data type.
> 
> Right, except through the macros provided by <stdio.h>, the only reason
> why the details are visible.

 Indeed, so these parts have become a part of the ABI.  This has fixed 
`_IO_read_ptr', `_IO_read_end', `_IO_write_ptr', `_IO_write_end', and 
`_flags' AFAICT, but not `_flags2' or anything else.

  Maciej