Re: [PATCH 1/1] ungetc: Cast to unsigned char before push back to stream

[Rich Felker <dalias@libc.org>]

On Fri, Oct 18, 2019 at 09:11:49AM -0400, Rich Felker wrote:
> On Fri, Oct 18, 2019 at 12:35:45PM +0000, wangjianjian (C) wrote:
> > >From 4d24e6fee85ed878dc632dd29aabeb7c7454952e Mon Sep 17 00:00:00 2001
> > From: Wang Jianjian <wangjianjian3@huawei.com>
> > Date: Fri, 18 Oct 2019 20:28:29 +0800
> > Subject: [PATCH 1/1] ungetc: Cast to unsigned char before push back to stream
> > 
> > Per Posix standard, casting to unsigned char is need before returning C
> > and pushing C back to stream.
> > 
> > Signed-off-by: Wang Jianjian <wangjianjian3@huawei.com>
> > ---
> >  src/stdio/ungetc.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/src/stdio/ungetc.c b/src/stdio/ungetc.c
> > index 180673a4..9733091a 100644
> > --- a/src/stdio/ungetc.c
> > +++ b/src/stdio/ungetc.c
> > @@ -12,9 +12,9 @@ int ungetc(int c, FILE *f)
> >                 return EOF;
> >         }
> > 
> > -       *--f->rpos = c;
> > +       *--f->rpos = (unsigned char)c;
> 
> This line does not change anything.
> 
> >         f->flags &= ~F_EOF;
> > 
> >         FUNLOCK(f);
> > -       return c;
> > +       return (unsigned char)c;
> >  }
> > --
> > 2.17.1
> 
> I believe this is actually a functional change, and a needed one for
> conformance (to ISO C, not specific to POSIX). The issue it seems to
> solve, which is what the change needs to be documented as, is the
> return value when a negative value not equal to EOF is passed to
> ungetc. In this case, the right value is already pushed back, but the
> function wrongly returns the original negative value passed in rather
> than the value that was pushed back.

Committing simplified version with just the second change. Thanks for
catching this.

Rich