Re: [PATCH] newlib: fix fseek optimization with SEEK_CUR

Corinna Vinschen <[email protected]> Wed, 29 Jan 2020 19:02:17 +0100

Newsgroups	gmane.comp.lib.newlib
Message-ID	<[email protected]>

On Nov 18 11:11, Corinna Vinschen wrote:
> On Nov  9 17:28, Bastien Bouclet wrote:
> > The call to fflush was invalidating the read buffer, preventing relative
> > seeks to positions that would have been inside the read buffer from
> > being optimized. The call to srefill would then re-read mostly the same
> > data that was initially in the read buffer.
> > ---
> >  newlib/libc/stdio/fseeko.c     | 31 ++++++-------------------------
> >  newlib/libc/stdio64/fseeko64.c | 31 ++++++-------------------------
> >  2 files changed, 12 insertions(+), 50 deletions(-)
> 
> Pushed.

Sorry, but I had to revert this patch.  It breaks gnulib's autoconf
test.  The attached conftest.c returns 5, rather than 0 as before
because lseek and ftello return different results.

While this is expected on BSD systems, it's not expected on at least
Linux and Cygwin.  Since this breaks backward compatibility and
leads to gnulib wrongly providing its own fflush, fseek and fseeko
implementations when building for newlib/Cygwin.

I attached the gnulib testcase for completeness.

Many thanks to Takashi Yano for figuring this out after the CYgwin
octave build was broken.

Thanks,
Corinna

-- 
Corinna Vinschen
Cygwin Maintainer
Red Hat

conftest.c (text/plain, 1.4 KB)

#include <stdio.h>
# include <unistd.h>

int
main ()
{
FILE *f = fopen ("conftest.txt", "r");
         char buffer[10];
         int fd;
         int c;
         if (f == NULL)
           return 1;
         fd = fileno (f);
         if (fd < 0 || fread (buffer, 1, 5, f) != 5)
           { fclose (f); return 2; }
         /* For deterministic results, ensure f read a bigger buffer.  */
         if (lseek (fd, 0, SEEK_CUR) == 5)
           { fclose (f); return 3; }
         /* POSIX requires fflush-fseek to set file offset of fd.  This fails
            on BSD systems and on mingw.  */
         if (fflush (f) != 0 || fseek (f, 0, SEEK_CUR) != 0)
           { fclose (f); return 4; }
         if (lseek (fd, 0, SEEK_CUR) != 5)
           { fclose (f); return 5; }
         /* Verify behaviour of fflush after ungetc. See
            <http://www.opengroup.org/austin/aardvark/latest/xshbug3.txt>  */
         /* Verify behaviour of fflush after a backup ungetc.  This fails on
            mingw.  */
         c = fgetc (f);
         ungetc (c, f);
         fflush (f);
         if (fgetc (f) != c)
           { fclose (f); return 6; }
         /* Verify behaviour of fflush after a non-backup ungetc.  This fails
            on glibc 2.8 and on BSD systems.  */
         c = fgetc (f);
         ungetc ('@', f);
         fflush (f);
         if (fgetc (f) != c)
           { fclose (f); return 7; }
         fclose (f);
         return 0;

  ;
  return 0;
}

conftest.txt (text/plain, 13 B)

hello world

signature.asc (application/pgp-signature, 833 B)