[PATCH v6 02/11] stdio-common: Introduce buffers for implementing printf

[Noah Goldstein]

On Fri, Dec 16, 2022 at 2:32 PM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Noah Goldstein:
>
> > On Fri, Dec 16, 2022 at 12:59 PM Florian Weimer <fweimer@redhat.com> wrote:
> >>
> >> * Noah Goldstein:
> >>
> >> >> +/* Switch to the file buffer if possible.  If the file has write_ptr
> >> >> +   == write_end, use the stage buffer instead.  */
> >> >> +void
> >> >> +__printf_buffer_to_file_switch (struct __printf_buffer_to_file *buf)
> >> >> +{
> >> >> +  if (buf->fp->_IO_write_ptr < buf->fp->_IO_write_end)
> >> >> +    {
> >> >
> >> > Think maybe this should have a minimum bound so we don't end up
> >> > in a situation where the strnlen / memcpy loop is only doing 1/2
> >> > bytes at a time.
> >> > Probably something like 32/64 would make sense.
> >>
> >> I think this would only matter with setvbuf and a small buffer.  We can
> >> use half of the staging buffer size (PRINTF_BUFFER_SIZE_TO_FILE_STAGE),
> >> that would be your 64 number.
> >
> > I don't follow, is it impossible for `_IO_write_ptr < _IO_write_end` and for
> > the buffer to be less than 64?
>
> Typical buffers are 8192 bytes large, I believe, but when printf is
> called, an arbitrary amount can already be consumed.
>
> > Otherwise I it seems `write_ptr` will be set to `_IO_write_ptr` and
> > `write_end` to `_IO_write_end` so the loop:
> > ```
> > +      if (buf->write_ptr == buf->write_end && !Xprintf_buffer_flush (buf))
> > +        return;
> > +      assert (buf->write_ptr != buf->write_end);
> > +      size_t to_copy = STRNLEN (s, buf->write_end - buf->write_ptr);
> > +      MEMCPY (buf->write_ptr, s, to_copy);
> > +      buf->write_ptr += to_copy;
> > +      s += to_copy;
> > ```
> >
> > Will be doing 1/2 bytes at a time.
>
> There are two buffers here: the staging buffer, and the (usually larger,
> but optional) FILE * buffer.  If there is no FILE * buffer (yet), we
> must use the staging buffer.  But we can instruct printf to write
> directly to the FILE * buffer if it is there.  Once the buffer is full,
> we need to use the staging buffer for a bit, but flushing that into the
> FILE * buffer will most likely make the FILE * buffer available again.

Si,

I guess my point is maybe better to use the staging buffer -> IO buffer
if there is only minimal room left in the FILE * buffer.

But thinking more on it, it doesn't really make sense.
>
> The core issue here is that there is no real way to flush the FILE *
> once it's full.  We always have to write a character:
>
>       /* The buffer in buf->fp has been filled.  This should just call
>          __overflow (buf->fp, EOF), but flush-only overflow is obscure
>          and not always correctly implemented.  See bug 28949.  Be
>          conservative and switch to a one-character buffer instead, to
>          obtain one more character for a regular __overflow call.  */
>
> This is basically a backwards-compatibility kludge for certain C++
> programs compiled with GCC 2.95.
>
> Thanks,
> Florian
>