Re: [Libguestfs] [PATCH 4/5] fuse: Fix read for empty files.

On 17/11/09 17:07, Richard W.M. Jones wrote: >> From 3c5d63be17d281634f036f53aa32103ad4ab1c41 Mon Sep 17 00:00:00 2001 > From: Richard Jones<rjones(a)redhat.com> > Date: Tue, 17 Nov 2009 17:02:45 +0000 > Subject: [PATCH 4/5] fuse: Fix read for empty files. > > Error handling for the guestfs_pread call was incorrect, which > meant that empty files could produce spurious error messages. > --- > fuse/guestmount.c | 8 +++++++- > 1 files changed, 7 insertions(+), 1 deletions(-) > > diff --git a/fuse/guestmount.c b/fuse/guestmount.c > index 9d16cef..669bf80 100644 > --- a/fuse/guestmount.c > +++ b/fuse/guestmount.c > @@ -621,8 +621,14 @@ fg_read (const char *path, char *buf, size_t size, off_t offset, > if (size> limit) > size = limit; > > + /* Note the correct error handling here is tricky, because in the > + * case where the call returns a zero-length buffer, it might return > + * NULL. However it won't adjust rsize along the error path, so we > + * can set rsize to something beforehand and use that as a flag. > + */ > + rsize = 1; > r = guestfs_pread (g, path, size, offset,&rsize); > - if (r == NULL) > + if (rsize == 1&& r == NULL) > return error (); > > /* This should never happen, but at least it stops us overflowing > -- 1.6.5.2 I've stared at this til my head hurts, and I don't get it. Why is 1 not a valid returned size from guestfs_pread? Why is NULL not an indicator of failure?

> + if (rsize == 1&& r == NULL)

The documentation says: This function returns a buffer, or NULL on error. The size of the returned buffer is written to *size_r. The caller must free the returned buffer after use. If NULL isn't sufficient to indicate error, guestfs_pread needs to be fixed, not this funtion.