Re: [Qemu-devel] [PATCH 03/10] qemu-ga: qmp_guest_file_*: improve error reporting

[Eric Blake <eblake@redhat.com>]

> > >      if (ferror(fh)) {
> > > +        error_setg_errno(err, errno, "failed to read file");
> > >          slog("guest-file-read failed, handle: %ld", handle);
> > > -        error_set(err, QERR_QGA_COMMAND_FAILED, "fread()
> > > failed");
> > >      } else {
> > 
> > I'm not sure about relying on errno for FILE/f*() functions. C99
> > doesn't
> > appear to require setting it for implementations

Correct that C99 doesn't require it, but POSIX _does_ require it.

Windows is the biggest system out there where errno is unreliable after
failure on FILE operations (but as we DO support mingw, we ARE impacted
by the lameness of Microsoft's C library being C89 but not POSIX).

> However, the other FILE functions seem safe to me. I'd be very
> surprised
> if some implementation doesn't set errno on fopen() failure for
> example

Then you probably haven't experimented much with mingw :)