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

[mdroth <mdroth@linux.vnet.ibm.com>]

On Wed, Nov 28, 2012 at 04:26:29PM -0500, Eric Blake wrote:
> 
> > > >      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).

Well, if it's primarilly an issue with windows, then I think we're okay
relying on it for anything in qga/commands-posix.c at least, as those
implementations get swapped out for the implementations in
qga/commands-win32.o for win32/mingw builds. We'll need to be wary of
this in the future if we end up sharing more code with the win32 port
becomes more feature-full though.

The comment elsewhere about setmntent() might still apply however.

> 
> > 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 :)
>