[Libguestfs] [PATCH nbdkit 1/2] include: Fix NBDKIT_HANDLE_NOT_NEEDED for C90 compilers.

[Richard W.M. Jones]

On Fri, Jan 18, 2019 at 08:36:07AM -0600, Eric Blake wrote:
> On 1/14/19 6:15 AM, Richard W.M. Jones wrote:
> > When an ANSI/C90 plugin compiled with ‘-pedantic’ uses
> > NBDKIT_HANDLE_NOT_NEEDED it gets the error:
> > 
> >   ISO C forbids conversion of function pointer to object pointer type
> 
> While POSIX requires it to work. But such is life.
> 
> > 
> > This is because the existing macro worked by returning a function
> > pointer but in C90 function pointers cannot be cast to data pointers
> > since on some ancient architectures code and data pointers were
> > incompatible.
> > 
> > We only need a convenient global data pointer here, and the address of
> > ‘errno’ should be fine.
> 
> errno is often implemented as a dereference of a function call in order
> to get thread-safe semantics; for example, your patch produces
> '&(*__errno_location())' on glibc.  That should be okay (especially
> since it silenced the warning).

Yeah, I was aware of this and looking for some other global variable
but couldn't think of one at the time.

Hmm, how about optind?

Rich.

> Another option, instead of referencing an actual variable, could be
> writing ((void*)(intptr_t)1).
> 
> Either way, this patch makes sense.
> 
> > @@ -78,7 +79,7 @@ extern char *nbdkit_realpath (const char *path);
> >  /* A static non-NULL pointer which can be used when you don't need a
> >   * per-connection handle.
> >   */
> > -#define NBDKIT_HANDLE_NOT_NEEDED ((void *) &nbdkit_error)
> > +#define NBDKIT_HANDLE_NOT_NEEDED (&errno)
> >  
> >  #ifdef __cplusplus
> >  }
> > 
> 
> -- 
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3226
> Virtualization:  qemu.org | libvirt.org
> 




-- 
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