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

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).

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