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

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.