Hi,
On Fri, 31 Oct 2008, Shawn O. Pearce wrote:
> Nicolas Pitre <nico@xxxxxxx> wrote:
> > On Fri, 31 Oct 2008, Shawn O. Pearce wrote:
> >
> > > > Both the negative code and errno style are lightweight in the
> > > > common "no error" case. The errno style is probably more handy
> > > > for those functions returning a pointer which should be NULL in
> > > > the error case.
Unfortunately, errno would not be thread-safe, unless you can guarantee
that errno is a thread-local variable.
> > >
> > > I'm sticking with return a negative code for now, to the extent that
> > > some functions which return a pointer but also have many common
> > > failure modes (e.g. git_odb_open) use an output parameter as their
> > > first arg, so the error code can be returned as the result of the
> > > function.
> >
> > Actually, the pointer-returning functions can encode error cases into a
> > "negative" pointer. See include/linux/err.h for example.
> >
> > void *ptr = git_alloc_foo(...);
> > if (IS_ERR(ptr))
> > die("git_alloc_foo failed: %s", git_strerr(PTR_ERR(ptr)));
>
> Oh, good point. We could also stagger the errors so they are
> always odd, and never return an odd-alignment pointer from a
> successful function. Thus IS_ERR can be written as:
>
> #define IS_ERR(ptr) (((intptr_t)(ptr)) & 1)
>
> which is quite cheap, and given the (probably required anyway)
> aligned allocation policy means we still have 2^31 possible
> error codes available.
Oh boy, both solutions are ugly as hell. Although the &1 method does not
limit the memory space as much (except if you plan to work in
space-contrained environments, where you do not want to be forced to
word-align structs).
The only pointer game I would remotely consider clean is if you had
const char *errors[] = {
...
};
inline int is_error(void *ptr) {
return ptr >= errors && ptr < errors + ARRAY_SIZE(errors);
}
although that would be less performant.
Ciao,
Dscho
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