Re: libgit2 - a true git library

From: Nicolas Pitre <nico@cam.org>
Date: Nov 1, 2008, 13:50 UTC
Message-ID: <alpine.LFD.2.00.0811010944000.13034@xanadu.home>
In-Reply-To: <20081101110120.GA3819@artemis.corp>

On Sat, 1 Nov 2008, Pierre Habouzit wrote:

Show 9 quoted lines

> Well, you can't return _sanely_ an error through a pointer. The &1
> method is broken as soon as you return a char* (there is an alignment
> requirement for malloc, not for any pointer out there), hence shall not
> be used, as it would not be the sole way to test for error.
> 
> Another option, that is _theorically_ not portable, but is ttbomk on all
> the platforms we intend to support (IOW POSIX-ish and windows), is to
> use "small" values of the pointers for errors. [NULL .. (void *)(PAGE_SIZE - 1)[
> cannot exist, which gives us probably always 512 different errors, and

4095 actually.  You don't need to align error codes.

> the test is ((uintptr_t)ptr < (PAGE_SIZE)) which is cheap. It's butt
> ugly, but encoding errors into pointers is butt ugly in the first place.

Or use "negative" pointers. Again, please have a look at include/linux/err.h. The pointer range from 0xffffffff (or 0xffffffffffffffff on 64-bit machines) down to the range you want is for errors, and the top of the address range is almost certain to never be valid in user space either.

Nicolas