Re: some issues with #defines

[Justin Cormack <justin@specialbusservice.com>]

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On 2 Dec 2012 03:26, "Rich Felker" <dalias@aerifal.cx> wrote:
>
> On Sat, Dec 01, 2012 at 11:52:19PM +0000, Justin Cormack wrote:
> > PAGE_SIZE is defined in include/bits/limits.h but I am not sure it
should
> > be. As far as I can see glibc does not define it at all any more fo
> > userspace (see man page for getpagesize).
>
> I'm aware that glibc does not define it because they consider it
> variable. musl considers it constant because increasing it just wastes
> large amounts of memory. The OS is still free to use larger hardware
> pages transparently when possible; all PAGE_SIZE really represents the
> the granularity of mmap.
>
> I've only run into one program which had a problem due to musl
> defining PAGE_SIZE; it was using PAGE_SIZE for its own purposes. This
> usage is non-portable since POSIX defines PAGE_SIZE. Per POSIX, if
> PAGE_SIZE is defined it's a contant, and if it's not defined, it might
> be variable, and the application must use sysconf() to query the page
> size. Portable applications can always use sysconf() if desired, but
> portable applications wanting to be more efficient can conditionally
> use PAGE_SIZE directly if it's defined (allowing a lot of compile-time
> constant propagation optimizations) and only fallback to the slower
> and more bloated runtime calculations on systems that lack the macro.

Thanks for that. It is including it because it is also including kernel
headers which is always a mess.

> > Running into other issues too with includes but suspect they are include
> > bugs and need to look at them further, was looking at compiling native
kvm
> > tool.
>
> There are still lots of issues with very-low-level programs that do
> hackish trace/debugger/jit type stuff not liking musl's headers. I'm
> working on improving the situation in some respects, but I'm not sure
> how successful things will be without some help from their side too.
>

Yes it is basically defining userspace kernel drivers so liable to be
messy. However it only seems to be a few places so will have a look at
patching it. It gives a way to run KVM without qemu and with much less code
(5k lines) so would be nice to have.

Justin

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