Re: libcruft/sysconf_cpus.c patch

Hello,

On Tue, November 23, 2010 04:41, Felix von Leitner wrote:
>> __sc_nr_cpus() [ == sysconf(_SC_NPROCESSORS_ONLN) ] can run off the end
>> of its /proc/cpuid buffer and (depending on process memory layout) dump
>> core.  This caused an intermittent dump in diet `x86info`.
>
> Thanks, added patch.
>
>> This will still get a wrong answer if "processor" or one of the
>> other-arch strings happens to get cut by the buffer end.  Not important
>> for my application.  I would suggest a malloc'd buffer the full stat'd
>> size of /proc/cpuid, but that seems a bit scary with 160-core x86 boxes
>> running around out there this year...
>
> The problem is that stat always returns size 0.
> So you'd have to have a loop and try bigger and bigger sizes.  That's
> too much effort to get the number of CPUs.  I'm sure there must be some
> smarter way to do this than parsing /proc/cpuinfo.
>
> Maybe we should use /proc/stat.  There is no sysctl for this,
> unfortunately, and it's not in the ELF auxvec.  Too bad, really.
> I'd prefer a way that works if /proc if not mounted.

You can use sched_getaffinity() to figure out the number of cpus.
Just count the number of bits set. Advantage of this is that it
gives the number of useable cpus, not the theoretical maximum.
Disadvantage is that it only works for Linux 2.6.

Greetings,

Indan