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Date: Sun, 30 Nov 2014 12:38:46 +0100
From: Szabolcs Nagy <>
Subject: Re: __sched_cpucount returns garbage

* Isaac Dunham <> [2014-11-29 15:36:33 -0800]:
> I noticed that nproc ended up on the toybox TODO list (via Tizen), and went
> poking about via strace and ltrace to see where it got the cpu count from.
> In the process, I discovered that __sched_cpucount is returning garbage;

works here as expected:

#define _GNU_SOURCE
#include <sched.h>
int main()
	cpu_set_t s = {0};
	CPU_SET(3, &s);
	CPU_SET(7, &s);
	CPU_SET(24, &s);
	return __sched_cpucount(sizeof s, &s);

returns 3

> on Alpine Linux on my N270-based netbook (1 physical core but 
> hyperthreading makes it look like 2),
> nproc
> outputs a random number of CPUs ranging from 413 to 472.

see where the cpu_set_t argument comes from
(most likely sched_getaffinity syscall)
then see why that is broken

__sched_cpucount just counts bit flags

> ltrace indicates that this is calling __sched_cpucount() and printing 
> its return value.
> nproc --all
> calls sysconf(_SC_NPROCESSORS_CONF) and gets the proper number of CPUs.
> Thanks,
> Isaac Dunham

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