As far as I can tell, syscall() is supposed to set errno (using
__sycall_ret), but maybe src/misc/syscall.c isn't what I think it is?
I indeed got the return value backwards, and I'll fix that, along with the
#ifdef.

On Mon, Feb 29, 2016 at 6:23 PM Alexander Monakov <amonakov@ispras.ru>
wrote:

> On Mon, 29 Feb 2016, Nathan Zadoks wrote:
>
> > This is a GNU extension, but a fairly minor one, for a system call that
> > otherwise has no libc wrapper.
> >
> > Adding it was discussed previously, without any objections:
> > http://www.openwall.com/lists/musl/2015/05/08/24
> > ---
> >  include/sched.h          |  3 +++
> >  src/sched/sched_getcpu.c | 10 ++++++++++
> >  2 files changed, 13 insertions(+)
> >  create mode 100644 src/sched/sched_getcpu.c
> >
> > diff --git a/include/sched.h b/include/sched.h
> > index 3e34a72..17f5e06 100644
> > --- a/include/sched.h
> > +++ b/include/sched.h
> > @@ -76,6 +76,9 @@ void free(void *);
> >
> >  typedef struct cpu_set_t { unsigned long __bits[128/sizeof(long)]; }
> cpu_set_t;
> >  int __sched_cpucount(size_t, const cpu_set_t *);
> > +#ifdef _GNU_SOURCE
>
> This code is already under the same #ifdef.
>
> > +int sched_getcpu(void);
> > +#endif
> >  int sched_getaffinity(pid_t, size_t, cpu_set_t *);
> >  int sched_setaffinity(pid_t, size_t, const cpu_set_t *);
> >
> > diff --git a/src/sched/sched_getcpu.c b/src/sched/sched_getcpu.c
> > new file mode 100644
> > index 0000000..070d6e7
> > --- /dev/null
> > +++ b/src/sched/sched_getcpu.c
> > @@ -0,0 +1,10 @@
> > +#define _GNU_SOURCE
> > +#include <stdlib.h>
>
> Do you need this include?
>
> > +#include <sched.h>
>
> (this include could also be dropped; I think it's a matter of policy
> whether
> such includes are desirable or not, so please wait for comment from Rich)
>
> > +#include "syscall.h"
> > +
> > +int sched_getcpu(void) {
> > +  int c, s;
> > +  s = syscall(SYS_getcpu, &c, NULL, NULL);
> > +  return (s == 0) ? c : s;
>
> This is wrong, as it doesn't set errno on error, and does not produce -1.
> This
> should be something like 'return s ? __syscall_ret(s) : c;' or maybe
> 'return __syscall_ret(s ? s : c);'.
>
> Alexander
>