Messages in this thread | | | Date | Wed, 27 Nov 2013 16:35:19 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 08/14] sched: add latency tracing for -deadline tasks. |
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* Peter Zijlstra <peterz@infradead.org> wrote:
> On Wed, Nov 27, 2013 at 03:34:35PM +0100, Ingo Molnar wrote: > > > > * Peter Zijlstra <peterz@infradead.org> wrote: > > > > > On Wed, Nov 27, 2013 at 09:16:47AM -0500, Steven Rostedt wrote: > > > > On Wed, 27 Nov 2013 14:43:45 +0100 > > > > Juri Lelli <juri.lelli@gmail.com> wrote: > > > > diff --git a/kernel/trace/trace_selftest.c b/kernel/trace/trace_selftest.c > > > > > index f76f8d6..ad94604 100644 > > > > > --- a/kernel/trace/trace_selftest.c > > > > > +++ b/kernel/trace/trace_selftest.c > > > > > @@ -1023,16 +1023,16 @@ trace_selftest_startup_nop(struct tracer *trace, struct trace_array *tr) > > > > > static int trace_wakeup_test_thread(void *data) > > > > > { > > > > > /* Make this a -deadline thread */ > > > > > - struct sched_param2 paramx = { > > > > > + static const struct sched_param2 param = { > > > > > .sched_priority = 0, > > > > > + .sched_flags = 0, > > > > > .sched_runtime = 100000ULL, > > > > > .sched_deadline = 10000000ULL, > > > > > .sched_period = 10000000ULL > > > > > - .sched_flags = 0 > > > > > > > > Assigning structures like this, you don't need to set the zero fields. > > > > all fields not explicitly stated, are set to zero. > > > > > > Only because its static. Otherwise unnamed members have indeterminate > > > value after initialization. > > > > I think for 'struct' C will initialize them to zero, even if they are > > not mentioned and even if they are on the stack. > > > > It will only be indeterminate when it's not initialized at all. > > Language spec: ISO/IEC 9899:1999 (aka C99) section 6.7.8 point 9 > says: > > 9. Except where explicitly stated otherwise, for the purpose of this > subclause unnamed members of objects of structure and union type do no > participate in initialization. Unnamed members of structure objects have > indeterminate value even after initialization. > > Later points (notably 21) make such an exception for aggregate objects > of static storage. > > Of course, its entirely possible I read the thing wrong; its 31 points > detailing the initialization of objects.
So why does GCC then behave like this:
triton:~> cat test.c
struct foo { int a; int b; };
int main(void) { struct foo x = { .a = 1 };
return x.b; }
triton:~> gcc -Wall -Wextra -O2 -o test test.c; ./test; echo $? 0
I'd expect -Wall -Wextra to warn about as trivial as the uninitialized variable use that you argue happens.
I'd also expect it to not return 0 but some random value on the stack (which is most likely not 0).
Thanks,
Ingo
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