Messages in this thread | | | Subject | Re: [PATCH] TRACING: Fix a copmile warning | From | Steven Rostedt <> | Date | Mon, 25 Jul 2011 20:08:54 -0400 |
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On Mon, 2011-07-25 at 19:58 -0400, Arnaud Lacombe wrote: > Hi,
> > [...] > > In which case the warning is fully valid. I'm not sure what's the C > > standard guarantee in term of conditional test order. > I'd assume that the following apply: > > 6.5 Expressions > > 3 The grouping of operators and operands is indicated by the > syntax.72) Except as specified > later (for the function-call (), &&, ||, ?:, and comma operators), > the order of evaluation > of subexpressions and the order in which side effects take place are > both unspecified. > > > in which case gcc is free to do whatever it wants :(
No it does not! Read what you wrote: "Except as specified later (for the function-call(), &&, ||, ?:...)"
&& and || must be short cuts. That is, it must evaluate the earlier statements before the later, and exit when it can. We use that all over the kernel (and in all C code):
if (ptr && ptr->field)
If it were to switch that to:
if (ptr->field && ptr)
we would have segfaults everywhere.
This looks like a serious gcc bug.
-- Steve
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