On Tue, Oct 24, 2017 at 01:39:03AM +0100, Mark Thompson wrote: > On 24/10/17 00:52, Michael Niedermayer wrote: > > Hi > > > > On Mon, Oct 23, 2017 at 04:43:19PM +0100, Mark Thompson wrote: > >> On 23/10/17 03:56, Michael Niedermayer wrote: > >>> the initialization should be thread safe as it never writes a different > >>> value in the same spot > >> > >> This is not true; please be very careful with assumptions like this. > >> > >> The C standard calls this a data race and it is undefined behaviour. > > > > Thats interresting, we definitly do not want undefined behavior > > can you quote the relevant parts of the C standard > > which make writing the same value, a data race ? > > > > I was not aware of this if its true. > > Also i dont immedeatly see this in the "latest" draft i am looking at > > atm > > > > What i see is this: > > 4 Two expression evaluations conflict if one of them modifies a memory > > location and the > > other one reads or modifies the same memory location. > > ... > > 25 The execution of a program contains a data race if it contains two > > conflicting actions in > > different threads, at least one of which is not atomic, and neither > > happens before the > > other. Any such data race results in undefined behavior. > > > > This seems carefully worded to not include writing the same value. > > As "modification" implies change which does not occur when writing the > > same value. > > All writes are modifications. > > See section 3.1 note 2: > """ > 3 NOTE 2 "Modify" includes the case where the new value being stored is > the same as the previous value. > """
That resolves the difference between our interpretation thanks [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB Elect your leaders based on what they did after the last election, not based on what they say before an election.
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