Re: exception to Paul's Second Rule?

[Robert Mecklenburg]

> From: "Noel Yap" <address@hidden>
> Robert Mecklenburg wrote:
> > %.o: %.c %.mkdir
> >
> > So I was thinking the target would be, say, "out/bar.o".  Now, with the
> > "slow" version it has prereq of "out/bar.mkdir" (In the slow version we
omit
> > the .. path component.)  Then the command script executes:
> >
> >     @mkdir -p $(dir out/bar.mkdir)
> >     @touch out/bar.mkdir
>
> I see.  Doing this would create one directory per object file

I must be off today, I don't understand.  This would execute mkdir for each
object file, but it would not create a new directory each time (or did I
miss something?):

    @mkdir -p $(dir out/bar.mkdir)
=>
    @mkdir -p out

If many .o files live in out, then you would still get only one directory.
You _would_ get many zero-length timestamp files out/bar.mkdir,
out/foo.mkdir, etc.

> and would still have a race condition when using --jobs.

Again, I'm not sure I understand you.  Directory creation on unix is atomic.
There can be no race conditions with mkdir.  There can be with the execution
of the touch, but that is non-critical and there would be no ill effects of
having a second touch "beat" the first touch.

What did I miss?

Thanks,
Robert