On Wed, 2013-10-16 at 12:49 +0200, Markus Armbruster wrote:
> Ian Campbell <ian.campb...@citrix.com> writes:
>
> > On Wed, 2013-10-16 at 10:54 +0100, Jan Beulich wrote:
> >> >>> On 16.10.13 at 08:30, "Gonglei (Arei)" <arei.gong...@huawei.com> wrote:
> >> > --- hvmloader/acpi/Makefile 2013-10-16 11:51:53.000000000 +0800
> >> > +++ hvmloader_new//acpi/Makefile 2013-10-16 11:51:58.000000000 +0800
> >> > @@ -36,18 +36,34 @@
> >> >
> >> > dsdt_anycpu_qemu_xen.asl: dsdt.asl mk_dsdt
> >> > awk 'NR > 1 {print s} {s=$$0}' $< > $@
> >> > - ./mk_dsdt --dm-version qemu-xen >> $@
> >> > + sed -i 's/AmlCode/dsdt_anycpu_qemu_xen/g' $@
> >>
> >> This must never be done - if someone hits Ctrl-C in the middle of
> >> this, you'll have a modified but incomplete generated file. You
> >> either need to use properly chained rules, or do all output to a
> >> temporary file which you rename as the last step.
> >>
> >> I realize that the problem existed before your change, but you
> >> making it worse requires doing it properly now.
> >
> > The correct way to do this is to generate to a temporary file and then
> > use $(call move-if-changed,$@.tmp,$@). There are a bunch of examples in
> > tools/libxl/Makefile and elsewhere.
>
> Quoting the GNU make manual[*]:
>
> So generally the right thing to do is to delete the target file if
> the recipe fails after beginning to change the file. make will do
> this if .DELETE_ON_ERROR appears as a target. This is almost always
> what you want make to do, but it is not historical practice; so for
> compatibility, you must explicitly request it.
>
> In my opinion, every Makefile should request it.
>
> [*] https://www.gnu.org/software/make/manual/html_node/Errors.html
I think this is somewhat orthogonal to the use of move-if-changed which
is there to ensure that the timestamp doesn't change gratuitously and
cause a load of knock on rebuilds even though the regenerated content is
identical.
Ian.
