On 27.07.2023 11:15, Anthony PERARD wrote: > On Fri, Jun 23, 2023 at 10:07:02AM +0200, Jan Beulich wrote: >> On 22.06.2023 17:30, Anthony PERARD wrote: >>> --- a/xen/Makefile >>> +++ b/xen/Makefile >>> @@ -11,10 +11,10 @@ export XEN_FULLVERSION = >>> $(XEN_VERSION).$(XEN_SUBVERSION)$(XEN_EXTRAVERSION) >>> -include xen-version >>> >>> export XEN_WHOAMI ?= $(USER) >>> -export XEN_DOMAIN ?= $(shell ([ -x /bin/dnsdomainname ] && >>> /bin/dnsdomainname) || ([ -x /bin/domainname ] && /bin/domainname || echo >>> [unknown])) >>> -export XEN_BUILD_DATE ?= $(shell LC_ALL=C date) >>> -export XEN_BUILD_TIME ?= $(shell LC_ALL=C date +%T) >>> -export XEN_BUILD_HOST ?= $(shell hostname) >>> +export XEN_DOMAIN ?= $(eval XEN_DOMAIN := $(shell ([ -x >>> /bin/dnsdomainname ] && /bin/dnsdomainname) || ([ -x /bin/domainname ] && >>> /bin/domainname || echo [unknown])))$(XEN_DOMAIN) >>> +export XEN_BUILD_DATE ?= $(eval XEN_BUILD_DATE := $(shell LC_ALL=C >>> date))$(XEN_BUILD_DATE) >>> +export XEN_BUILD_TIME ?= $(eval XEN_BUILD_TIME := $(shell LC_ALL=C >>> date +%T))$(XEN_BUILD_TIME) >>> +export XEN_BUILD_HOST ?= $(eval XEN_BUILD_HOST := $(shell >>> hostname))$(XEN_BUILD_HOST) >> >> Interesting approach. It looks like it should be independent of make's >> internal workings, but I still wonder: Is this documented somewhere, >> so we won't be caught by surprise of it not working anymore because of >> some change to make's internals? > > So, this is a makefile trick that I've seen on someone's blog post. > > But I tried to find evidence in the GNU make manual if variable expansion is > expected to work like that, and I can't. So I can imagine one day make > doing expansion of variable in parallel, or were the result of the eval > would happen only on the next line. So I don't know if this approach is > always going to work. > > Maybe we could go for a safer alternative: > > Simply replacing ?= by something actually documented in the manual, and > then replacing = by := . > > ifeq ($(origin XEN_BUILD_DATE), undefined) > export XEN_BUILD_DATE := $(shell LC_ALL=C date) > endif > > An alternative, with a macro could be: > > set-shell-if-undef = $(if $(filter undefined,$(origin $(1))),$(eval $(1) > := $(shell $(2)))) > > $(call set-shell-if-undef,XEN_BUILD_DATE,LC_ALL=C date) > export XEN_BUILD_DATE > > But this kind of hide that a shell command is been called. But having > $(shell) as parameter of call $(call) mean the shell command is always > called even when unneeded. > > But then, with the macro, I'm not sure where to put it, to be able to > use it here and in Config.mk for the next patch, another file in > xen.git/config/*.mk ? > > Should I replace all the eval with "ifeq (); endif" ?
I think that would be best (and I prefer that form anyway for being more clear as to what it does). Jan
