ideas towards requiring VPATH build

ideas towards requiring VPATH build

[Eric Blake]

I know we've talked about enforcing a VPATH build, but haven't yet 
flipped the switch.  This week, I've played with using a VPATH build (cd 
qemu; mkdir -p build; cd ./build; ../configure ...; make ...), but find 
my old habits of expecting an in-tree build to just work (cd qemu; make 
...) hard to overcome.  So this is what I've come up with: if you place 
the following file in-tree, then any 'make ...' command you type in-tree 
without using -C will have the same effect as if you had typed the same 
command in the build directory, but without having to manually remember 
to switch to the build directory.

Perhaps this can be a starting point for a patch to actually include 
this file in qemu.git as part of the larger effort to force VPATH 
builds, while still having the convenience of in-tree make working for 
those who were used to it.  (I places an echo and sleep in my file to 
remind myself when I forgot to use the build directory, but that is not 
mandatory if we want GNUmakefile stored in qemu.git).

Presumably, any full switch to force a VPATH build would also include 
creating the build directory as needed (my hack assumes that it already 
exists).

$ cat GNUmakefile
# Hack for redirecting while reminding myself to use distinct builddir
%: force
	@echo 'changing directory to build...'
	@sleep 2
	@$(MAKE) -C build -f Makefile $(MAKECMDGOALS)
force: ;
GNUmakefile: ;

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

Re: ideas towards requiring VPATH build

[Eric Blake]

On 10/11/19 4:28 PM, Eric Blake wrote:
> I know we've talked about enforcing a VPATH build, but haven't yet 
> flipped the switch.  This week, I've played with using a VPATH build (cd 
> qemu; mkdir -p build; cd ./build; ../configure ...; make ...), but find 
> my old habits of expecting an in-tree build to just work (cd qemu; make 
> ...) hard to overcome.  So this is what I've come up with: if you place 
> the following file in-tree, then any 'make ...' command you type in-tree 
> without using -C will have the same effect as if you had typed the same 
> command in the build directory, but without having to manually remember 
> to switch to the build directory.
> 
> Perhaps this can be a starting point for a patch to actually include 
> this file in qemu.git as part of the larger effort to force VPATH 
> builds, while still having the convenience of in-tree make working for 
> those who were used to it.  (I places an echo and sleep in my file to 
> remind myself when I forgot to use the build directory, but that is not 
> mandatory if we want GNUmakefile stored in qemu.git).
> 
> Presumably, any full switch to force a VPATH build would also include 
> creating the build directory as needed (my hack assumes that it already 
> exists).
> 
> $ cat GNUmakefile
> # Hack for redirecting while reminding myself to use distinct builddir
> %: force
>      @echo 'changing directory to build...'
>      @sleep 2
>      @$(MAKE) -C build -f Makefile $(MAKECMDGOALS)
> force: ;
> GNUmakefile: ;

Works for 'make all' or 'make check', but doesn't quite work for 'make'. 
  For that, I had to add:

ifeq ($(MAKECMDGOALS),)
recurse: all
endif

prior to the %: line (the name 'recurse' is not special, it merely has 
to be something unlikely to be in the actual Makefile, and appear as the 
first rule with a dependency on the name of the real first rule in 
Makefile, so that when make is invoked without a target, we still end up 
invoking the actual Makefile rather than our GNUmakefile shim claiming 
that 'force' has nothing to do).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org