Re: only run submake if that submake needs to be run?

[David Boyce]

At 01:53 PM 8/20/2004, David Wuertele wrote:
> The reason I chose recursive in the first place was for a couple of
> reasons:
>
> 1.  I want to be able to run make in the sub-directory and have it
>     work
>
> 2.  I want to use the same variable namespace for all the makefiles.
>     For example, I don't want one of my users to have to create
>     variable names like:
>
>       my_program_OBJ := this.o that.o
>
>     I want them to be able to write:
>
>       OBJ := this.o that.o
>
>     This is an oversimplified example for the purposes of making the
>     point.  I want them to be able to copy their neighbor's Makefile
>     and not have to change all the variable names.
>
> Is there a way to write makefiles in a non-recursive fashion that will
> enable me to acheive both goals?

I've had success with a circular-include design. E.g. there's a Makefile 
per directory, just like in the recursive model. Each Makefile includes the 
"base makefile", and the base makefile in turn includes all subtree 
Makefiles (this takes a bit of macro/ifdef logic to prevent infinite 
include loops). The net result is a single logical Makefile accessible via 
a different physical "entry point" in each subdir.

This preserves two of the major benefits of the recursive model: (1) the 
user can simply type "make" in any directory (as opposed to "make -f 
../../../Makefile") and (2) a developer adding a new source file need only 
modify the Makefile in the same dir, which usually is just a list of object 
files, without having to delve into the arcanities of the full build system.

Since slashes are a legal part of a variable name in make, I tend to prefix 
local variables with the subdir path, e.g. "my/sub/dir_OBJS=obj1.o obj2.o". 
At the time I did this it had to be compatible with GNU make 3.75 or so; 
it's quite possible that newer versions offer help with generating unique 
names, similar to what Noel Yap mentioned.

Though the ability of a non-recursive system to detect and deal with 
out-of-dateness in other parts of the tree is generally considered a 
feature, a colleague subsequently added a feature such that "make local" 
would build only objects local to the cwd and thus emulate recursive behavior.

-David Boyce