Recursive make using a variable with parallel builds

[Paul D. Smith]

William S Fulton writes:
 > The testcase below is a demonstration of a problem I've got with a much 
 > larger build system with which I would like to start using parallel 
 > builds. In the example below, the ok and the bad targets are essentially 
 > the same, but the bad target uses a variable to invoke make. Why does 
 > this variable cause a warning as shown below and how can I fix this? I'm 
 > trying to avoid a huge rewrite by keeping the variables. The manual 
 > indicates that $(MAKE) should be prefixed with a +, but this just causes 
 > make to error out with:
 > 
 > /bin/sh: line 1: +make: command not found

The manual says that the command script line should be prefixed with
"+", not that the $(MAKE) variable needs to be so prefixed.  The "+"
is a special character for make, just like "@" (don't print).  All
make special characters MUST come at the beginning of the command
script; make will NOT go searching through the entire text looking for
a "+" or a "@" (that would be very bad).

So, if you have a rule like this:

 > common_stuff = @echo bad $*; $(MAKE) $*.xtest

 > %.bad:
 >          $(common_stuff);

you have to use:

  common_stuff = address@hidden bad $*; $(MAKE) $*.xtest

or else:

 %.bad:
          +$(common_stuff);

(why do you have a semicolon here?  that will just slow make down)

The reason you need this in this case but not the other case is that
if the variable $(MAKE) appears DIRECTLY in the rule text, then make
can figure out for itself that the command invokes a sub-make.

If you hide the $(MAKE) variable in another variable, as above, then
make can't figure it out and you have to use "+" to tell it what
you're doing.

-- 
-------------------------------------------------------------------------------
 Paul D. Smith <address@hidden>          Find some GNU make tips at:
 http://www.gnu.org                      http://make.paulandlesley.org
 "Please remain calm...I may be mad, but I am a professional." --Mad Scientist