Question

I need to export a user-defined GNU make function (i.e. a recursively expanded makefile variable) to a sub-make. However, make seems to expand all such variables into simply expanded variables, which makes them useless.

Here is an example:

Contents of “parent.make”:

export myfunc = my arg is $1
$(info parent testing myfunc: $(call myfunc,parent))
all: ; $(MAKE) -f child.make

Contents of “child.make”:

$(info child testing myfunc: $(call myfunc,child))
nullrule: ; @true

Running “make -f parent.make” produces the following output:

parent testing myfunc: my arg is parent
make -f child.make
child testing myfunc: my arg is 

Parent exports myfunc as a simple expanded variable containing “my arg is” to the sub-make, making it useless as a function.

Answer 1

I’m not sure if this will satisfy your requirements, but here goes:

parent.make:

export myfunc1 = my arg is $1
export myfunc2 = $(value myfunc1)
$(info parent testing myfunc1: $(call myfunc1,parent))
all: ; $(MAKE) -f child.make

child.make:

$(info child testing myfunc2: $(call myfunc2,child))
nullrule: ; @true

EDIT:
All right, how about this:

child.make:

myfunc1=$(myfunc2)

$(info child testing myfunc1: $(call myfunc1,child))
nullrule: ; @true

EDIT:

Not so fast. Take a look at this:

parent.make:

myfunc_base = my arg is $1

export myfunc = $(value myfunc_base)

$(info parent testing myfunc: $(call myfunc_base,parent))
all: ; @$(MAKE) -f child.make

child.make:

$(info child testing myfunc: $(call myfunc,child))
nullrule: ;  @$(MAKE) -f grandchild.make

grandchild.make:

$(info grandchild testing myfunc: $(call myfunc,grandchild))
nullrule: ; @true

This works with no modification at all to child.make or grandchild.make. It does require that parent.make call myfunc_base rather than myfunc; if that’s a problem there’s a way around it, maybe more than one.

One possibility, we move the definition up into whatever calls parent, where it won’t actually be called at all:

grandparent.make:

myfunc_base = my arg is $1

export myfunc = $(value myfunc_base)

all: ; @$(MAKE) -f child.make

parent.make:

$(info parent testing myfunc: $(call myfunc,parent))
all: ; @$(MAKE) -f child.make

I realize this might not be workable, since the definition of myfunc might require other things defined in parent.make. See if this will suit your situation; there may be another way…