How does gmake parse and execute template definitions - gnu-make

Question

So, I have a make template. I invoke it like:
$(eval $(call PRIVATE_LIBRARY_TEMPLATE,privatelib1,64))
And it is defined like:
define PRIVATE_LIBRARY_TEMPLATE
# Evaluate the condition multiple times because of way make processes templates
$(if $(2)=='',$(eval $(call LIBRARYBUILD_TEMPLATE,$(1),32)))
$(if $(2)=='',$(eval $(call LIBRARYBUILD_TEMPLATE,$(1),64)))
$(if $(2)!='',$(eval $(call LIBRARYBUILD_TEMPLATE,$(1),$(2))))
# More stuff that doesn't matter here
endef
or like:
define PRIVATE_LIBRARY_TEMPLATE
# Evaluate the condition multiple times because of way make processes templates
$(if $(2)=='',$(call LIBRARYBUILD_TEMPLATE,$(1),32))
$(if $(2)=='',$(call LIBRARYBUILD_TEMPLATE,$(1),64))
$(if $(2)!='',$(call LIBRARYBUILD_TEMPLATE,$(1),$(2)))
# More stuff that doesn't matter here
endef
previously it was defined as:
define PRIVATE_LIBRARY_TEMPLATE
$$(eval $$(call LIBRARYBUILD_TEMPLATE,$(1),32))
$$(eval $$(call LIBRARYBUILD_TEMPLATE,$(1),64))
before I add the $(if, $(1) is passed to the LIBRARYBUILD_TEMPLATE intact, but once I add the $(if, $(1) becomes an empty string.
I've tried various combinations of $$ $(eval $$eval etc, but there is something fundamental I'm just not understanding about the way gmake is parsing this template definition.
What I am trying to do is to make $(2) optional in this template, and use it if provided, or if not provided build both 32 and 64 bit libraries.
How is the template definition being initially parsed, and then evaluated.

Answer 1

First off the function $(if condition,then-part,else-part) is not the same as the other conditionals like ifeq. The $(if) function condition is simply checked for empty string or not (c.f. manual):
If it expands to any non-empty string, then the condition is considered to be true. If it expands to an empty string, the condition is considered to be false.
When it comes to $(eval $(call ...), things can become tricky in which order things must be evaluated. I usually think like this:
If the evaluated result of an operation depends on an argument (like $1), then the operation needs to be delayed with secondary expansion.
So in your case I think this is what you want:
.SECONDEXPANSION:
define LIBRARYBUILD_TEMPLATE
$$(info >> LIBRARYBUILD_TEMPLATE: $1 $2)
endef
define PRIVATE_LIBRARY_TEMPLATE
$$(info > PRIVATE_LIBRARY_TEMPLATE $1 $2)
ifeq ($2,)
$$(eval $$(call LIBRARYBUILD_TEMPLATE,$1,32))
$$(eval $$(call LIBRARYBUILD_TEMPLATE,$1,64))
else
$$(eval $$(call LIBRARYBUILD_TEMPLATE,$1,$2))
endif
endef
$(eval $(call PRIVATE_LIBRARY_TEMPLATE,privatelib1,64))
$(eval $(call PRIVATE_LIBRARY_TEMPLATE,privatelib2,))
Which gives the output:
> PRIVATE_LIBRARY_TEMPLATE privatelib1 64
>> LIBRARYBUILD_TEMPLATE: privatelib1 64
> PRIVATE_LIBRARY_TEMPLATE privatelib2
>> LIBRARYBUILD_TEMPLATE: privatelib2 32
>> LIBRARYBUILD_TEMPLATE: privatelib2 64

Answer 2

Let's use this definition of PRIVATE_LIBRARY_TEMPLATE
define PRIVATE_LIBRARY_TEMPLATE
# Evaluate the condition multiple times because of way make processes templates
$(if $(2)=='',$(eval $(call LIBRARYBUILD_TEMPLATE,$(1),32)))
$(if $(2)=='',$(eval $(call LIBRARYBUILD_TEMPLATE,$(1),64)))
$(if $(2)!='',$(eval $(call LIBRARYBUILD_TEMPLATE,$(1),$(2))))
endef
It is instructive to look in detail at what make does when it encounters
$(eval $(call PRIVATE_LIBRARY_TEMPLATE,privatelib1,64))
Clearly, before it can expand the $eval, make must first expand the $call:
1 is set to privatelib1
2 is set to 64
PRIVATE_LIBRARY_TEMPLATE is now expanded.
First off, a $(if …) needs expanding:
Make looks at the condition in $(if $(2)=='',$(call LIBRARYBUILD_TEMPLATE,$(1),32)), and so expands $(2)==''. You will note that 64=='' not an empty string, and so is considered true. To complete the expansion of the $(if …) , make thus chooses the true branch and goes on to expand $(call LIBRARYBUILD_TEMPLATE,$(1),32)
1 becomes privatelib1
2 becomes 32
$(call LIBRARYBUILD_TEMPLATE,privatelib1,32) becomes some text. No idea what, but since it will eventually be passed to $eval it must be valid make syntax. Let's assume it is something simple like LIB_privatelib1_32 := 1.
A second $(if …) is similarly expanded.
A third $(if …) is similarly expanded.
For the sake of argument, let's say the final expansion of the $(call PRIVATE_LIBRARY_TEMPLATE,…) is this text:
LIB_privatelib1_32 := 1
LIB_privatelib1_64 := 1
LIB_privatelib1_64 := 1
These three lines are passed to $eval.
As a side-effect, three new simple variables are defined.
The expansion of the $(eval …) though is empty.
Phew.
One obvious mistake here is that == is not valid make syntax.
Truthyness is merely whether a string has characters in it.
You probably want something like:
$(if $2,$(eval $(call LIBRARYBUILD_TEMPLATE,$1,32)))
$(if $2,$(eval $(call LIBRARYBUILD_TEMPLATE,$1,64)))
$(if $2,,$(eval $(call LIBRARYBUILD_TEMPLATE,$1,$2)))
(check out that last one for negations.)