How can I pass and return a value from user defined function in MAKEFILE?

Question

I would like create a function that takes a list as an argument, performs some operation and returns a list(or a scalar value). I would like to define the function using 'define'. The function:

1) Performs an operation on the input list
2) Checks if the resultant list is empty, if the list is empty raise an error. 3) Otherwise return the resultant list

This is easily possible in languages like C/C++. I am facing issues when I try to do this in MAKEFILE.

a) Can you point me to examples or post an example here?.
b) How does the MAKEFILE function returns value?

I checked the makefile documentation and few other places on the web but could not find anything useful. This would give me an idea on starting with functions. Thank you for your help!

define myfuntest
fnames := $(filter %pattern, $(1))
ifneq ($(fnames),)
$(error this is an error)
endif
endef

Caller function is something like:

abc := Documents Downloads
return_value := $(call mytestfun,$(abc))

I want 'fnames' to be returned in 'return_value'

Answer 1

User-defined macros must be a single "expression". The returned value is the result of expanding the expression. You definitely cannot use ifneq or variable assignments or other similar things in a user-defined macro.

You can create a makefile piece which is used alongside call, but it can only be used with eval, and that means it's a separate section of makefile, not really a "function" as normally defined.

So, if you can construct your user-defined macro with just make functions such that the result of the expansion is the result you want then you can do it as a macro; for example:

myfuntest = $(or $(filter %pattern,$(1)),$(error this is an error))

results := $(call myfuntest,foo barpattern biz baz)

If the result of the filter will either be a list of matching words and that will be assigned to results, or else it will run the error function.

However, if your function is more complex and cannot be expressed in an expression format, you will have to use eval, and pass in the name of the variable to be assigned, like this:

define myfuntest
  ... compute fnames from $(2) ...
$(1) := $$(fnames)
endef

You must be very careful with $ vs. $$, as always when using eval and call together. You then invoke this like:

$(eval $(call myfuntest,return_value,$(abc)))