Re: How does $@ work in a make conditional?

[Krzysztof Cieniuch]

JeffS wrote:
> Hi!  I'm using GNU Make 3.80.  In my Makefile, I use automatic 
> variable $@  to refer to the current target, as shown below.
>
>        |     @echo current target is ... address@hidden
>        ifeq ($@,sms)
>             @echo yep, they are equal
>        else
>             @echo no, they are not equal
>        endif
>        |
>
> It seems that $@  expands to sms , as shown in the output below.
>
> Output is:
>
>        |current target is ... [sms]
>        no, they are not equal
>        |
>
> My question: since $@  (apparently) expands to sms , shouldn't the 
> "true" branch of the ifeq  conditional be executed (with the 
> consequence that the output should read yep, they are equal)? [I am at 
> a loss as to why the output is no, they are not equal.]
>
> Thank you for any advice you can provide.
The ifeq are evaluated during makefile parsing step they control what 
make 'sees' in the makefile.  $@ (and all automatic variables)
is only available in command script  and commands evaluation is deferred 
until second phase (that is not 100% correct see secondary expansion).
The analogy (not exact) is as  you would expect C preprocesor macros 
being evaluated during runtime.

If you do not understand what I'm talking about :-) see following 
chapters of make manual:
3.9 How make Reads a Makefile
7. Conditional Parts of Makefile (first paragraph exactly explains why 
it doesn't work)
10.5.3 Automatic Variables

Anyways to achieve what you want use if function instead i.e.:

comma=,
all: sms nosms
sms:
   @echo current target is ... address@hidden
   @echo $(if $(filter-out xx,x$(subst $@,,sms)$(subst 
sms,,$@)x),no$(comma) they are not equal,yep$(comma) they are equal)
nosms:
   @echo current target is ... address@hidden
   @echo $(if $(filter-out xx,x$(subst $@,,sms)$(subst 
sms,,$@)x),no$(comma) they are not equal,yep$(comma) they are equal)

note commas are argument separarors so thats why I had to escape them 
with variable (another useful trick :-)

Thanks
Chris