Question on patten matching and GNUmake

[Praveen Jain]

I intend to do:

gmake sup/libbmp.so
gmake lc/libbmp.so


1) Following works for me:

1.1) a global rule:

%.so: 
    @mkdir -p $(@D)
    $(LD) $(LIBLDOPTS) $^ -o $@

1.2) In makefile I do:

sup/libbmp.so: sup/a.o sup/b.o
lc/libbmp.so: lc/a.o lc/b.o


2)  I've few targets (such as sup, lc) but too many libs in various 
directories. So to reduce permutations, I tried to do following instead

2.1) global rule:
sup/%.so lc/%.so: 
    @mkdir -p $(@D)
    $(LD) $(LIBLDOPTS) $^ -o $@

2.2) In makefile now I do:

%/libbmp.so: %/a.o %/b.o

when I do gmake sup/libbmp.so or lc/libbmp.so, it tells me target is undefined.

3) If I modify as follows:

a global rule: same as (2.1)

3.1) In makefile, I do:

%/libbmp.so: %/a.o %/b.o ;   # i.e. add empty command

while I don't get target undefined, now make merges (2.1) and (3.1) and 
resultant command becomes empty.


Bottom line, looks to me using % pattern matching as in (2.2) requires a 
command. But providing empty (or any other) command causes merge with global 
rule in-effective.

A rule which has no patten matching doesn't require a command to be present.

Any suggestions/pointers will be highly appreciated. I'm using 3.80 version of 
GNU make.

Thanks
Praveen