Make oddity [longish]

[Ferguson, Neale]

I have discovered something odd about make's behavior. I have the following
test case:

Nat
|-Makefile
+-/src1
  |-Makefile
  |-x11.c
  |-x12.c
+-/src2
  |-Makefile
  |-x21.c
  |-x22.c

The toplevel Makefile contains:

all :
        @echo "Process Nat/src1"
        cd src1;$(MAKE) -$(MAKEFLAGS)
        @echo "Process Nat/src2"
        cd src2;$(MAKE) -$(MAKEFLAGS)

The src1/Makefile:

lib=../nat.a

files = x11.c \
        x12.c

.c~.a: ;
.c.a: ;
.c~.o: ;
.c.o: ;

ALL : $(lib)

$(lib) : $(lib)(x11.o) $(lib)(x12.o)
        ar rv $(lib) $?
        rm -f $?

$(lib)(x11.o) : x11.o
$(lib)(x12.o) : x12.o

x11.o : x11.c
        rm -f x11.o
        $(CC) -c $(CFLAGS) x11.c

x12.o : x12.c
        rm -f x12.o
        $(CC) -c $(CFLAGS) x12.c

The src2/Makefile:

lib=../nat.a

files = x21.c \
        x22.c

.c~.a: ;
.c.a: ;
.c~.o: ;
.c.o: ;

ALL : $(lib)

$(lib) : $(lib)(x21.o) $(lib)(x22.o)
        ar rv $(lib) $?
        rm -f $?

$(lib)(x21.o) : x21.o
$(lib)(x22.o) : x22.o

x21.o : x21.c
        rm -f x21.o
        $(CC) -c $(CFLAGS) x21.c

x22.o : x22.c
        rm -f x22.o
        $(CC) -c $(CFLAGS) x22.c

(The x11.c files etc. contain int x11() { return 0; })

If I run make from the top level I see:

Process Nat/src1
cd src1;make -
make[1]: Entering directory `/FS/fs0300/usanefe/Nat/src1'
rm -f x11.o
cc -c  x11.c
rm -f x12.o
cc -c  x12.c
ar rv ../nat.a x11.o x12.o
a - x11.o
a - x12.o
rm -f x11.o x12.o
make[1]: Leaving directory `/FS/fs0300/usanefe/Nat/src1'
Process Nat/src2
cd src2;make -
make[1]: Entering directory `/FS/fs0300/usanefe/Nat/src2'
ar rv ../nat.a
rm -f
make[1]: Leaving directory `/FS/fs0300/usanefe/Nat/src2'

If I change the lower level makefiles to be:

lib=../nat.a

files = x11.c \
        x12.c

.c~.a: ;
.c.a: ;
.c~.o: ;
.c.o: ;

ALL : dummy

dummy : $(lib)(x11.o) $(lib)(x12.o)
        ar rv $(lib) $?
        rm -f $?

$(lib)(x11.o) : x11.o
$(lib)(x12.o) : x12.o

x11.o : x11.c
        rm -f x11.o
        $(CC) -c $(CFLAGS) x11.c

x12.o : x12.c
        rm -f x12.o
        $(CC) -c $(CFLAGS) x12.c

I then get:

Process Nat/src1
cd src1;make -
make[1]: Entering directory `/FS/fs0300/usanefe/Nat/src1'
rm -f x11.o
cc -c  x11.c
rm -f x12.o
cc -c  x12.c
ar rv ../nat.a x11.o x12.o
a - x11.o
a - x12.o
rm -f x11.o x12.o
make[1]: Leaving directory `/FS/fs0300/usanefe/Nat/src1'
Process Nat/src2
cd src2;make -
make[1]: Entering directory `/FS/fs0300/usanefe/Nat/src2'
rm -f x21.o
cc -c  x21.c
rm -f x22.o
cc -c  x22.c
ar rv ../nat.a x21.o x22.o
a - x21.o
a - x22.o
rm -f x21.o x22.o
make[1]: Leaving directory `/FS/fs0300/usanefe/Nat/src2'

Apparently, other makes (i.e. non GNU make) do not exhibit this behavior
(i.e. the 1st version of the makefile will produce the desired results).