Question

I have been working on a C++ project that links to a .so(dynamic library) file.
Let’s assume that I have a target PROG which needs to link to a.so, and a.so is also built by me, specified in the following Makefile.

PROG_SRCS = prog.cpp
PROG_OBJS = $(PROG_SRCS: %.cpp:%.o)

all: PROG
PROG: $(PROG_OBJS) liba.so
    $(LINK.cpp) -o $@ $(PROG_OBJS) -la

LIBA_SRCS = liba/a.cpp
LIBA_OBJS = $(LIBA_SRCS: %.cpp:%.o)

liba.so: $(LIBA_OBJS)
    $(LINK.cpp) -shared -Wl.-soname,$@ -o $@ $^

I have used auto-dependency-generation to get the .cpp files get their own dependency on
the .h files. And prog.cpp includes a.h.

But by this way, once I change a.cpp, liba.so would be remake, then PROG would be
remake(relink), which is not what I want. I just change the implementation of liba.so,
but not any interface definitions. PROG should just remake after I change a.h.

I want to make that a.so should be built before PROG is built, but the changes of a.so would not incur the building of PROG.

The following Makefile is the method I figured out, but with a slightly side-effect
(generate a temporary file).

ORDER = /tmp/.ORDER

all: PROG
PROG: $(PROG_OBJS) $(ORDER)
    $(LINK.cpp) -shared -Wl,-soname,$@ -o $@ $(PROG_OBJS) -la

$(ORDER): liba.so
    test -e $@ || touch $@

In this way, every time liba.so gets remake, $(ORDER) gets remake too. But it only touch
the file if it doesn’t exist.

Is there any way to specify this kind of dependency without any side-effect, e.g. creating a tmp file.

Answer 1

If you’re willing to rely on non-portable aspects of GNU make, you can use order-only prerequisites order-only prerequisites for this. That’s the only way to do it other than the stamp file method you’ve already discovered.