How do I specify in a Makefile.am script that I only want to compile object .o files?

Question

I have a Makefile.am which will be responsible for building a final application binary:

project/src/Makefile.am

Also in the src directory is a sub-directory called ctrnn which contains an additional Makefile.am:

project/src/ctrnn/Makefile.am

Now, ctrnn/Makefile.am should only generate object .o files with the idea being that the top-level Makefile.am should use the object files generated in subdirectory ctrnn to build the binary.

This is the ctrnn/Makefile.am:

SOURCES = network.cpp\
    neuron.cpp

AM_CPPFLAGS=  @CXXFLAGS@

Based on this Makefile.am file, I want to end up with network.o and neuron.o. I am generating the according Makefile using Automake etc, yet when I try and then execute the make file, it doesn't do anything and just says:

make: Nothing to be done for `all'

I know why this is, I need to specify the build target. But how do I do this in the ctrnn/Makefile.am script given that I don't want to build a binary which would require bin_PROGRAMS but actual object files network.o and neuron.o?

(Note if I do specify a bin_PROGRAMS name, it rightly ends up complaining of an undefined reference to main).

What am I do wrong?

Answer 1

Automake cannot build objects without an explicit target (program, library) that will use these objects. One reason is that compilation options are specified per-target. If two targets, e.g. two binaries use the same object but have different compilation option, the same object may have to be compiled twice.

You have three ways to do what you want, they all involve tying your source files to some target.

1. Do not use a src/ctrnn/Makefile.am, just make reference to the subdirectory source files from your src/Makefile.am:

   bin_PROGRAMS = foo
   foo_SOURCES = main.c crtnn/network.cpp crtnn/neuron.cpp
   

   Note that this will build network.o and neuron.o in the same directory as main.o. If you want objects in subdirectories, use AUTOMAKE_OPTIONS = subdir-objects.

2. Use a convenience library. In src/crtnn/Makefile.am make a library of your two objects:

   noinst_LIBRARIES = libcrtnn.a
   libcrtnn_a_SOURCES = network.cpp neuron.cpp
   

   and in src/Makefile.am, link your executable to the library:

   bin_PROGRAMS = foo
   foo_SOURCES = main.c
   foo_LDADD = crtnn/libcrtnn.a
   SUBDIRS = crtnn
   

   It's called "convenience" when it is not going to be installed (you can tell because of the noinst_ prefix): it is just used during the build. And this is a static library, so the result is the same as if you had replaced crtnn/libcrtnn.a by crtnn/network.o and crtn/neuro.o when linking foo.

3. Use a Libtool convenience library. This requires more setup if you are not using Libtool already. You should add a call LT_INIT in configure.ac and rerun autoreconf to install the libtool files. Then you can update src/crtnn/Makefile.am to make a library of your two objects as follows:

   noinst_LTLIBRARIES = libcrtnn.la
   libcrtnn_la_SOURCES = network.cpp neuron.cpp
   

   and src/Makefile.am as follows:

   bin_PROGRAMS = foo
   foo_SOURCES = main.c
   foo_LDADD = crtnn/libcrtnn.la
   SUBDIRS = crtnn
   

   What is the difference? you may ask, almost none. One advantage of using Libtool convenience libraries is that they can be nested: a Libtool library can include another Libtool library (this is convenient when you have a deep hierarchy of source code and you are building a library at each level). Also Libtool convenience libraries can be used to build a shared library if you want. Automake's static libraries cannot.