Laboratory Exercises, C++ Programming

14 Tools for Practical C++ Development
1.3 Rules for Linking
In our example makefile, we used an explicit rule for linking. Actually, there is an implicit rule
for linking, which looks (after some variable expansions) like this:
$(CC) $(LDFLAGS) $^ $(LOADLIBES) $(LDLIBS) -o $@
LDFLAGS are options to the linker, such as -Ldirectory. LOADLIBES and LDLIBS 6 are variables
intended to contain libraries, such as -llab1. So this is a good rule, except for one thing: it
uses $(CC) to link, and CC is by default gcc, not g++. But if you change the definition of CC, the
implicit rule works also for C++:
# Define the linker
CC = g++
1.4 Generating Prerequisites Automatically
While you’re working with a project the prerequisites are often changed. New #include directives
are added and others are removed. In order for make to have correct information of the
dependencies, the makefile must be modified accordingly. The necessary modifications can be
performed automatically by make itself.
A3. Read the make manual, section 4.14 “Generating Prerequisites Automatically”, and implement
such functionality in your makefile. Hint: Copy the %.d rule (it is not necessary that
you understand everything in it) to your makefile and make suitable modifications. Do not
forget to include the *.d files.
The first time you run make you will get a warning about .d files that don’t exist. This
is normal and not an error. Look at the .d files that are created and see that they contain
the correct dependencies.
One useful make facility that we haven’t mentioned earlier is wildcards: as an example,
you can define the variable SRC as a list of all .cc files with SRC = $(wildcard *.cc). And
from this you can get all .o files with OBJ = $(SRC:.cc=.o).
From now on, you must update the makefile when you add new programs, so that you
can always issue the make command in the build directory to build everything. When you
add a new program, you should only have to add a rule that defines the .o-files that are
necessary to link the program, and maybe add the program name to a list of executables.
This is important — it will save you many hours of command typing.
(Optional) Collect all object files not containing main functions into a library, and link
executables against that library. If you know how to create shared libraries, use a shared
library libcxx.so, otherwise create a static archive libcxx.a.
1.5 Writing a Good Makefile
A makefile should always be written to run in sh (Bourne Shell), not in csh. Do not use special
features from, e.g., bash, zsh, or ksh. The tools used in explicit rules should be accessed through
make variables, so that the user can substitute alternatives to the defaults.
A makefile should also be written so that it doesn’t have to be located in the same directory
as the source code. You can achieve this by defining a SRCDIR variable and make all rules refer
to the source code from that prefix. Even better is to use the variable VPATH, which specifies a
search path for source files. Vpath is described in section 4.5 of the make manual.
6 There doesn’t seem to be any difference between LOADLIBES and LDLIBS — they always appear together and are
concatenated. Use LDLIBS.