122Chapter4 • Using GNU make
Let’s try to build the default target for the first time and see the output of these
echo
commands.
[root@conformix make]# make
gcc -g -O2 -c -o ftp.o ftp.c
gcc -g -O2 -c -o common.o common.c
ftp.o common.o ftp.h common.h
ftp
ftp.o
gcc ftp.o common.o -o ftp
[root@conformix make]#
The first two lines in the output use implicit rules to build object files. The third line in the
output is generated by the first
echo
command and it has a list of all the dependencies. The sec-
ond echo command displays the fourth line in the output, which is just the name of the target, i.e.
ftp
. The last
echo
command prints the first dependency in the fifth line of the output.
Now let us modify the
common.c
file and run
make
once again. The output will be as
follows:
[root@conformix make]# make
gcc -g -O2 -c -o common.o common.c
common.o
ftp
ftp.o
gcc ftp.o common.o -o ftp
[root@conformix make]#
This time
make
used an implicit rule to build
common.o
. The important thing to note is
the second line in the output, which is displayed by the first
echo
command in the makefile.
This time it displayed only one file in the dependency list because
common.o
is the only file
that is changed in the dependencies.
Automatic variables are very useful in control structures and in the decision-making pro-
cess when you carry out an operation based upon the result of some comparison.
4.4Working with Multiple Makefiles and Directories
Until now we have used simple makefiles to build projects or targets. We used only one makefile
in the project and all of the source files were present in the same directory. In real-life software
development projects, we have multiple directories containing different parts of a software prod-
uct. For example, one directory may contain all library files, another header files and a third one
common files used by different parts. Similarly every part of the software project may have its
own subdirectory.
This section deals with the same source files and targets that we used earlier in this chap-
ter. The project contains three targets to be built. These targets are
ftp
,
tftp
and
dnsre-
solver
. We are using the same set of source files. However to demonstrate the use of multiple
directories and multiple makefiles, we have created each component in a separate directory. This