The Linux Development Platform Configuring, Using, and ... - Classes

CH04.fm Page 118 Monday, October 7, 2002 8:54 PM
118 Chapter 4 • Using GNU make
Now let us clean the files we have created using the following rule.
clean:
rm -f $(TARGETS) $(OBJS)
The result of running the command make clean will be as follows:
[root@rr2 test]# make clean
rm -f ftp tftp dnsresolver ftp.o tftp.o dnsresolver.o common.o
[root@rr2 test]#
As you can see from the above output, make has replaced variables with values present in
the variables before running the rm command. This is what is done every time make invokes a
command.
4.2.4 Explicit Rules
There are two types of rules: explicit rules and implicit rules. All of the rules that we have
been using in this chapter until now are explicit rules. An explicit rule has three parts: target,
dependencies and commands. Explicit rules are defined in detail and they perform exactly as
they are written. In general, all rules present in a makefile are explicit rules.
4.2.5 Implicit Rules
Implicit rules are used by make to build certain targets by itself. These rules are usually
language-specific and operate depending upon file extension. For example, make can build .o
files from .c files using an implicit rule for C language compilation. Consider the basic makefile
we used earlier in this chapter as shown below:
# Variable definition
OBJS = ftp.o common.o
HDRS = ftp.h common.h
CFLAGS = -g -O2
TARGETS = ftp
CC = gcc
# Default Target
ftp: $(OBJS) $(HDRS)
$(CC) $(OBJS) -o ftp
ftp.o: ftp.c $(HDRS)
$(CC) $(CFLAGS) -c ftp.c
common.o: common.c common.h
$(CC) $(CFLAGS) -c common.c
clean:
rm -f $(TARGETS) $(OBJS)