Datasheet

Book VIII
Chapter 1
Programming
in Linux
539
Exploring the Software-Development Tools in Linux
This rule specifies an abstract target named clean that doesn’t depend on
anything. This dependency statement says that to create the target clean,
GNU make invokes the command rm -f *.o, which deletes all files that
have the .o extension (namely, the object files). Thus, the effect of creating
the target named clean is to delete the object files.
Variables (or macros)
In addition to the basic capability of building targets from dependents, GNU
make includes many features that make it easy for you to express the depen-
dencies and rules for building a target from its dependents. If you need to
compile a large number of C++ files by using GCC with the same options, for
example, typing the options for each file is tedious. You can avoid this repet-
itive task by defining a variable or macro in make as follows:
# Define macros for name of compiler
CXX= g++
# Define a macro for the GCC flags
CXXFLAGS= -O2 -g –mcpu=i686
# A rule for building an object file
form.o: form.C form.h
$(CXX) -c $(CXXFLAGS) form.C
In this example, CXX and CXXFLAGS are make variables. (GNU make prefers
to call them variables, but most UNIX make utilities call them macros.)
To use a variable anywhere in the makefile, start with a dollar sign ($)
followed by the variable within parentheses. GNU make replaces all occur-
rences of a variable with its definition; thus, it replaces all occurrences of
$(CXXFLAGS) with the string -O2 -g -mcpu=i686.
GNU make has several predefined variables that have special meanings.
Table 1-2 lists these variables. In addition to the variables listed in Table 1-2,
GNU make considers all environment variables (such as PATH and HOME) to
be predefined variables as well.
Table 1-2 Some Predefined Variables in GNU make
Variable Meaning
$%
Member name for targets that are archives. If the target is
libDisp.a(image.o), for example, $% is image.o.
$*
Name of the target file without the extension.
$+
Names of all dependent files with duplicate dependencies, listed
in their order of occurrence.
$<
The name of the first dependent file.
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