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2 Building from Source use none none generator toencode none with upc-a generating that their value none for none can change over the course of the build. Automatic variables are vital for implicit rules but can be very useful in explicit rules as well. Consider this common Makefile pattern for creating a program:.

.NET Framework 3.5 program: $(OBJS) $(CC) -o $@ $^ The command port none for none ion consists of a predefined variable (CC) as well as two automatic variables. Unfortunately, there is no way to encapsulate this in an implicit rule because there isn t a clear pattern. The program can be any name, and the OBJS can contain any number of arbitrarily named objects.

As far as make is concerned, there is no pattern to follow. If you want to use this same rule to create additional programs in the same Makefile, you are resigned to cutting and pasting these commands for every program, as follows:. program1: $(PROG1_OBJS) $(CC) -o $@ $^ program2: $(PROG2_OBJS) $(CC) -o $@ $^ # etc... At least with th none none e use of automatic variables, you can cut and paste with no modifications to the command portion of the rule. It is a little cryptic, but you can do better. By embedding the automatic variables in a more intuitively named variable, you can make the pattern easier to follow.

For example:. build_C_program= $(CC) -o $@ $^ program1: mod1.o mod2.o $(build_C_program) program2: mod3.

o mod4.o $(build_C_program). build_C_program,. Now the ugly aut omatic variables are part of the definition for but they behave as expected when build_C_program appears as a command. The rule is a little easier to follow, and other developers don t need to know what build_C_program looks like unless they want to..

Build Tools 2.2.2.

6 Manipula ting Variables To understand why you need to manipulate variable values, let s look at a typical example of a text substitution pattern used in UNIX make:. SRCS=foo.c bar.c none none OBJS=$(SRCS:.

c=.o). This syntax is c alled a substitution reference and is available in all flavors of make. Here, we take a list of source files and use it to create a list of object files. In this example, we take the value of the SRCS variable, substitute .

o for all the .c extensions, and store the result in the OBJS variable. Because we have two source modules, named foo.

c and bar.c, we would like OBJS to contain foo.o and bar.

o. The text substitution is preferable to typing every name again on a second line, which would be tedious and error prone. Makefiles usually contain many such lists, making text substitutions indispensable.

The substitution reference works in this example because all the source modules have the same extension. What if you have mixed C and C++ sources in your SRCS variable For example:. SRCS=foo.c bar.c none for none pp OBJS=$(SRCS:.

c=.o). The substitution reference syntax does not allow more than one file extension pattern. So in our example, the substitution does not take place for bar.cpp because it doesn t match the .

c pattern. OBJS then contains foo.o bar.

cpp, which is not what you want. A typical solution in UNIX make would be to create an additional variable as follows:. CSRCS=foo.c CXXS none for none RCS=foo.cpp OBJS=$(CSRCS:.

c=.o) $(CXXSRCS:.cpp=.

o). This technique t ends to increase clutter, especially when you are dealing with numerous lists. Next, I will look at the GNU function extensions, which are much more flexible..

2 Building from Source 2.2.2.

7 Manipula ting Variables with Functions As you have seen, the substitution reference syntax can be very handy, but it is a bit limited. GNU make s function extensions are versatile tools for manipulating variable contents. Here are two ways to do the same thing:.

OBJS=$(SRCS:.c=. none for none o) OBJS:=$(patsubst %.

c, %.o, $(SRCS))). The first line u ses the familiar substitution reference. The second line is the equivalent of the substitution reference using the patsubst function. The patsubst function has the same limitations as the substitution reference: Only one extension at a time is supported.

Functions can be nested, however, which is something that you can t do with a substitution reference. To include additional extensions, for example, you can use the following syntax:. OBJS:=$(patsubst none none %.cpp, %.o, $(patsubst %.

c, %.o, $(SRCS))). Now we can trans form multiple file extensions without having to create extra variables. Here is another technique using different functions. It is a bit more concise and generic:.

OBJS:=$(addsuffi none for none x .o, $(basename $(SRCS))). The basename fun ction takes a list of filenames (in this case, the contents of SRCS) and strips the extension from each of them. It doesn t matter what the extension is; basename just strips off all the characters following the last period in the text. We take the output and glue .

o to the end of each basename, using the addsuffix function. This gives us the list of objects we want. Another useful function is the shell function.

Recall the earlier example in which we used backticks to create a filename based on the date and time:.
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