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<h1 class="section">Variables Make Makefiles Simpler</h1> <p>In our example, we had to list all the object files twice in the rule for <samp>edit</samp> (repeated here): </p> <div class="example"> <pre class="example">edit : main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
cc -o edit main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
</pre>
</div> <p>Such duplication is error-prone; if a new object file is added to the system, we might add it to one list and forget the other. We can eliminate the risk and simplify the makefile by using a variable. <em>Variables</em> allow a text string to be defined once and substituted in multiple places later (see <a href="using-variables">How to Use Variables</a>). </p> <p>It is standard practice for every makefile to have a variable named <code>objects</code>, <code>OBJECTS</code>, <code>objs</code>, <code>OBJS</code>, <code>obj</code>, or <code>OBJ</code> which is a list of all object file names. We would define such a variable <code>objects</code> with a line like this in the makefile: </p> <div class="example"> <pre class="example">objects = main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
</pre>
</div> <p>Then, each place we want to put a list of the object file names, we can substitute the variable’s value by writing ‘<samp>$(objects)</samp>’ (see <a href="using-variables">How to Use Variables</a>). </p> <p>Here is how the complete simple makefile looks when you use a variable for the object files: </p> <div class="example"> <pre class="example">objects = main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
edit : $(objects)
cc -o edit $(objects)
main.o : main.c defs.h
cc -c main.c
kbd.o : kbd.c defs.h command.h
cc -c kbd.c
command.o : command.c defs.h command.h
cc -c command.c
display.o : display.c defs.h buffer.h
cc -c display.c
insert.o : insert.c defs.h buffer.h
cc -c insert.c
search.o : search.c defs.h buffer.h
cc -c search.c
files.o : files.c defs.h buffer.h command.h
cc -c files.c
utils.o : utils.c defs.h
cc -c utils.c
clean :
rm edit $(objects)
</pre>
</div><div class="_attribution">
<p class="_attribution-p">
Copyright © 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021, 2022 Free Software Foundation, Inc. <br>Licensed under the GNU Free Documentation License.<br>
<a href="https://www.gnu.org/software/make/manual/html_node/Variables-Simplify.html" class="_attribution-link">https://www.gnu.org/software/make/manual/html_node/Variables-Simplify.html</a>
</p>
</div>
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