make

<BODY > <HR><center> <table border=1> <td> <A NAME="tex2html41" HREF="node3.html"><img height=36 width=36 alt="PREVIOUS" src="../../../icons/latex2html/previous.gif"></A></td> <td> <A NAME="tex2html47" HREF="unix_utilities.html"><img height=36 width=36 alt="UP" src="../../../icons/latex2html/up.gif"></A></td> <td> <A NAME="tex2html49" HREF="node5.html"><img height=36 width=36 alt="NEXT" src="../../../icons/latex2html/next.gif"></A></td> <td><a href="contents-node4.html"><img height=36 width=36 alt="CONTENTS" src="../../../icons/latex2html/toc.gif"></a></td> </table> </center> <HR>  <H1><A NAME="SECTION00040000000000000000"> make</A> </H1> The <B>make</B> utility is used to update files that depend on other files. In this course we will use <B>make</B> to compile programs. You can use <B>make</B> without any further thought, if you simply follow the pattern given here. However, if you plan to use Unix quite a bit in your careers, it is recommended that you learn more about how it works. See, for example the online man pages and B.W. Kernighan and R. Pike, <EM>The Unix Programming Environment</EM> (Prentice Hall, London, 1984), p. 241 ff. <P> Suppose you have a source file called <code>runge.cc</code> and routinely compile it to make an executable <code>runge</code> using the command: <PRE> g++ -O runge.cc -o runge </PRE> The output executable file <B>runge</B> depends on the source file <B>runge.cc</B>. If you make a change to <B>runge.cc</B> you should recompile the executable to bring it up to date. Unix tracks the date and time a file is modified. So by comparing the dates of <B>runge.cc</B> and <B>runge</B> one can tell whether it is necessary to update <B>runge</B>. It takes two pieces of information to accomplish this - first, the dependency file <B>runge</B> depends on file <B>runge.cc</B>, and second, the rule for updating the dependent file, i.e. the compilation statement above. These two pieces of information are encoded in a special file called called the ``makefile''. By default that file is named <B>Makefile</B> or <B>makefile</B>. For this example the makefile consists of two lines: <PRE> runge: runge.cc <Tab> g++ -O runge.cc -o runge </PRE> Please note that the <code><Tab></code> means ``hit the tab key''. After you create the makefile, it is a simple matter to compile your program. From the Unix prompt, type <PRE> make runge </PRE> The <B>make</B> utility looks at your file <B>Makefile</B> and sees that the executable <B>runge</B> depends on the source <B>runge.cc</B>. It then compares the date and time stamps on these files. If the source program is more recent, which would be the case if you made a change in the source program after the last compilation, then <B>make</B> uses the next command to make the new version. Please note that the <B><Tab></B> is necessary in the makefile to signify that the line is a shell command. The file should end with an end-of-line character. That is, be sure after typing the last line in the file, that you hit <B>Enter</B>. (If you don't do this, the make utility is apt to ignore the last line, and the cause of the problem will be invisible and baffling.) <P> The above example actually happens to be the default compilation procedure for the <B>make</B> utility. That is, if your makefile is missing, but you type <B>make runge</B> anyway, then <B>make</B> will do the compilation exactly as specified in this example, compiling with the optimization option <B>-O</B>, linking with the standard libraries (but not the commonly used C math library), and producing an executable with the name <B>runge</B>. If your source program name ends with <B>.cc</B>, <B>make</B> uses <B>g++</B>, and if it ends with <B>.c</B>, <B>make</B> uses <B>gcc</B> to do the compilation. However, if your executable programs involve more than one object file, or require any of the nonstandard libraries or a different set of compiler options, then you have to create a makefile and you will find that <B>make</B> is indeed your friend. <P> If you are writing C, you would replace the <B>g++</B> command with the corresponding <B>gcc</B> command and, of course, change the name of the source file to a <B>.c</B> file. <P> Here is an example of a makefile that involves two source files: <PRE> runge: runge.cc functn.cc <Tab> g++ -O runge.cc functn.cc -o $@ </PRE> In this example, the executable <B>runge</B> depends on two source files, and you want the whole thing recompiled when you change either source file. The <code>$@</code> is a special make variable that gets replaced by the ``target'', in this case <B>runge</B>. <P> You can be more sophisticated and efficient if you understand that there are actually two steps to creating an executable from a source file. First it is necessary to translate the source files such as <B>runge.cc</B> to object files, conventionally <B>runge.o</B>, and then take the object files and ``link'' them with the libraries to produce the executable. The make rule above is inefficient, since if you change just one source file, all source files are recompiled in order to create the executable. Why not just recompile the one you need, making a new object file, and the link it with the other object files that didn't have to be recompiled. Here is the way we do it: <PRE> OBJECTS= runge.o functn.o .cc.o: <Tab> g++ -O -c $*.cc runge: ${OBJECTS} <Tab> g++ ${OBJECTS} -o $@ </PRE> Here a <B>make</B> variable (macro) called <B>OBJECTS</B> is defined to be a list of object files. In the next two lines a generic rule is given for converting any <B>.cc</B> file into the corresponding <B>.o</B> file. The <B>-c</B> option tells g++ to do the compilation and produce the <B>.o</B> file, but not to link the file into an executable file yet. The <code>$*.cc</code> says that the source file name is the same as the object file, except that it ends in <B>.cc</B>. Finally, <B>make</B> is told that the executable file <B>runge</B> depends on the object files in the list <B>OBJECTS</B>, and a rule is given for making the executable. Since in this case <B>g++</B> is being given a list of <EM>object</EM> files, make will simply link them all to make the executable file, recompiling only where necessary. <P> <HR><center> <table border=1> <td> <A NAME="tex2html41" HREF="node3.html"><img height=36 width=36 alt="PREVIOUS" src="../../../icons/latex2html/previous.gif"></A></td> <td> <A NAME="tex2html47" HREF="unix_utilities.html"><img height=36 width=36 alt="UP" src="../../../icons/latex2html/up.gif"></A></td> <td> <A NAME="tex2html49" HREF="node5.html"><img height=36 width=36 alt="NEXT" src="../../../icons/latex2html/next.gif"></A></td> <td><a href="contents-node4.html"><img height=36 width=36 alt="CONTENTS" src="../../../icons/latex2html/toc.gif"></a></td> </table> </center> <HR>  <HR><P><ADDRESS> <I></I> </ADDRESS> </BODY>