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The Art of Unix Programming
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Unix Programming - make: Automating Your Recipes

Program sources by themselves don't make an application. The way you put them together and package them for distribution matters, too. Unix provides a tool for semi-automating these processes; make(1). Make is covered in most introductory Unix books. For a really thorough reference, you can consult Managing Projects with Make [Oram-Talbot]. If you're using GNU make (the most advanced make, and the one normally shipped with open-source Unixes) the treatment in Programming with GNU Software [Loukides-Oram] may be better in some respects. Most Unixes that carry GNU make will also support GNU Emacs; if yours does you will probably find a complete make manual on-line through Emacs's info documentation system.

Ports of GNU make to DOS and Windows are available from the FSF.

If you're developing in C or C++, an important part of the recipe for building your application will be the collection of compilation and linkage commands needed to get from your sources to working binaries. Entering these commands is a lot of tedious detail work, and most modern development environments include a way to put them in command files or databases that can automatically be re-executed to build your application.

Unix's make(1) program, the original of all these facilities, was designed specifically to help C programmers manage these recipes. It lets you write down the dependencies between files in a project in one or more ‘makefiles’. Each makefile consists of a series of productions ; each one tells make that some given target file depends on some set of source files, and says what to do if any of the sources are newer than the target. You don't actually have to write down all dependencies, as the make program can deduce a lot of the obvious ones from filenames and extensions.

For example: You might put in a makefile that the binary myprog depends on three object files myprog.o, helper.o, and stuff.o. If you have source files myprog.c, helper.c, and stuff.c, make will know without being told that each .o file depends on the corresponding .c file, and supply its own standard recipe for building a .o file from a .c file.

Make originated with a visit from Steve Johnson (author of yacc, etc.), storming into my office, cursing the Fates that had caused him to waste a morning debugging a correct program (bug had been fixed, file hadn't been compiled, cc *.o was therefore unaffected). As I had spent a part of the previous evening coping with the same disaster on a project I was working on, the idea of a tool to solve it came up. It began with an elaborate idea of a dependency analyzer, boiled down to something much simpler, and turned into Make that weekend. Use of tools that were still wet was part of the culture. Makefiles were text files, not magically encoded binaries, because that was the Unix ethos: printable, debuggable, understandable stuff.

-- Stuart Feldman

When you run make in a project directory, the make program looks at all productions and timestamps and does the minimum amount of work necessary to make sure derived files are up to date.

You can read a good example of a moderately complex makefile in the sources for fetchmail . In the subsections below we'll refer to it again.

Very complex makefiles, especially when they call subsidiary makefiles, can become a source of complications rather than simplifying the build process. A now-classic warning is issued in Recursive Make Considered Harmful.[136] The argument in this paper has become widely accepted since it was written in 1997, and has come near to reversing previous community practice.

No discussion of make(1) would be complete without an acknowledgement that it includes one of the worst design botches in the history of Unix. The use of tab characters as a required leader for command lines associated with a production means that the interpretation of a makefile can change drastically on the basis of invisible differences in whitespace.

Why the tab in column 1? Yacc was new, Lex was brand new. I hadn't tried either, so I figured this would be a good excuse to learn. After getting myself snarled up with my first stab at Lex, I just did something simple with the pattern newline-tab. It worked, it stayed. And then a few weeks later I had a user population of about a dozen, most of them friends, and I didn't want to screw up my embedded base. The rest, sadly, is history.

-- Stuart Feldman

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The Art of Unix Programming
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