Running Linux, Third Edition

Makefiles

Sometime during your life with Linux you will probably have to deal with make, even if you don’t plan to do any programming. It’s likely you’ll want to patch and rebuild the kernel and that involves running make. If you’re lucky, you won’t have to muck with the makefiles—but we’ve tried to direct this book toward unlucky people as well. So in this section, we’ll explain enough of the subtle syntax of make so that you’re not intimidated by a makefile.

For some of our examples, we’ll draw on the current makefile for the Linux kernel. It exploits a lot of extensions in the powerful GNU version of make, so we’ll describe some of those as well as the standard make features. A good introduction to make is provided in Managing Projects with make by Andrew Oram and Steve Talbott. GNU extensions are well documented by the GNU make manual.

Most users see make as a way to build object files and libraries from sources and to build executables from object files. More conceptually, make is a general-purpose program that builds targets from prerequisites. The target can be a program executable, a PostScript document, or whatever. The prerequisites can be C code, a text file, and so on.

While you can write simple shell scripts to execute gcc commands that build an executable program, make is special in that it knows which targets need to be rebuilt and which don’t. An object file needs to be recompiled only if its corresponding source has changed.

For example, say you have a program ...