Debugging Makefiles

Here is some information on debugging Makefiles:

showing symbol values, and showing file dependencies

There's a good Dr. Dobbs article on this topic, by John Graham-Cumming, at:

• Debugging Makefiles

or printable, all-in-one page, at http://www.embedded.com/columns/technicalinsights/197003517?printable=true

This article describes some macros you can put in your Makefiles to show you the values of symbols, and to show the dependencies that led to a particular target being built. This is handy if you have a complex Makefile that you are trying to debug.

Makefile Debugger

John has also written a Makefile Debugger.

The following articles describe the debugger and how to use it:

• An Interactive GNU Make Debugger
• Dynamic Breakpoints in the GNU Make Debugger

The following resources are needed to use the debugger:

• Download gmsl (a library of gnu make stuff), from http://gmsl.sf.net/
• Download gmd (the debugger itself), from http://gmd.sf.net/

Expand the tarballs from these, and place __gmsl, gmsl and gmd in the directory with the Makefile you wish to debug.

Add the following lines to your Makefile:

include gdm

and

all:
        $(__BREAKPOINT)

Debugging a kernel build

Verbose options

There are several Kernel makefile tricks which are helpful for debugging a kernel build.

Try "make V=1 [<rest of make args...>]" to see all the full-length commands used to build the kernel.

Sometimes, if a particular step is breaking and you don't understand why, you can copy-and-paste from this verbose command output (to the shell command line). To do this you need to make sure you are in the correct source directory, and that all required files are present.

To see why a particular target is being build, use "make V=2". This is quite useful when a small change results in much more being build than you expected.

Building in a separate directory

Use the variable KBUILD_OUTPUT either as an environment variable or on the make command line, if you wish to put the build output into a separate directory from the source. This can be useful for several reasons, including when you want to build for multiple architectures from the same source, or when you wish to avoid cluttering up the source tree with object files, libraries and other generated files (to make diffs easier).