Debugging GNU make

Question

Is there a command line way in make to find out which of the prerequisites of a target is not updated?

Answer 1

make -d

should give you more than enough information to debug your makefile.

Be warned: it will take some time and effort to analyze the output but loading the output into your favorite editor and doing searches will assist a lot.

You can greatly reduce the amount of debugging output if you specify the specific target you're interested in. So if you're only interested in the dodgy target, instead of just make -d which may make a hundred different things, try:

make clean
make -d dodgy

(assuming you have a clean target of course).

The make --debug is identical to make -d but you can also specify:

make --debug=FLAGS

where flags can be:

• a for all debugging (same as make -d and make --debug).
• b for basic debugging.
• v for slightly more verbose basic debugging.
• i for implicit rules.
• j for invocation information.
• m for information during makefile remakes.

It looks like make --debug=b is the best option for what you need, as shown in the following transcript:

pax@paxbox> cat makefile
c:a b
    touch c

pax@paxbox> touch a b ; make
touch c

pax@paxbox> make
make: 'c' is up to date.

pax@paxbox> touch a ; make --debug=b
GNU Make 3.81
Copyright (C) 2006  Free Software Foundation, Inc. Blah, blah, blah.
Reading makefiles...
Updating goal targets....
 Prerequisite 'a' is newer than target 'c'.
Must remake target 'c'.
touch c
Successfully remade target file 'c'.