Makedepend: Error: Failed to Read ...

I complained a few months ago about makedepend not picking up dependencies, and gave a recipe for avoiding makedepend.

I'm stuck with it for a project, and a couple of times I've gotten a makedepend failure, with only this useless warning:

makedepend: error:  failed to read [some directory name]

The problem ended up being an #include somewhere, where the opening double-quote (") was missing. One time I accidentally had a single-quote at the beginning, closed with a double-quote; the other time a co-worker left off the opening quote, but did have the closing quote.

No help from makedepend on which file has the faulty include: it just gives up on the whole directory. Thanks, makedepend. Notice that if you leave off both quotes, or have both as single-quotes, makedepend is kind enough to give you a meaningful message. Or, if you open with a double-quote, and forget the closing quote, it exits without an error! Though I'm not sure if it gets the dependencies right in that case.

Do yourself a favor, use gcc's dependency-generating scheme, and never use makedepend again.

Stupid, stupid makedepend

One of the most frustrating, time-wasting debugging experiences you can have is when the code you thought you compiled didn't really get compiled. Or something you didn't even think about because someone else updated it didn't get compiled.

This is why the utility makedepend was introduced, so that compile dependencies get taken care of automatically, every time. So why is the Linux makedepend so stupid that it doesn't work every time?!?!

In the interest of speed, the Linux makedepend was written to parse include files only once per run (usually this is per directory). That's usually OK, but when the include file is preprocessed differently by different files (or multiple times in one file), it doesn't work. I thought the first rule of optimization was: make the common case fast, but make every case work.

Moral: use the dependency scheme described in the GNU make info pages. It goes like this:

# In the variables area:
sources = foo.cpp bar.cpp ...
OBJS = $(sources:.cpp=.o)

# In the targets area, somewhere after "all:"
ifneq ($(MAKECMDGOALS),clean)
include $(sources:.cpp=.d)
endif

%.d: %.cpp
  @(set -o pipefail; \
    $(CXX) -MM $(CXXFLAGS) $< \
     | sed 's,\($*\)\.o[ :]*,\1.o $@ : ,g' \
      > $@;) || $(RM) $@

Of course this assumes that CXX = g++. By the way, that little gem about not building the dependencies during "clean" is in the info pages, but not the page that describes the generic dependency rules. Yes, I have read every make info page.

The "pipefail" bit is my own little contribution; it saves you some headaches when the g++ command fails when you're not watching, because the sed command will succeed and allow make to continue ignorantly on its way. That's a bash option, so it might not work on platforms other than Linux.