Advanced Auto-Dependency Generation. A slight improvement.

[Nick Patavalis]

Hi,

I just read the excellent article `Advanced Auto-Dependency
Generation`_ by Paul D. Smith. The proposed technique of generating
dependency lists when compiling the sources, and using the dependency
lists created by the *previous build*, is truly ingenious.

.._Advanced Auto-Dependency Generation:
    http://make.paulandlesley.org/autodep.html

There is, though, a minor glitch. With the proposed implementation, if
a dependency file somehow gets deleted while the corresponding object
file remains standing, in this case, dependency tracking *silently
stops working*. The orphaned ``.o`` file will be rebuild *only* when
the primary source file (the corresponding ``.c`` source) changes,
regardless of what happens to the secondary source files (the included
headers). This is bad.

Luckily there seems to be a simple solution:

- Make the object files depend on the corresponding dependency files

- Provide rules with empty command-sets targeting the dependency
  files.

The following Makefile demonstrates this solution. Apart from the
trick discussed above it deviates from Smith's implementation at the
following:

- Uses a single GCC invocation per object-target to build both: the
  dependency list, and the object file.

- Suffixes the dependency files by ``.d`` instead of by ``.P``

- Make the timestamps of the dependency files identical to the
  timestamps of the corresponding object files using the "touch"
  command.

- If a build command fails, or gets canned, it removes the
  corresponding ``.d``, ``.P`` and ``.o`` files

Well here it is: ::

  ##################################################################
  
  CC=gcc -c
  
  SRCS = file1.c file2.c
  
  all: $(SRCS:.c=.o)
  
  %.o : %.c %.d
          trap "rm -f '$@' '$*.d' '$*.P'; exit 1" ERR HUP INT TERM; \
          $(CC) -MMD $(CPPFLAGS) $(CFLAGS) -o '$@' '$<'; \
          sed 's%^$@ *:%$*.d $@:%' '$*'.d > '$*'.P; \
          cp '$*'.P '$*'.d; \
          sed -e 's/#.*//' -e 's/^[^:]*: *//' -e 's/ *\\$$//' \
              -e '/^$$/d' -e 's/$$/:/' < '$*'.P >> '$*'.d; \
          rm -f '$*'.P; \
          touch -r '$@' '$*'.d;

  %.d: ;
  -include $(SRCS:.c=.d)
  
  ##################################################################

As I'm far from being a Make expert, I'm eagerly expecting the
comments and feedback of more experienced users. Is what I'm doing
correct? Could I do do something better? Do you see any problems with
my approach? The "trap" command looks particularly ugly, but I could
not think of any other way to clean-up the left-overs when the command
fails or gets canned by the user.

Regards
/npat

P.S. A couple of hours ago, I tried to post the same message using
     GMANE (http://www.gmane.org/). It seems that I have failed since
     the message disappeared without a trace. I also tried the same
     thing yesterday with similar results. Has anyone else observed
     this? Or is it some configuration problem at my side?
     Anyway... sorry if this gets to you twice.

-- 
Act from reason, and failure makes you rethink and study harder. Act
from faith, and failure makes you blame someone and push harder.
  -- Erik Naggum