Re: SUFFIXES bug

[Paul D. Smith]

%% Patrick Craig <address@hidden> writes:

  pc> GNU Make version 3.79.1
  pc> Built for i686-pc-cygwin

  pc> My project has some autogenerated C files. I use a different
  pc> suffix for these files (.agc) so that I can delete them in the
  pc> clean rule (rm -f *.agc).  Here is a simplified Makefile:

  pc> .SUFFIXES: .agc

  pc> all::boot test.o

  pc> boot::
  pc>  cp test.pre test.agc

  pc> #test.agc::

  pc> .agc.o:
  pc>  gcc -c -xc $?

  pc> clean::
  pc>  rm -f test.agc test.o

  pc> If I have just the test.pre file and run make, it generates
  pc> test.agc and then says it doesn't know how to make test.o. If I
  pc> then run make again it correctly generates test.o. If I uncomment
  pc> out the empty test.agc rule, make works correctly, generating both
  pc> test.agc and test.o in one pass.  Microsoft nmake works correctly
  pc> even without the test.agc rule.

This doesn't have anything to do with suffixes; you could get the same
behavior in other ways.

The issue is GNU make's directory cache feature.

When GNU make first needs to find a file in a directory it caches the
contents internally.  Then as it runs rules which create new files, it
adds them to its cache.  This allows a significant performance
improvement.

The problem is that, if you create files as a side-effect of a rule and
make doesn't know about it, then make can't know to add that new file to
the cache, so it can't be found later even though it actually exists on
the disk.

That's what happens here: first the "boot" rule is run which creates
test.agc, but make doesn't know that.  Then make wants to find a rule
that can create test.o; it sees the implicit rule to build the .o from
the .agc, _but_ you have no rule telling it how to build a .agc file
and, looking at the cached version of the directory, no file named
"test.agc" exists.

So it fails.

The next time you run make, it sees that the test.agc file exists, so it
knows how to use it to build test.o.

What you really want to do is include a rule that tells make how to
build a test.agc; you say that these files are autogenerated, so you
should tell make how to generate them, like this:

  %.agc : %.in # or whatever you build them from
        <autogenerate a $@ from a $<>

  %.o : %.agc
        gcc -c -xc $<

(you don't want $? here, you want $<).  You can do this with suffix
rules too, of course, but pattern rules are simpler to read and
understand.

-- 
-------------------------------------------------------------------------------
 Paul D. Smith <address@hidden>          Find some GNU make tips at:
 http://www.gnu.org                      http://www.paulandlesley.org/gmake/
 "Please remain calm...I may be mad, but I am a professional." --Mad Scientist