2. Lots of tips

2.1 Automating dependency creation

There is a neat utility called makedepend which can go through your source code and see which files #include which other ones. It then tacks a bunch of dependency lines to the bottom of your makefile with a commented line that says ``Do not delete this line.'' You can use it again later to update those dependency lines. Makedepend is used like this:

makedepend [options] source-files...

For options, -Idirectory tells it to look in directory for include files, and -Ydirectory tells it not to look in directory. Just -Y by itself means not to look at any of the system include directories. You can give as many -I and -Y options as you need. Makedepend can ignore any options it doesn't understand if you put them between double dashes (--); this is useful as you'll see below.

If you give it -Y, it will probably spit out a bunch of stuff about not being able to find iostream.h and so on. Just ignore these messages; they are harmless. Note that you could get rid of them by omitting -Y, but then your makefile would have all sorts of long rules stating that foo.o depends on iostream.h and ios.h and streambuf.h and strbuf.h and .... Since those system header files almost never change, you don't want make to waste time scanning them, so you generally leave in the -Y.

After makedepend scans your source code, it tacks the resulting dependency rules onto the end of your makefile and saves you a ton of typing. It's very useful to have a ``depend'' target in your makefile which does this automatically:

depend:
   ->   makedepend -- $(CXXFLAGS) -- -Y $(SRC_FILES)

We give makedepend the CXXFLAGS because that macro contains the -I commands which are normally passed to g++ to tell it where the various .h files are, but those same commands are useful here. Makedepend happily ignores any other options in CXXFLAGS such as debugging flags because of the double dashes --.

2.2 Troubleshooting

(Problems and suggestions, in increasing order of unlikeliness.)

• Did you use gmake?
• Do all your command lines begin with tab characters?
• Does the last line of the makefile end in a newline character?
• If you've made lots of drastic changes, you should probably run the commands:

  gmake clean
  gmake depend
  gmake
  

• If g++ complains that it can't find an include file, did you leave out a -I option out somewhere? Remember that C compilation uses the macros CC and CFLAGS and that C++ compilation uses CXX and CXXFLAGS.
• If it tells you ``symbol undefined,'' did you forget a .o file or perhaps a library? Are you doing templates correctly? (That's another whole story...) Another tricky point is that the built in rule for linking of .o files uses the macro CC, which defaults to gcc. If you use gcc to link C++ object files, it doesn't know to add in the C++ library, which has the code for ostreams and such. The quickest fix is to make sure that you do this:

  CXX = g++ 
  CC = $(CXX)
  

• Did you define a macro and discover that it's mysteriously changing value? Make has this feature where it reads your environment variables (strings such as your path and X display which you change with the setenv command) and defines macros based on them. Just be forewarned that occasionally something funny happens to makefile macros that have the same name as environment variables. This feature can be useful, but also confusing.

  You're more likely to notice this if you make a typo or forget the name of your variable:

  PRYNTER = teerlp2   # OOPS.
  PRINT_COMMAND = print
  
  print:
     ->   $(PRINTCMD) -p$(PRINTER) $(FILES)   # OOPS.
  

• Pseudo-targets can be strange. If you accidentally create a file named, say, ``clean,'' it will mess up gmake clean.
• Also, beware of doing fancy shell commands inside a dependency rule. The command interpreter, usually tcsh or bash, is called separately for each command. This means that if you set an environment variable in one line of the command, it doesn't exist in the next one. If you change directories in one line, that change is lost in the next one:

  printout: $(SRC) $(HEADERS)
     ->   setenv PRINTER teerlp2
     ->   print -p$PRINTER $WHATEVER ...         #OOPS.
  

  If you need to do anything that fancy, write a shell script instead, and have make call it.