Question

What can I do to fix this problem? I’m new to emacs, Unix, and makefiles. Please explain what I’m doing wrong and how I can fix it. I’m sure it’s a simple problem

progA: yesno.h yesno.cpp
       g++ -DDEBUG -c yesno.cpp yesno.h
progB: guess.cpp
       g++ -DDEBUG -c guess.cpp
progC: yesno.o guess.o
       g++ -DDEBUG -o guess guess.o yesno.o

My error is:

Your makefile does too much work when only guess.cpp has been changed:
g++ -c yesno.cpp yesno.h
g++ -c guess.cpp
g++ -o guess guess.o yesno.o

Answer 1

Your makefile would recompile everything each time.

You don’t need the first four lines; make knows how to create object files from source. Or, alternatively, you need to revise those 4 lines into 2, and put them at the end, and change the target names:

guess: yesno.o guess.o
        g++ -o guess yesno.o guess.o
guess.o: yesno.h
yesno.o: yesno.h

The first target is the one that is built by default, so that ensures that guess (the program) is built. The command line omits the -DDEBUG since that only affects source code compilation and there is no source in that link line (though, if using macros as one would in an advanced makefile, I’d be fine with the options such as -DDEBUG appearing in the link line).

The third line says that guess.o depends on yesno.h; this is a guess (on my part). The make program already knows how to convert guess.cpp to guess.o; it just needs the extra information that it also uses the yesno.h header. (If it doesn’t, how does it know about the functions defined in yesno.cpp?)

The last line says that yesno.o depends on yesno.h; again, make knows how to compile yesno.cpp to yesno.o but needs the extra information about the header.

This should only recompile guess.o when only guess.cpp changes; it should only recompile yesno.o when yesno.cpp changes; it should recompile both object files when yesno.h changes. If any of the source files changes, the program will be relinked; if no source files change, then the make command should do nothing (or simply report that there is nothing to do).