Makefile. How to exclude one particular file from compilation?

I am trying to exclude main.cpp file from the list of files to be compiled defined by the rule below:

$(TMPDIRPATH)%.o: %.cpp
    @echo compile $<
ifneq ($(notdir $<), main.cpp)
        @$(COMPILE.cpp) $(OUTPUT_OPTION) $<
endif

This 'ifneq' condition always evaluates to true, which is bizarre. What am I doing wrong? Is there a better way to exlude one file from an explicit rule?

Why don't you try using the filter-out text function if you're using GNU Make.

Returns all whitespace-separated words in text that do not match any of the pattern words, removing the words that do match one or more. This is the exact opposite of the filter function.

For example, given:

objects=main1.o foo.o main2.o bar.o
mains=main1.o main2.o 

the following generates a list which contains all the object files not in ‘mains’:

$(filter-out $(mains),$(objects))

That isn't the best way to do it, but if you do it along these lines, write it as a shell condition, not using GNU make conditionals:

$(TMPDIRPATH)%.o: %.cpp
    @echo compile $<
    @if [ $(notdir $<) != main.cpp ]; \
    then $(COMPILE.cpp) $(OUTPUT_OPTION) $<; \
    fi

The continuation markers (backslashes) are needed. So are the semicolons. The values prefixed with $ will be expanded by make before the shell is invoked to interpret them. You probably don't want the echo where it is, either. You probably need:

$(TMPDIRPATH)%.o: %.cpp
    @if [ $(notdir $<) != main.cpp ]; \
    then echo compile $<; \
         $(COMPILE.cpp) $(OUTPUT_OPTION) $<; \
    fi

The way I would expect to do it is with a list of the files to be compiled. Using any wild card mechanism leads to problems when extra files are added - other tests, or stray files that aren't really part of the system.

The comment says "But the GNU Make Manual says ifneq should work".

The ifneq would work if it were positioned correctly, which means 'not indented as part of the commands associated with a rule'. You could, therefore, write something like (an appallingly bad example, but my brain's on the fritz):

ifneq (${CFLAGS}, -Wall)
CFLAGS += -Wall
endif

file1.o: file1.c
    ${CC} ${CFLAGS} -c $<

But when the ifneq is indented as in the question, it is just a command that actually isn't found on the system when the make runs the shell to process the command.

The ifneq line is evaluated only once, when make starts up and parses the makefile. In that context, $< is empty.

To get different behavior for each of the targets matched by your pattern rule, you could do something like

$(TMPDIRPATH)%.o: %.cpp
    @echo compile $<
    @$(if $(filter main.cpp,$<),$(COMPILE.cpp) $(OUTPUT_OPTION) $<)

It might help you to think of the difference between ifneq and $(if) in a makefile as like the difference between #if and if() in C code.

Taking a step back, though: If you don't want main.cpp to be compiled by this rule, then you probably want to provide an explicit rule with $(TMPDIRPATH)main.o as its target, which will be preferred to the pattern rule always. Or, if you don't want$(TMPDIRPATH)main.o to get made at all, you should be looking for rules that have it on the right sight of the :, and removing it from there.

Make doesn't really have a good way to handle conditionals within a rule. You could put the conditional in the command, but in this case there's a much cleaner way:

$(TMPDIRPATH)main.o:
    @echo compile $< (but not really)

$(TMPDIRPATH)%.o: %.cpp
    @echo compile $<
    @$(COMPILE.cpp) $(OUTPUT_OPTION) $<

EDIT:
I didn't realize you didn't have a main.cpp. The solution is simple: remove main.cpp as the prerequisite of the main.o rule (I've removed it above). Now the makefile doesn't need it, and won't try to build it.

But you're still running the rule, which means that something is still trying to build main.o, as either an explicit target or a prerequisite of something else. That is a symptom of confusion, which this change to the makefile will not fix. If you tell us more about the situation, maybe we can propose a better solution. What calls for main.o? Do you have a main.o? What target do you specify when you call Make?