Question

I have the following makefile which is a slight modification on others that I have used in the past. There is an odd issue though with my variable ${CXXOPTS} not being used in the .c.o makefile rule. When I execute the makefile, this is what is executed g++ -c -o SeqPrep2.o SeqPrep2.cpp when I expect this to be executed: g++ -Iseqan-03-02-2012 -c -o SeqPrep2.o SeqPrep2.cpp. (Edit: I solved the problem but see my updated question below about why this makefile worked at all in the first place)

L=-lm -lz
SEQANINC=seqan-03-02-2012
DESTDIR=$(HOME)/
BINDIR=bin
CXXOPTS=-I${SEQANINC}
CXX=g++
A=SeqPrep2
USEROPTS=
O=$(patsubst %.cpp,%.o,$(wildcard *.cpp))
SOURCES=$(wildcard *.cpp)

all: ${A} ${O} ${SOURCES}

install: ${O} ${MYLIBS} ${SOURCES}
    ${CXX} ${USEROPTS} -o ${DESTDIR}${BINDIR}/${A} ${O} ${L}

${A}: ${O} ${MYLIBS} ${SOURCES}
    ${CXX} ${USEROPTS} -o ${A} ${O} ${L}

clean::
    rm -f ${A} ${O}

.c.o:
    ${CXX} ${CXXOPTS} ${USEROPTS} -c  $< -o $@

check-syntax:
    ${CXX} ${CXXOPTS} ${USEROPTS} -c -o .nul -S ${CHK_SOURCES}

UPDATE:

I changed .c.o to .cpp.o. Is this a case of gnu make guessing that when I asked for a .o file in one of my rules, that it should make it just by running g++ -c -o SeqPrep2.o SeqPrep2.cpp even though I didn’t tell it to do that? I guess that is my new question, why did the above makefile work at all, and why did it have the odd behavior I observed. One thing to note is that even though it didn’t come though in the formatting, there is a lot of white space between the g++ and the -c, kind of like it was trying to put in my variables, but it didn’t. That is partially what originally lead me to believe that it was seeing my rule at all, even though it seems like it didn’t now. Thanks for helping me understand how this stuff works.

Answer 1

You’ve got the rule .c.o but you’re compiling cpp files, when I use .cpp.o: it works great! With files: me.cpp us.cpp you.cpp

I get:

Building file me.cpp
g++ -Iseqan-03-02-2012  -c  me.cpp -o me.o
Building file us.cpp
g++ -Iseqan-03-02-2012  -c  us.cpp -o us.o
Building file you.cpp
g++ -Iseqan-03-02-2012  -c  you.cpp -o you.o
g++  -o SeqPrep2 me.o us.o you.o -lm -lz