makefile error: undefined reference to main

Question

I am trying to compile my piece of code using make. Normally I compile my code like this:

mipsisa32r2el-timesys-linux-gnu-g++ -o testing -I/usr/include/libxml2 -L/develop/xc4/rootfs/parsecpp/lib -L/develop/xc4/rootfs/parsecpp/sqlite-mips2/lib -I/develop/xc4/rootfs/parsecpp/sqlite-mips2/include db.cpp main.cpp networkinterfacemodule.cpp network.cpp multiplex.cpp program.cpp service.cpp -lsqlite3 -lxml2

To get rid of this long command I tried to write a makefile:

CC= mipsisa32r2el-timesys-linux-gnu-g++

export LD_LIBRARY_PATH=:/parsecpp/sqlite-mips2/lib:/parsecpp/lib:/tmp/vixs_temp/DirectFB/single_core/lib


CFLAGS=-I/usr/include/libxml2 -I/develop/xc4/rootfs/parsecpp/sqlite-mips2/include

LDFLAGS=-L/develop/xc4/rootfs/parsecpp/lib -L/develop/xc4/rootfs/parsecpp/sqlite-mips2/lib

LIBS = -lsqlite3 -lxml2

PROG=testing

all: main.o db.o mod.o multiplex.o network.o networkinterfacemodule.o program.o service.o
    $(CC) -o $(PROG) $(CFLAGS) $(LDFLAGS)  main.o db.o mod.o multiplex.o network.o networkinterfacemodule.o program.o service.o $(LIBS) 

main.o: main.cpp 
    $(CC) $(CFLAGS) $(LDFLAGS) main.cpp db.cpp networkinterfacemodule.cpp mod.cpp multiplex.cpp network.cpp program.cpp service.cpp $(LIBS)

db.o: db.cpp 
    $(CC) $(CFLAGS) $(LDFLAGS) db.cpp $(LIBS)


mod.o: mod.cpp
    $(CC) $(CFLAGS) $(LDFLAGS) mod.cpp $(LIBS)

multiplex.o: multiplex.cpp
    $(CC) $(CFLAGS) $(LDFLAGS) multiplex.cpp $(LIBS)

network.o: network.cpp
    $(CC) $(CFLAGS) $(LDFLAGS) network.cpp $(LIBS)

networkmoduleinterface.o: networkinterfacemodule.cpp
    $(CC) $(CFLAGS) $(LDFLAGS) networkinterfacemodule.cpp $(LIBS)

program.o: program.cpp
    $(CC) $(CFLAGS) $(LDFLAGS) program.cpp $(LIBS)

service.o: service.cpp
    $(CC) $(CFLAGS) $(LDFLAGS) service.cpp $(LIBS)

clean:
    rm -rf *o testing

Then I get this error:

/opt/timesys/linux-gnu/toolchain/bin/../../toolchain/lib/crt1.o: In function `__start':
(.text+0xc): undefined reference to `main'
/opt/timesys/linux-gnu/toolchain/bin/../../toolchain/lib/crt1.o: In function `__start':
(.text+0x10): undefined reference to `main'
collect2: ld returned 1 exit status
make: *** [db.o] Error 1

Can anybody help me?

Answer 1

Whenever you are just compiling a file and not linking it, use the "-c" flag.

For example :-

db.o: db.cpp 
$(CC) -c $(CFLAGS) $(LDFLAGS) db.cpp $(LIBS)

Also, while compiling, there's no need to provide "$(LIBS)" to the compiler, only provide them when linking. Nor do you need the linker flags since linker is not called when using the "-c" flag.

So you could write,

 db.o: db.cpp 
 $(CC) -c $(CFLAGS) db.cpp

---

UPDATED (based on comments):-

When linking the files, the linker expects one and only one main function. In the above case, the main function is not defined in db.cpp and hence although compilation succeeds, the linker throws an error as it cannot find the main function.

Answer 2

When you compile without the -c flag, gcc tries to link the program. Since the main function is most probably in the main.c and not in db.c, the linker fails when searching for the main function in db.c. This means that you need to tell the compiler that you just want output that is not yet linked, but translated into an object file and this is exactly what the -c flag does.

this results in what rajatkhanduja said:

db.o: db.cpp 
$(CC) -c $(CFLAGS) db.cpp