Undefined reference to 'main' error in crt1.o function _start

hjazz6 · 04-13-2011, 03:34 AM

Hi all,

I've having problems with my Makefile.

I'm trying to create a program from 2 files - main.cpp that contains the main function, and modules.c that contains the definitions of the functions that are called in main(). modules.c only contain function definitions, no main function.

My Makefile is as follows:

Code:

CC := gcc
CXX := g++
LINK := g++ -Wall
CFLAGS := -g 
CXXFLAGS := -g 
    
TARGET = program
    
$(TARGET): modules.o main.o
   $(LINK) -o $@ $< -lpcap
    
clean:
   rm *.o $(TARGET)
    
modules.o:
   $(CC) $(CFLAGS) -c modules.c -o $@ $<
    
main.o:
   $(CXX) $(CXXFLAGS) -c main.cpp -o $@ $<

I have included "modules.h", which contains all the function declarations, in my main.cpp.

When I try to make using this Makefile, I get the error

Code:

/usr/lib/gcc/x86_64-redhat-linux/4.4.4/../../../../lib64/crt1.o: In function '_start':
    (.text+0x20): undefined reference to 'main'

If I switch the order of modules.o and main.o in my $(TARGET) line, then I get errors that say "undefined reference to" the functions I have defined in modules.c, in main.cpp.

I don't know what is wrong.

Thank you.

Regards,
Rayne

dwhitney67 · 04-13-2011, 06:19 AM

The issue is with the Makefile, when you attempt to link the object files. You are not obtaining both object files because you are using $<. You should have used $^.

There's some other minor issues with the Makefile; for example, -Wall is a compiler option, and thus is not applicable when linking. Also you should use LDFLAGS to specify the linker options, versus hard coding them when linking your program. You can also simplify your rules with regards to compiling your source files, or omit them altogether. Note that "make" has built-in rules that oftentimes suffice for most projects; thus there is no need to redefine the rules.

Here's a simple Makefile that will satisfy your goals:

Code:

TARGET   = program

CFLAGS   = -g -Wall
CXXFLAGS = -g -Wall
LDFLAGS  = -lpcap


$(TARGET) : modules.o main.o
        $(CXX) -o $@ $^ $(LDFLAGS)

clean :
        $(RM) *.o $(TARGET)

P.S. I never specify a wildcard string when removing files; a simple typo can cost you a lot, especially if you remove special files. Consider the following:

Code:

TARGET   = program

C_SRCS   = modules.c
CXX_SRCS = main.cpp

OBJS     = $(C_SRCS:.c=.o)
OBJS    += $(CXX_SRCS:.cpp=.o)

CFLAGS   = -g -Wall
CXXFLAGS = -g -Wall
LDFLAGS  = -lpcap


$(TARGET) : $(OBJS)
        $(CXX) -o $@ $^ $(LDFLAGS)

clean :
        $(RM) $(OBJS) $(TARGET)

dunlopjp · 07-28-2011, 01:53 PM

I'm getting a similar error when attempting to build OpenFOAM 1.7.x.

14:41:39$ g++ -DlinuxIA64 -DWM_DP -Wextra -Wno-unused-parameter -Wold-style-cast -O3 -DNoRepository -ftemplate-depth-40 -IlnInclude -I. -I/app2/openfoam/platforms/linux.altix/OpenFOAM-1.7.x/src/OpenFOAM/lnInclude -I/app2/openfoam/platforms/linux.altix/OpenFOAM-1.7.x/src/OSspecific/POSIX/lnInclude -fPIC -Xlinker --add-needed Make/linuxIA64GccDPOpt/ansysToFoam.o -L/app2/openfoam/platforms/linux.altix/OpenFOAM-1.7.x/lib/linuxIA64GccDPOpt -lOpenFOAM -ldl -lm -o /app2/openfoam/platforms/linux.altix/OpenFOAM-1.7.x/applications/bin/linuxIA64GccDPOpt/ansysToFoam
/usr/lib/crt1.o: In function `_start':
init.c

.text+0x41): undefined reference to `main'
collect2: ld returned 1 exit status
14:41:52$

And another attempt with the -Wall argument

14:49:50$ g++ -DlinuxIA64 -DWM_DP -Wall -Wextra -Wno-unused-parameter -Wold-style-cast -O3 -DNoRepository -ftemplate-depth-40 -IlnInclude -I. -I/app2/openfoam/platforms/linux.altix/OpenFOAM-1.7.x/src/OpenFOAM/lnInclude -I/app2/openfoam/platforms/linux.altix/OpenFOAM-1.7.x/src/OSspecific/POSIX/lnInclude -fPIC -Xlinker --add-needed Make/linuxIA64GccDPOpt/ansysToFoam.o -L/app2/openfoam/platforms/linux.altix/OpenFOAM-1.7.x/lib/linuxIA64GccDPOpt -lOpenFOAM -ldl -lm -o /app2/openfoam/platforms/linux.altix/OpenFOAM-1.7.x/applications/bin/linuxIA64GccDPOpt/ansysToFoam
/usr/lib/crt1.o: In function `_start':
init.c

.text+0x41): undefined reference to `main'
collect2: ld returned 1 exit status
14:51:05$

Any assistance would be greatly appreciated.

Thanks
Jim Dunlop

knudfl · 07-30-2011, 06:51 PM

Post #3, @dunlopjp

Which Linux OS are you using ? Like Fedora 15, CentOS 6, etc.

And "g++ -DlinuxIA64" looks like you are compiling for ia64.
Is that correct ?

dunlopjp · 08-01-2011, 07:06 AM

08:04:11$ uname -a
Linux hawk-0 2.6.16.60-0.66.1-default #1 SMP Fri May 28 12:10:21 UTC 2010 ia64 ia64 ia64 GNU/Linux

Yes it's ia64

The distribution is SuSE