Linking not done in Makefile

Question

I tried to make a Makefile using files main.cpp, factorial.cpp, hello.cpp and function.h On typing 'make' on Linux command window, it shows:

g++ -c -o hello main.o factorial.o hello.o
g++: main.o: linker input file unused because linking not done
g++: factorial.o: linker input file unused because linking not done
g++: hello.o: linker input file unused because linking not done

I am making the Makefile for first time. Please give suggestions what can be the problem? The Makefile contains following code->

hello: main.o factorial.o hello.o
        g++ -c -o hello main.o factorial.o hello.o

main.o: main.cpp
        g++ -c -o main.o main.cpp

factorial.o: factorial.cpp
        g++ -c -o factorial.o factorial.cpp

hello.o: hello.cpp
        g++ -c -o hello.o hello.cpp

The individual file contents if you want to see are: 1) main.cpp

#include<iostream>
#include"functions.h"
using namespace std;
int main()
{
    print_hello();
    cout << endl;
    cout << "The factorial of 5 is " << factorial(5) << endl;
    return 0;
}

2) hello.cpp

#include<iostream>
#include "functions.h"
using namespace std;

void print_hello()
{
    cout << "Hello World!";
}

3) factorial.cpp

#include "functions.h"

int factorial(int n)
{
    if(n!=1)
    {
        return(n * factorial(n-1));
    }
    else return 1;
}

4) function.h

 void print_hello();  
 int factorial(int n);

Answer 1

The -c parameter to g++ tells it not to link:

-c Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file.

You definitely don't want the -c here:

hello: main.o factorial.o hello.o
        g++ -c -o hello main.o factorial.o hello.o

Answer 2

You could also use rules and patterns to make it more generic:

SRC_DIR = ./src
OBJ_DIR = ./bin/obj
BIN_DIR = ./build/bin

# List all the sources
SRCS = A.cpp B.cpp

# Define the rule to make object file from cpp
$(OBJ_DIR)/%.o : $(SRC_DIR)/%.cpp
    g++ -o $@ $(INCLUDES) $(CPPFLAGS) -c $^

TARGET_BIN = $(BIN_DIR)/test

all : make_dirs $(TARGET_BIN)

$(TARGET_BIN) : $(SRCS:%.cpp=$(OBJ_DIR)/%.o)
    g++ $(LDFLAGS) -o $@ $^ $(LDLIBS)

make_dirs :
    mkdir -p $(OBJ_DIR)
    mkdir -p $(BIN_DIR)

With this approach you have several benefits:

• Easy to use: you specify source files once, and you don't care about processing of each object file: the job is done by a single rule.

• More maintainable: every time you need to change the compiler or linker options you do it in the single rule, not for each translation unit.