CODEWITHSUNDEEP
c++makefile
Namelikme Saiyotan
Namelikme Saiyotan

Reputation: 25

Compiling a multiple cpp files Error undefined reference to `main'

I am currently learning how to use makefile and currently dividing my main.cpp to multiple files.

here is my current structure

├── makefile
└── src
    ├── Graphics
    │   ├── Graphics.cpp
    │   └── Graphics.h
    └── main.cpp

and my makefile

CC = g++
CFLAGS = -c
LFLAGS = -Wall
OUTPUT = game

game : main.o Graphics.o
                $(CC) $(LFLAGS) main.o Graphics.o -lSDL2 -lSDL2_image -o $(OUTPUT)
main.o : src/main.cpp
                $(CC) $(CFLAGS) src/main.cpp
Graphics.o : src/Graphics/Graphics.cpp
                $(CC) $(CFlAGS) src/Graphics/Graphics.cpp -lSDL2

Everytime I compile. I always get this Error. But I have my main declared. is it looking for a main inside my Graphics.cpp?

/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_line): relocation 0 has invalid symbol index 2
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/crt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
make: *** [Graphics.o] Error 1

Here is my main function

#include "Graphics/Graphics.h"

int main(int argc , const char * argv[]){
  Graphics graphics;
  while(true){
    //do things here later
  }
  return 0;
}

//Edit.

Btw. is there also a way not to link sdl2 on Graphics.cpp, If remove -lSDL2. it would give me all that sdl2 function undefined

Upvotes: 1

Views: 1402

Answers (2)

Filip Roséen
Filip Roséen

Reputation: 63892

You have a typo in your makefile; $(CFlAGS) is not the same as $(CFLAGS), meaning that -c is not passed as a command-line option to the compiler when trying to compile src/Graphics/Graphics.cpp.

CC = g++
CFLAGS = -c
LFLAGS = -Wall
OUTPUT = game

game : main.o Graphics.o
                $(CC) $(LFLAGS) main.o Graphics.o -lSDL2 -lSDL2_image -o $(OUTPUT)
main.o : src/main.cpp
                $(CC) $(CFLAGS) src/main.cpp
Graphics.o : src/Graphics/Graphics.cpp
                $(CC) $(CFlAGS) src/Graphics/Graphics.cpp -lSDL2

Variables in makefiles are case-sensitive, which means that even though you wanted to include the contents of CFLAGS you instead got the contents of an undeclared variable named CFlAGS.

Note: Without -c you are telling g++ to run the linker, and as such it will look for a function named main so that it can produce an executable; graphics.cpp does not contain such function, and the diagnostic you have presented is emitted.

Note: After you have fixed the issue with $(CFlAGS) you will be able to remove -lSDL2 from the relevant line.

Upvotes: 2

melpomene
melpomene

Reputation: 85887

In your error message it says:

make: *** [Graphics.o] Error 1

That means this error occurred while compiling Graphics.o, not while linking game.

Your command for compiling Graphics.o is

   $(CC) $(CFlAGS) src/Graphics/Graphics.cpp -lSDL2

-lSDL2 doesn't make sense here: You want to compile an object file, so the linker is not involved. But you're getting linker errors - why?

Because to compile without linking, you need the -c option. This is part of your CFLAGS variable. But you're not using CFLAGS, you're using CFlAGS (lowercase l). So I bet the command that make is running looks like

g++  src/Graphics/Graphics.cpp -lSDL2

without -c.

Fix: remove -lSDL2, capitalize L in variable name.

Upvotes: 1

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