What does "linker input file unused because linking not done" mean? (C makefile)

Question

I have created a makefile to compile and link my program, however, I can't figure out why I am getting this error. Is it to do with SDL?

GCC = gcc
CFLAGS = -c -std=c99 -lm -Wall -Wextra -pedantic -O3 -Wfloat-equal -g
SDL = -lSDL2 -lSDL2_ttf -lSDL2_image -lSDL2_mixer

all: ./game

game: global.o display.o player.o entities.o controls.o sound.o menu.o
    $(GCC) $(CFLAGS) global.o display.o player.o entities.o controls.o sound.o menu.o -o game

global.o: global.c
    $(GCC) $(CFLAGS) $(SDL) global.c

display.o: display.c
    $(GCC) $(CFLAGS) $(SDL) display.c

player.o: player.c
    $(GCC) $(CFLAGS) $(SDL) player.c

entities.o: entities.c
    $(GCC) $(CFLAGS) $(SDL) entities.c

controls.o: controls.c
    $(GCC) $(CFLAGS) $(SDL) controls.c

sound.o: sound.c
    $(GCC) $(CFLAGS) $(SDL) sound.c

menu.o: menu.c
    $(GCC) $(CFLAGS) $(SDL) menu.c

clean:
    rm *o game

Answer 1

Your linking command expands to:

gcc -c -std=c99 -lm -Wall -Wextra -pedantic -O3 -Wfloat-equal -g global.o display.o player.o entities.o controls.o sound.o menu.o -o game

which, as you can see, has the -c flag in it. The -c flag tells gcc not to do linking. So it has nothing to actually do. (.o files can only be used for linking, and you've disabled linking, which is why you get that message)

You don't want to use the same flags for compiling and linking. For compiling you probably want -c -std=c99 -Wall -Wextra -pedantic -O3 -Wfloat-equal -g, and for linking you want -lm -lSDL2 -lSDL2_ttf -lSDL2_image -lSDL2_mixer -g.