C/GTK - Makefile error: undefined reference to main when invoking make

I am trying to write a Makefile with a C file that utilizes GTK3.0 but I am running into the following:

/usr/bin/ld: /usr/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../lib/Scrt1.o: in function `_start':
(.text+0x1b): undefined reference to `main'
collect2: error: ld returned 1 exit status
make: *** [makefile:13: sudoku] Error 1

Here is the Makefile:

CC := gcc
PKGCONFIG = $(shell which pkg-config)
CFLAGS = $(shell $(PKGCONFIG) --cflags gtk+-3.0)
LIBS = $(shell $(PKGCONFIG) --libs gtk+-3.0)

SRC = src/sudoku.c
OBJS = bin/sudoku.o

$(OBJS): $(SRC)
    $(CC) -c $(CFLAGS) -o $(OBJS) $(SRC)

sudoku: $(OBJS)
    $(CC) -o $(OBJS) $(LIBS)

.PHONY: clean

clean:
    rm -f $(OBJS)
    rm -f sudoku

Not entirely sure what is the problem here. According to some solutions online, the source code may not have a main function but a main function is clearly defined in my src/sudoku.c file.

Another issue is that the sudoku.o file is placed incorrectly into the same directory that the makefile is in (I want it to be placed into the bin directory).

Solution

This rule:

sudoku: $(OBJS)
    $(CC) -o $(OBJS) $(LIBS)

is wrong, because you have omitted the argument to the -o compiler option. As a result, the first (and in this case, only) file among those designated by $(OBJS) is taken as the name of the output file instead of one to include in the link. You presumably wanted this, instead:

sudoku: $(OBJS)
    $(CC) -o $@ $(OBJS) $(LIBS)

($@ expands to the name of the target being built).

Additionally, although this rule will work well enough when there is only one source file:

$(OBJS): $(SRC)
    $(CC) -c $(CFLAGS) -o $(OBJS) $(SRC)

, it will break if there are two or more. I would actually remove it altogether, as it does not do anything that make's built-in rule for constructing object files from C source files doesn't also do.