make: *** No rule to make target 'obj/main.o', needed by 'myftp'. Stop

February 25, 2023

I’m making an FTP server from scratch in C. I need a makefile to compile.
his is the architecture of my project :

/
    include /
        header file (*.h)
    src /
        potential sub directories /
            *.c
        *.c
    main.c
    Makefile

this is my makefile :

SRC         := main.c $(wildcard src/*.c) $(wildcard src/**/*.c)
OBJ         := $(SRC:%.c=obj/%.o)
DEP         := $(DEP:%.c=dep/%.d)

CC          ?= gcc
CPPFLAGS    := -Iinclude
CFLAGS      :=
LDFLAGS     :=
LDLIBS      :=

all: myftp

myftp: $(OBJ)
    @$(CC) -o $@ $(CFLAGS) $^ $(LDFLAGS) $(LDLIBS)
    @echo -e "\e[1;32mLinked '$@'\e[0m"

obj/%.o dep/%.d: src/%.c
    mkdir -p $(@D) $(@D:obj/%=dep/%)
    @$(CC) -c -o 'obj/$*.o' $(CFLAGS) $(CPPFLAGS) -MD -MF 'dep/$*.d' -MQ 'obj/$*.o' $<
    @echo -e "\e[32mBuilt '$@'\e[0m"

clean:
    @echo -ne '\e[31m'
    @rm -vr obj dep
    @echo -ne '\e[0m'

fclean: clean
    @echo -ne '\e[1;31m'
    @rm -v my_ftp
    @echo -ne '\e[0m'

re: fclean all

-include $(DEP)

When I do the make command this is the error message :

make: *** No rule to make target 'obj/main.o', needed by 'myftp'. Stop.

I try to modify the line :
OBJ := $(SRC:%.c=obj/%.o)
like this :
OBJ := $(SRC:%.c=%.o).

With this modification, the compilation work but all the .o and .d files are in the same directories as the .c file and not in other directories that recreate the project’s architecture but only for the .o and .d files. Like that :

/
    obj /
        main.o
        src /
            the other .o files
    dep /
        main.d
        src /
            the other .d files
    include /
        header file (*.h)
    src /
        potential sub directories /
            *.c
        *.c
    main.c
    Makefile

>Solution :

The rule

obj/%.o dep/%.d: src/%.c

only is for the source files in the src directory. But main.c isn’t in src so it’s not included in that rule.

You need to create a rule for main.c as well:

obj/main.o dep/main.o: main.c

Also note that your current rule for source files also doesn’t include subdirectories of src.

And unless creating an actual Makefile yourself is part of your assignment or exercise, then I recommend you use tools like CMake or Meson or similar. They will make things like handling multiple directories and dependencies much easier.