How to generate object files in the build directory?

Question

I am currently working on a C project to implement a Linux userspace application. I wrote this Makefile, which allows me to compile :

PROJ_DIR := ../
SRC_DIR := $(PROJ_DIR)/src
BUILD_DIR := $(PROJ_DIR)/build

# define the executable file
TARGET := tcp_proxy

# include paths list
INC_DIRS := $(SRC_DIR)
INC_DIRS += $(SRC_DIR)/rpmsg
INC_DIRS += $(SRC_DIR)/rpmsg/rx
INC_DIRS += $(SRC_DIR)/rpmsg/tx
INC_DIRS += $(SRC_DIR)/tcp
INC_DIRS += $(SRC_DIR)/tcp/rx
INC_DIRS += $(SRC_DIR)/tcp/tx

INC_FLAGS := $(addprefix -I,$(INC_DIRS))

# source file list
SRCS := $(shell find $(SRC_DIR) -name *.c)

# object file list
OBJS := $(SRCS:.c=.o)

LDFLAGS := -pthread

all: tcp_proxy

tcp_proxy: $(OBJS)
    $(CC) $(CFLAGS) $(INC_FLAGS) -o $(TARGET) $(OBJS) $(LDFLAGS)

%.o: %.c
    $(CC) $(CFLAGS) $(INC_FLAGS) -c $< -o $@

.PHONY: clean

clean:
    $(RM) *.o *~ $(TARGET)

However, the object files are generated in the same place as the source files. So I would like to know how to duplicate the folders under "src" in a "build" directory and generate inside this directory the object files and the executable.

I would also like, when I do a make clean, to be able to delete all the object files and the executable.

My project :

There is no need to write a %.o: %.c rule. make has default rules for common things like this; use them. — William Pursell
– William Pursell, Commented Jan 13, 2022 at 17:08

MadScientist · Accepted Answer · 2022-01-13 15:41:02Z

1

Searching would definitely find you a lot of examples and information on this.

First tell make what objects you want to build; change your setting of OBJS to this:

# object file list
OBJS := $(SRCS:$(SRC_DIR)/%.c=$(OBJ_DIR)/%.o)

Then tell make how to build it; change your pattern rule for building objects to this:

$(OBJ_DIR)/%.o: $(SRC_DIR)/%.c
        @mkdir -p $(@D)
        $(CC) $(CFLAGS) $(INC_FLAGS) -c $< -o $@

answered Jan 13, 2022 at 15:41

MadScientist

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7 Comments

Martin Denion Over a year ago

Thanks for your help. However, it doesn't compile, I get "mkdir: missing operand"

MadScientist Over a year ago

It seems like you must have mistyped something, or else maybe your value of OBJ_DIR variable is something weird? However, that's a very strange error message. What version of Linux are you using? You can try removing the @ from @mkdir so you can see the command line to see what it says.

Martin Denion Over a year ago

I am working on Ubuntu 20.04.3 LTS. When I remove the @ from @mkdir I only see the command mkdir -p. @D seems to be empty.

MadScientist Over a year ago

It's just not possible for the @D variable to be empty. Make sets it to a value before it expands your recipe. Please be sure you haven't typo'd something about the variable name, or added some kind of weird non-printing character, or something like that. What is the value you set your OBJ_DIR variable to? What is the value you set your SRC_DIR variable to? You don't show these in your makefile.

MadScientist Over a year ago

Also, you can try adding the --warn-undefined-variables option when invoking make, and make will tell you if you've used the wrong variable name.

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