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I'm trying to learn how to write makefiles for my projects. Given that I'm using a text editor like sublime-text and the terminal.

I know how to write a makefile to compile a single file and it's relevant header files IF all files are in the same directory.

Imagine that we have a directory and some sub-directories which are structured like this:

the Headers contains all the .h files Objects is a directory to hold the .o files and Sources is a directory that has the source codes .cpp The Makefile is in the top level directory.

.
├── Headers
│   └── function.h
├── Makefile
├── Objects
│   └── main.o
└── Sources
    ├── function.cpp
    └── main.cpp

I have tried to put together a Makefile by following the GNU make and the answer to this post.

At the moment my Makefile looks like this:

INC_DIR = Headers
SRC_DIR = Sources
OBJ_DIR = Objects
CXXFLAGS = -c -Wall -I.
CC = g++

SRCS = $(SRC_DIR)/*.cpp 
OBJS = $(OBJ_DIR)/*.o

DEPS = $(INC_DIR)/*.h

output: $(OBJ_DIR)/main.o $(OBJ_DIR)/function.o
    $(CC) $^ -o $@

$(OBJ_DIR)/%.o: $(SRC_DIR)/%.cpp $(DEPS)
    $(CC) $(CXXFLAGS)  $< -o $@

clean: 
    rm $(OBJ_DIR)/*.o output

Which will generate the following error when make is executed

g++ -c -Wall -I.  Sources/function.cpp -o Objects/function.o
Sources/function.cpp:2:22: fatal error: function.h: No such file or directory
compilation terminated.
Makefile:30: recipe for target 'Objects/function.o' failed
make: *** [Objects/function.o] Error 1

I know that I'm doing something wrong but can't figure out what and how to fix it.

I'd appreciate any help and guidance.

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Amir
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1 Answers1

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Add Headers to your CXXFLAGS:

 CXXFLAGS = -c -Wall -I. -IHeaders
                       # ^^^^^^^^^

Also you rather might want to generate header dependencies using GCC's -M<x> option family and include these into your Makefile.

Here's a pretty good explanation for the technique from GNU make's documentation:

4.14 Generating Prerequisites Automatically

more info can be also found here:

Makefile (Auto-Dependency Generation)

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πάντα ῥεῖ
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  • what is the `-M` option and how do i use it? – Amir Jul 09 '16 at 13:53
  • I'm trying to learn the -M (Auto prerequisites). So what does `-IHeaders` do exactly in the `CXXFLAGS`? – Amir Jul 09 '16 at 14:07
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    @Amir It adds the directory path for searching header files to GCC's build options. So you can write `#include "function.h"` in your source code and GCC will automatically lookup any directory passed with the `-I` option. – πάντα ῥεῖ Jul 09 '16 at 14:11
  • So if the `Headers` file name changes the `-IHeaders` will change respectively? – Amir Jul 09 '16 at 14:20
  • @Amir If you mean the directory name changes from e.g. `Headers` to `Includes`, then yes you have to change that accordingly. – πάντα ῥεῖ Jul 09 '16 at 14:25