c++ - Makefile: How to correctly include header file and its directory?

Question

I have the following makefile:

CC=g++
INC_DIR = ../StdCUtil
CFLAGS=-c -Wall -I$(INC_DIR)
DEPS = split.h

all: Lock.o DBC.o Trace.o

%.o: %.cpp $(DEPS)
    $(CC) -o $@ $< $(CFLAGS)

clean:
    rm -rf *o all

This makefile and all three source files Lock.cpp, DBC.cpp, Trace.cpp are located in the current directory called Core. One of the source file Trace.cpp contains a line that includes a header file outside the current directory:

//in Trace.cpp
#include "StdCUtil/split.h"

The header file split.h is located at one level above the current directory and then in the subdirectory called StdCUtil. So that's why I added INC_DIR = ../StdCUtil in the makefile. The overall directory structure looks like the following:

root
  |___Core
  |     |
  |     |____Makefile
  |     |____DBC.cpp
  |     |____Lock.cpp
  |     |____Trace.cpp
  |
  |___StdCUtil
        |___split.h

But when I make it, it gives me the error:

Trace.cpp:8:28: fatal error: StdCUtil/split.h: No such file or directory
 #include "StdCUtil/split.h"
                            ^
compilation terminated.
<builtin>: recipe for target 'Trace.o' failed

Why this doesn't find the header file split.h even if I specify the INC_DIR in the makefile? How to correct this?

Answer 1

These lines in your makefile,

INC_DIR = ../StdCUtil
CFLAGS=-c -Wall -I$(INC_DIR)
DEPS = split.h

and this line in your .cpp file,

#include "StdCUtil/split.h"

are in conflict.

With your makefile in your source directory and with that -I option you should be using #include "split.h" in your source file, and your dependency should be ../StdCUtil/split.h.

Another option:

INC_DIR = ../StdCUtil
CFLAGS=-c -Wall -I$(INC_DIR)/..  # Ugly!
DEPS = $(INC_DIR)/split.h

With this your #include directive would remain as #include "StdCUtil/split.h".

Yet another option is to place your makefile in the parent directory:

root
  |____Makefile
  |
  |___Core
  |     |____DBC.cpp
  |     |____Lock.cpp
  |     |____Trace.cpp
  |
  |___StdCUtil
        |___split.h

With this layout it is common to put the object files (and possibly the executable) in a subdirectory that is parallel to your Core and StdCUtil directories. Object, for example. With this, your makefile becomes:

INC_DIR = StdCUtil
SRC_DIR = Core
OBJ_DIR = Object
CFLAGS  = -c -Wall -I.
SRCS = $(SRC_DIR)/Lock.cpp $(SRC_DIR)/DBC.cpp $(SRC_DIR)/Trace.cpp
OBJS = $(OBJ_DIR)/Lock.o $(OBJ_DIR)/DBC.o $(OBJ_DIR)/Trace.o
# Note: The above will soon get unwieldy.
# The wildcard and patsubt commands will come to your rescue.

DEPS = $(INC_DIR)/split.h
# Note: The above will soon get unwieldy.
# You will soon want to use an automatic dependency generator.


all: $(OBJS)

$(OBJ_DIR)/%.o: $(SRC_DIR)/%.cpp
  $(CC) $(CFLAGS) -c $< -o $@

$(OBJ_DIR)/Trace.o: $(DEPS)