How to properly add include directories with CMake

About a year ago I asked about header dependencies in CMake.

I realized recently that the issue seemed to be that CMake considered those header files to be external to the project. At least, when generating a Code::Blocks project the header files do not appear within the project (the source files do). It therefore seems to me that CMake consider those headers to be external to the project, and does not track them in the depends.

A quick search in the CMake tutorial only pointed to include_directories which does not seem to do what I wish...

What is the proper way to signal to CMake that a particular directory contains headers to be included, and that those headers should be tracked by the generated Makefile?

Solution 1:

Two things must be done.

First add the directory to be included:

target_include_directories(test PRIVATE ${YOUR_DIRECTORY})

In case you are stuck with a very old CMake version (2.8.10 or older) without support for target_include_directories, you can also use the legacy include_directories instead:

include_directories(${YOUR_DIRECTORY})

Then you also must add the header files to the list of your source files for the current target, for instance:

set(SOURCES file.cpp file2.cpp ${YOUR_DIRECTORY}/file1.h ${YOUR_DIRECTORY}/file2.h)
add_executable(test ${SOURCES})

This way, the header files will appear as dependencies in the Makefile, and also for example in the generated Visual Studio project, if you generate one.

How to use those header files for several targets:

set(HEADER_FILES ${YOUR_DIRECTORY}/file1.h ${YOUR_DIRECTORY}/file2.h)

add_library(mylib libsrc.cpp ${HEADER_FILES})
target_include_directories(mylib PRIVATE ${YOUR_DIRECTORY})
add_executable(myexec execfile.cpp ${HEADER_FILES})
target_include_directories(myexec PRIVATE ${YOUR_DIRECTORY})

Solution 2:

First, you use include_directories() to tell CMake to add the directory as -I to the compilation command line. Second, you list the headers in your add_executable() or add_library() call.

As an example, if your project's sources are in src, and you need headers from include, you could do it like this:

include_directories(include)

add_executable(MyExec
  src/main.c
  src/other_source.c
  include/header1.h
  include/header2.h
)

Solution 3:

Add include_directories("/your/path/here").

This will be similar to calling gcc with -I/your/path/here/ option.

Make sure you put double quotes around the path. Other people didn't mention that and it made me stuck for 2 days. So this answer is for people who are very new to CMake and very confused.