I'm in a situation where I should not disturb the existing CMakeLists.txt files, but I still should add some g++ system include directory to my build.
In other words, I need -isystem /path/to/my/include added to my compiler flags, but when calling something like cmake ...
Maybe something like cmake .. -DCMAKE_CXX_FLAGS="$CMAKE_CXX_FLAGS -isystem /path/to/my/include"? Is there a way to do this?
I have the same problem. I found two solutions:
The one proposed by sakra in a previous answer, i.e. setting an environment variable with C++ flags:
export CXXFLAGS=-isystem\ /path/to/my/include
cmake <path to my sources>
OR the same thing, but environment variable are set only for this CMake call:
CXXFLAGS=-isystem\ /path/to/my/include cmake <path to my sources>
IMPORTANT: you must clean your build directory (i.e. clean the CMake cache) before launching any of this form. Without cleaning the cache, CMake will continue using your cached CMAKE_CXX_FLAGS from the previous run.
Directly setting CMAKE_CXX_FLAGS in cmake string:
cmake -DCMAKE_CXX_FLAGS=-isystem\ /path/to/my/include <path to my sources>
I believe that it can be done by a more 'native' way, but I didn't find a variable responsible for paths to headers in CMake.
-I way, i. e. -isystem path0 -isystem path1)
You can set the environment variable CXXFLAGS before invoking CMake.
$ export CXXFLAGS=-isystem\ /path/to/my/include
$ cmake ..
CMake will the initialize the cache variable CMAKE_CXX_FLAGS with the flags from the environment variable. The variable affects all build types.
export form: CXXFLAGS=-isystem\ /path/to/my/include cmake ..Using -DCMAKE_CXX_STANDARD_INCLUDE_DIRECTORIES=<something> worked for me even without toolchain file. This avoids cluttering compiler flags.
Just an additional note to the other answers: with CMake 3.15.3 on macOS 10.14.5, only the solution using the CMake flag seems to work properly.
So, in my case, only this solution worked fine:
cmake -DCMAKE_CXX_FLAGS=-I\ /path/to/include <path/to/source>
or
cmake -DCMAKE_C_FLAGS=-I\ /path/to/include <path/to/source>
Another choice: https://cmake.org/cmake/help/latest/command/project.html#code-injection
For example
There has a empty file hdr/myhdr.h
The file hdr/inject.cmake below:
# hdr/inject.cmake
include_directories(hdr)
And the main CMakeLists.txt below:
# CMakeList.txt
cmake_minimum_required(VERSION 3.15)
project(hello CXX)
add_executable(main main.cpp)
The main.cpp below:
#include "myhdr.h"
int main()
{
return 0;
}
When we using the command cmake . -DCMAKE_PROJECT_INCLUDE=hdr/inject.cmake, we can let include_directories(hdr) to apply on all projects (project(hello) also) in CMakeLists.txt .
$CMAKE_CXX_FLAGS, would reference a shell variable. Probably not what you would want. Actually, it should be useless on the command line.