Adding include directories to CMake when calling it from the command line

Question

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?

so you want to add compilation flags and then remove them? Or do you just want to add them? — Marco A., Sep 15 '14 at 13:57
Note that the variable reference, `$CMAKE_CXX_FLAGS`, would reference a shell variable. Probably not what you would want. Actually, it should be useless on the command line. — Alexis Wilke, May 28 '15 at 07:52

score 35 · Accepted Answer · edited Feb 14 '19 at 19:24

35

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.

edited Feb 14 '19 at 19:24

Peter Mortensen

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answered Feb 25 '16 at 17:05

avtomaton

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That IMPORTANT tip saved my sanity! – Ryan H. Aug 06 '19 at 01:52
what if i need multiple directories added to include path from command line? – maximus Aug 30 '19 at 06:16
1

You can pass more than one -isystem parameter (like using a usual `-I` way, i. e. `-isystem path0 -isystem path1`) – avtomaton Aug 30 '19 at 12:04
Nothing works from this – mblw Dec 21 '22 at 09:05

score 17 · Answer 2 · answered Sep 15 '14 at 15:02

17

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.

answered Sep 15 '14 at 15:02

sakra

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or without `export` form: `CXXFLAGS=-isystem\ /path/to/my/include cmake ..` – avtomaton Feb 25 '16 at 16:35

score 11 · Answer 3 · answered Feb 26 '20 at 12:40

11

Using -DCMAKE_CXX_STANDARD_INCLUDE_DIRECTORIES=<something> worked for me even without toolchain file. This avoids cluttering compiler flags.

answered Feb 26 '20 at 12:40

scrutari

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I don't think you should override this. It is meant to be set by toolchain files. If you override it you might end up not being able to find standard C++ headers. – Timmmm Jun 16 '20 at 11:31
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This still can be used as a workaround for loading Cmake-based projects into IDEs. Also when LLVM toolchain is used, this option helps force Cmake to see standard headers from LLVM rather than those installed in the system (typically from GNU toolchain of a different version). – scrutari Jun 16 '20 at 12:01

score 3 · Answer 4 · answered Sep 27 '19 at 07:47

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>

Adding include directories to CMake when calling it from the command line

4 Answers4

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