CMake set_target_properties and system include directories

Question

When using CMake's include_directories command, there is a way to specify whether a given directory is a system include directory.

For various reasons, though, I have to resort to using set_target_properties to specify different include paths for different targets in the same scope, by setting INCLUDE_DIRECTORIES property of the target.

The problem is, however, that I could not figure out how to tell CMake that a directory is a system directory so it uses -isystem instead of -I when possible. Mainly because the property is simply a list of directory paths and does not have any flags.

I thought there could be SYSTEM_INCLUDE_DIRECTORIES, but I could not find any mention of that.

Any thoughts on how to go about this?

Answer 1

The correct way to add system directories is to use the SYSTEM option of the include_directories command:

include_directories(SYSTEM "/foo/bar")

If include_directories is not an option, you can specify a system include directory by directly modifying the target's COMPILE_FLAGS property:

set_target_properties(main PROPERTIES APPEND_STRING PROPERTY 
    COMPILE_FLAGS " ${CMAKE_INCLUDE_SYSTEM_FLAG_CXX} /foo/bar")

The variable CMAKE_INCLUDE_SYSTEM_FLAG_CXX usually resolves to -isystem on UNIX systems.