Add include directories to a target.

target_include_directories(<target> [SYSTEM] [BEFORE]
  [<INTERFACE|PUBLIC|PRIVATE> [items2...] ...])

Specify include directories to use when compiling a given target. The named <target> must have been created by a command such as add_executable() or add_library() and must not be an IMPORTED target.

If BEFORE is specified, the content will be prepended to the property instead of being appended.

The INTERFACE, PUBLIC and PRIVATE keywords are required to specify the scope of the following arguments. PRIVATE and PUBLIC items will populate the INCLUDE_DIRECTORIES property of <target>. PUBLIC and INTERFACE items will populate the INTERFACE_INCLUDE_DIRECTORIES property of <target>. The following arguments specify include directories.

Specified include directories may be absolute paths or relative paths. Repeated calls for the same <target> append items in the order called. If SYSTEM is specified, the compiler will be told the directories are meant as system include directories on some platforms (signalling this setting might achieve effects such as the compiler skipping warnings, or these fixed-install system files not being considered in dependency calculations - see compiler docs). If SYSTEM is used together with PUBLIC or INTERFACE, the INTERFACE_SYSTEM_INCLUDE_DIRECTORIES target property will be populated with the specified directories.

Arguments to target_include_directories may use “generator expressions” with the syntax $<...>. See the cmake-generator-expressions(7) manual for available expressions. See the cmake-buildsystem(7) manual for more on defining buildsystem properties.

Include directories usage requirements commonly differ between the build-tree and the install-tree. The BUILD_INTERFACE and INSTALL_INTERFACE generator expressions can be used to describe separate usage requirements based on the usage location. Relative paths are allowed within the INSTALL_INTERFACE expression and are interpreted relative to the installation prefix. For example:

target_include_directories(mylib PUBLIC
  $<INSTALL_INTERFACE:include/mylib>  # <prefix>/include/mylib

Note that it is not advisable to populate the INSTALL_INTERFACE of the INTERFACE_INCLUDE_DIRECTORIES of a target with paths for dependencies. That would hard-code into installed packages the include directory paths for dependencies as found on the machine the package was made on.

The INSTALL_INTERFACE of the INTERFACE_INCLUDE_DIRECTORIES is only suitable for specifying the required include directories of the target itself, not its dependencies.

That is, code like this is incorrect for targets which will be used to generate cmake-packages(7):

target_include_directories(mylib INTERFACE

Dependencies must provide their own IMPORTED targets which have their own INTERFACE_INCLUDE_DIRECTORIES populated appropriately. Those IMPORTED targets may then be used with the target_link_libraries() command for mylib.

That way, when a consumer uses the installed package, the consumer will run the appropriate find_package() command to find the dependencies on their own machine and populate the IMPORTED targets with appropriate paths. See Creating Packages for more. Note that many modules currently shipped with CMake do not currently provide IMPORTED targets.