cmake: add header dir to target

[russkel]

This simplifies use of the runtime in CMake projects. With this mod it's now possible to simply do:

add_subdirectory(external/kaitai_struct_cpp_stl_runtime)

add_executable(myproj main.cpp etcetcetc)
target_link_libraries(myproj PRIVATE
  etc etc etc et
  kaitai_struct_cpp_stl_runtime
)

Before I was getting this error, due to it not finding the includes correctly.

In file included from ksy/myproj_protocol.cpp:3:
ksy/myproj_protocol.h:6:10: fatal error: kaitai/kaitaistruct.h: No such file or directory
    6 | #include "kaitai/kaitaistruct.h"
      |          ^~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
[2/18] Building CXX object src/CMakeFiles/myproj.dir/myproj.cpp.o
FAILED: src/CMakeFiles/myproj.dir/myproj.cpp.o 
/usr/bin/c++  -I../external/PcapPlusPlus/Dist/header -Iksy -I../ -g -std=c++17 -MD -MT src/CMakeFiles/myproj.dir/myproj.cpp.o -MF src/CMakeFiles/myproj.dir/myproj.cpp.o.d -o src/CMakeFiles/myproj.dir/myproj.cpp.o -c ../src/myproj.cpp
../src/myproj.cpp:5:10: fatal error: kaitai/kaitaistream.h: No such file or directory
    5 | #include <kaitai/kaitaistream.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.

[LHLaurini]

Would you mind rebasing this pull request? Maybe we can get a maintainer to merge it.

[generalmimon]

Can you explain why you believe that CMAKE_CURRENT_SOURCE_DIR is the right variable to use here?

Some people use CMAKE_CURRENT_LIST_DIR in this context instead, see https://stackoverflow.com/a/40242257/12940655. And the official CMake tutorial uses PROJECT_SOURCE_DIR, see https://cmake.org/cmake/help/latest/guide/tutorial/Adding%20a%20Library.html#build-and-run:

Finally we need to specify the library's header file location. Modify target_include_directories() to add the MathFunctions subdirectory as an include directory so that the MathFunctions.h header file can be found.

TODO 4: CMakeLists.txt

target_include_directories(Tutorial PUBLIC
                          "${PROJECT_BINARY_DIR}"
                          "${PROJECT_SOURCE_DIR}/MathFunctions"
                          )

Intuitively, PROJECT_SOURCE_DIR sounds most predictable and robust to me and it's probably what we actually want (given that the official tutorial uses it too). The word "current" in CMAKE_CURRENT_* variable names suggests that the contents of these variables may change under some circumstances (which might be some other CMake way to include kaitai_struct_cpp_stl_runtime to someone else's application, I don't really know) and that could break things.

[russkel]

Honestly I have never seen PROJECT_SOURCE_DIR in the wild. If it works then great. Like with everything in Cmake there is 1e6 different ways to do the same thing.

[LHLaurini]

In this case, all three are equivalent. How I understand it:

• CMAKE_CURRENT_SOURCE_DIR is the source directory being processed. It is "current" because that can change with add_subdirectory;
• CMAKE_CURRENT_LIST_DIR is the directory containing the current CMake file. In this case, the listing is in the source dir;
• PROJECT_SOURCE_DIR is the source directory from the last call to project.

So, IMO using either CMAKE_CURRENT_SOURCE_DIR or PROJECT_SOURCE_DIR would be sensible (and equivalent) choices, but I do agree that PROJECT_SOURCE_DIR may be clearer.

[russkel]

Would you mind rebasing this pull request? Maybe we can get a maintainer to merge it.

I don't have anything to test it on anymore given this was 3 years ago.

[generalmimon]

@russkel @LHLaurini Thank you both!