Transitive include directories

Code
1

I’ve lost something in translation when reading this bit of documentation on Transitive Usage Requirements in cmake-buildsystem(7):

Usage requirements are propagated by reading the INTERFACE_ variants of target properties from dependencies and appending the values to the non-INTERFACE_ variants of the operand. For example, theINTERFACE_INCLUDE_DIRECTORIES of dependencies is read and appended to the INCLUDE_DIRECTORIES of the operand.

From this, I would expect that if I had 2 targets, libA and libB, and libA specified a PUBLIC or INTERFACE include dir using target_include_directory(libA PUBLIC includeA), if I were to specify

add_library(libB b.c)
target_link_libraries(libB INTERFACE libA)

that the source file b.c would get compiled with -I/path/to/includeA.

I don’t observe this and I don’t understand why. Please explain?

Thanks!

PS - as an aside, I’m trying to use INTERFACE for just the headers, as I would want libB to dlsym lookup the symbols from libA (or any other libA equivalent interface library).

2

You would need to use PUBLIC libA. INTERFACE libA only applies to consumers of libB, not parts of libB itself.

3

I don’t know if it’s conventional, but I found a solution that fit my needs (and your answer makes perfect sense now).

I created a separate INTERFACE library for libA that would communicate the include directory but not attempt to link against libA.

add_library(libA SHARED a.c)
add_library(libAInterface INTERFACE)
target_include_directories(libAInterface INTERFACE includeA)
#### elsewhere
target_link_libraries(libB PRIVATE libAInterface)

I now get it that target_link_libraries(foo INTERFACE bar) commutes the INTERFACE properties of bar ot the INTERFACE properties of foo. It doesn’t do anything to foo’s compilation.

4

I had a post somewhere about a $<COMPILE_ONLY:> genex that would do this, but yes, this is the best solution that I know of available today.