Precompile library, .a with header, included with the source? Embedded target.

Code
1

OK, I wanted to get some feedback on something.

I am using CMake to build for embedded targets, and ran into something that was surprisingly challenging to do.
It is not uncommon to be given pre-compiled libraries as .a files, along with an interface in .h files.

The first results that turn up on google, I just could NOT get them to work.

For example, lets say if I have a “libvendor”, distributed as a libvendor_arm.a and vendor.h.

Like this, as it’s own CMakeLists.txt files in libs/libvendor/,
with add_subdirectory( libs/libvendor ) added to my base cmake script.

add_library(vendor STATIC IMPORTED)
set_target_properties(cbt PROPERTIES
        IMPORTED_LOCATION ${CMAKE_CURRENT_SOURCE_DIR}/libvendor_arm.a
        INTERFACE_INCLUDE_DIRECTORIES ${CMAKE_CURRENT_SOURCE_DIR}
)

Using that, and linking vendor to my main exe target, include paths do not seem to get added, and builds fail with:
cannot find -lvendor: No such file or directory

Eventually, I settled on this:

add_library( vendor INTERFACE )
target_sources( vendor INTERFACE vendor.h )
target_include_directories( vendor INTERFACE
        ${CMAKE_CURRENT_SOURCE_DIR} )

add_library(vendor_arm STATIC IMPORTED)
set_target_properties(vendor_arm
        PROPERTIES IMPORTED_LOCATION  ${CMAKE_CURRENT_SOURCE_DIR}/libvendor_arm.a )
target_link_libraries(vendor INTERFACE vendor_arm )

Have I … overlooked something? Is this idiomatically correct, such that I can then do:
target_link_libraries( app.elf PUBLIC vendor ), and get the header interface AND linking to the .a ?

2

IMPORTED libraries are scoped. By making it in a subdirectory, it is not “visible” to the parent directory. You can either:

  • use a FindVendor.cmake file and find_package(Vendor) and do this logic (my recommendation);
  • use add_library(vendor STATIC IMPORTED GLOBAL) to make the target globally visible.
1 Like
3

Oh. Ok. Would I use find_package even if it is something inside the repo, and not a package installed somewhere else in the system, or provided by the OS?

I am trying some versions of this, but just cannot get it to work …

# Try to find the library
find_path(VENDOR_INCLUDE_DIR NAMES vendor.h PATHS ${PROJECT_SOURCE_DIR}/libs/libvendor )
find_library(VENDOR_LIBRARY NAMES libvendor_arm.a PATHS ${PROJECT_SOURCE_DIR}/libs/libvendor )

include(FindPackageHandleStandardArgs)
find_package_handle_standard_args(VENDOR DEFAULT_MSG VENDOR_LIBRARY VENDOR_INCLUDE_DIR)

if(VENDOR_FOUND)
    set(VENDOR_LIBRARIES ${VENDOR_LIBRARY})
    set(VENDOR_INCLUDE_DIRS ${VENDOR_INCLUDE_DIR})
endif()

However, it just refuses to pick up anything for VENDOR_INCLUDE_DIR, even when using an absolute path to search …

4

Oh, if you vendor it, just make it GLOBAL (there’s nothing to find_* as you “know” the paths). It’s basically “your” target anyways at that point.

5

This contains a typo. You’ve used a different target when you define the library (vendor) and when you set properties (cbt). Does your real project have that typo too?

6

whoops it does
in my script, it’s all libcbt, but I was anonymizing that for the post, changing everything back to “vendor” this and that.

BUT I think that typo is not too relevant - the issue there will be that the library is not exported to the global scope, as Ben informed me of.