[CMake] OSX RPATH linker flags not added on first build

[clinton at elemtech.com]

----- On Feb 3, 2017, at 11:25 AM, Doug Digglington <doug.digglington at gmail.com> wrote: 

> Hello,

> I am using ExternalProject to download and build a third-party library (SDL) in
> my project. I am running into an issue on OSX where the RPATH linker flags will
> not be added when my project is built and linked for the first time. As a
> result the built executable will not have any RPATH paths in it. However, when
> I rerun CMake or touch the main CMakeLists.txt file and rebuild then the linker
> flags will be added appropriately.

> I have tried explicitly setting the RPATH variables according to the "CMake
> RPATH handling" page both for the default and "always full" scenarios but that
> did not seem to change this behavior. Here is my current CMakeLists.txt file:

> cmake_minimum_required(VERSION 3.5)
> project(shared_object)

> set(CMAKE_MACOSX_RPATH TRUE)

> set(LIB_INSTALL_DIR ${CMAKE_INSTALL_PREFIX}/sdl/install)
> set(LIB_DIR ${LIB_INSTALL_DIR}/lib)
> set(LIB_PATH
> ${LIB_DIR}/${CMAKE_SHARED_LIBRARY_PREFIX}SDL2${CMAKE_SHARED_LIBRARY_SUFFIX})

> include(ExternalProject)
> ExternalProject_Add(
> libsdl
> PREFIX ${CMAKE_BINARY_DIR}/sdl
> URL [ https://www.libsdl.org/release/SDL2-2.0.5.tar.gz |
> https://www.libsdl.org/release/SDL2-2.0.5.tar.gz ]
> URL_MD5 d4055424d556b4a908aa76fad63abd3c
> CMAKE_ARGS
> -DCMAKE_INSTALL_PREFIX=${LIB_INSTALL_DIR}
> -DCMAKE_MACOSX_RPATH=${CMAKE_MACOSX_RPATH}
> -DSDL_STATIC=FALSE
> INSTALL_DIR ${LIB_INSTALL_DIR}
> )

> add_library(sdl SHARED IMPORTED GLOBAL)
> set_target_properties(sdl PROPERTIES
> IMPORTED_LOCATION ${LIB_PATH}
> )

You could try adding another property here. 
set_target_properties(sdl PROPERTIES IMPORTED_SONAME "@rpath/ ${CMAKE_SHARED_LIBRARY_PREFIX}SDL2${CMAKE_SHARED_LIBRARY_SUFFIX}") 

With that hint, CMake would not need to wait until the library exists before being able to find out if it uses @rpath. 

And I see you also set CMAKE_MACOSX_RPATH=TRUE. That will only have an effect on any add_library() calls within the current scope (excluding imported libraries). 

Clint 

> add_dependencies(sdl libsdl)
> add_executable(shared_object main.cpp)
> target_link_libraries(shared_object sdl)

> When I run CMake for the first time here are the contents of the
> CMakeFiles/shared_object.dir/link.txt file:

> /Applications/Xcode_6.4.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++
> -Wl,-search_paths_first -Wl,-headerpad_max_install_names
> CMakeFiles/shared_object.dir/main.cpp.o -o shared_object
> sdl/install/lib/libSDL2.dylib

> Notice that there are no -Wl,-rpath linker flags. When I run `make` to build the
> project there is no RPATH information in the built executable. I checked this
> with `otool -l shared_object | grep -A2 RPATH`.

> Now if I rerun CMake after everything has been built or if I touch the
> CMakeLists.txt file before running `make` again then the contents of the
> CMakeFiles/shared_object.dir/link.txt changes and the RPATH information is
> linked into the built application. Here is the contents of
> CMakeFiles/shared_object.dir/link.txt after I rerun CMake:

> /Applications/Xcode_6.4.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++
> -Wl,-search_paths_first -Wl,-headerpad_max_install_names
> CMakeFiles/shared_object.dir/main.cpp.o -o shared_object
> -Wl,-rpath,/var/tmp/so/sdl/install/lib sdl/install/lib/libSDL2.dylib

> After rebuliding the application it will contain the expected RPATH information:

> otool -l shared_object | grep -A2 RPATH
> cmd LC_RPATH
> cmdsize 40
> path /var/tmp/so/sdl/install/lib (offset 12)

> Is there something that I am configuring incorrectly about the external project
> or imported library that causes it to not be known or added to the linker flags
> the first time CMake is run?

> --

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