Path to target output file

I have a .so library target created by add_library, and need to pass an absolute path to this library to an external script. Now I have ${LIBRARY_OUTPUT_PATH}/${CMAKE_SHARED_LIBRARY_PREFIX}LangShared${CMAKE_SHARED_LIBRARY_SUFFIX} for that (LIBRARY_OUTPUT_PATH is defined in my CMakeLists.txt). This looks like hard-coding to me, because it will break as soon as the target is renamed or some of its properties are changed. Is there a way to get an absolute path to add_library's output?

You should use a generator expression for this.

From the docs for add_custom_command and the docs for generator expressions:

Arguments to COMMAND may use "generator expressions"...

Generator expressions are evaluted during build system generation to produce information specific to each build configuration.

In this case, assuming your library target is called "MyLib", the generator expression representing the full path to the built library would be:

$<TARGET_FILE:MyLib>

Try:

get_property(fancy_lib_location TARGET fancy_lib PROPERTY LOCATION)
message (STATUS "fancy_lib_location == ${fancy_lib_location}")

Where fancy_lib is the target created with add_library (fancy_lib SHARED ...).

I found that works directly with Makefile generators, but there is more work to be done for Visual Studio generators since the value of fancy_lib_location is not what you would expect:

1. fancy_lib_location will contain an embedded reference to a Visual-Studio-specific $(OutDir) reference that you will have to replace with the value of the CMAKE_BUILD_TYPE CMake variable (which resolves to something like Debug, or Release).
2. At least for CMake 2.8.1, and at least on Visual Studio targets, and if you have set the CMAKE_DEBUG_POSTFIX variable, then it will not be included in the value (which may or may not be a bug, I don't know).

To expand on the answer by @bgooddr, here is a CMake function to get the location of a target:

function(get_fancy_lib_location)
    set(options)
    set(multiValueArgs LIB)
    set(oneValueArgs LOCATION)
    cmake_parse_arguments(get_fancy_lib_location "${options}" "${oneValueArgs}" "${multiValueArgs}" ${ARGN} )
    message (STATUS "fancy_lib  == ${get_fancy_lib_location_LIB}")

    get_property(fancy_lib_location TARGET "${get_fancy_lib_location_LIB}" PROPERTY LOCATION)
    message (STATUS "fancy_lib_location == ${fancy_lib_location}")

    set(${get_fancy_lib_location_LOCATION} ${fancy_lib_location})
endfunction()