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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?

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3 Answers 3

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89

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>
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    This is very frustrating because none of the add_custom_[...] commands allow you to invoke macros - they can only run external programs.
    – Nathan Osman
    Commented Feb 1, 2015 at 6:19
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    Yeah, to invoke a macro you'd need to add it to a standalone CMake script file and invoke CMake itself from the command using the -P arg. The downside (apart from being overly complex) is that the macro in the script file doesn't have access to any of the current variables in the main CMake process - you need to pass them too when invoking CMake. So for example add_custom_command(... COMMAND ${CMAKE_COMMAND} -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE} -P"path/to/script_with_macro.cmake")
    – Fraser
    Commented Feb 1, 2015 at 9:58
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    Or, if used in a cmake-macro, $<TARGET_FILE:${target_macro_parameter}> .
    – Patrick B.
    Commented Mar 7, 2016 at 10:19
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    Generator expressions only work as arguments to a few functions related to the "generation" step. If you want this during the "configuration" step, generator expressions don't work. See this question: stackoverflow.com/questions/56470020/…
    – John Zwinck
    Commented Jun 7, 2019 at 4:29
33

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).
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    For point 1, you have available the per-configuration versions of LOCATION, i.e. LOCATION_DEBUG, LOCATION_RELEASE, etc. This resolves the path fully. For point 2, this seems to be fixed as at v2.8.10
    – Fraser
    Commented Apr 8, 2013 at 19:06
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    @Ose Unfortunately, ${CMAKE_BUILD_TYPE} is only supported in make-based generators, so your solution is likely to break when using an IDE like Visual Studio.
    – ComicSansMS
    Commented May 8, 2013 at 11:40
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    Policy CMP0026 in Cmake 3.0 and later) disallows using the LOCATION property in this manner. The generator expression method is preferred.
    – Mike DeSimone
    Commented Feb 18, 2015 at 21:33
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    with cmake v3.15.5, this gives following error: The LOCATION property may not be read from target. Use the target name directly with add_custom_command, or use the generator expression $<TARGET_FILE>, as appropriate.
    – harish
    Commented Nov 26, 2019 at 7:49
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    @harish I've not used cmake in years and so my answer may now be badly out of date, and thus now "stale".
    – bgoodr
    Commented Nov 27, 2019 at 1:25
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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()
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