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78 lines
3.1 KiB
78 lines
3.1 KiB
Step 10: Selecting Static or Shared Libraries
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=============================================
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In this section we will show how the :variable:`BUILD_SHARED_LIBS` variable can
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be used to control the default behavior of :command:`add_library`,
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and allow control over how libraries without an explicit type (``STATIC``,
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``SHARED``, ``MODULE`` or ``OBJECT``) are built.
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To accomplish this we need to add :variable:`BUILD_SHARED_LIBS` to the
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top-level ``CMakeLists.txt``. We use the :command:`option` command as it allows
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users to optionally select if the value should be ``ON`` or ``OFF``.
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Next we are going to refactor ``MathFunctions`` to become a real library that
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encapsulates using ``mysqrt`` or ``sqrt``, instead of requiring the calling
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code to do this logic. This will also mean that ``USE_MYMATH`` will not control
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building ``MathFunctions``, but instead will control the behavior of this
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library.
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The first step is to update the starting section of the top-level
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``CMakeLists.txt`` to look like:
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.. literalinclude:: Step11/CMakeLists.txt
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:caption: CMakeLists.txt
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:name: CMakeLists.txt-option-BUILD_SHARED_LIBS
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:language: cmake
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:end-before: # add the binary tree
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Now that we have made ``MathFunctions`` always be used, we will need to update
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the logic of that library. So, in ``MathFunctions/CMakeLists.txt`` we need to
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create a SqrtLibrary that will conditionally be built and installed when
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``USE_MYMATH`` is enabled. Now, since this is a tutorial, we are going to
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explicitly require that SqrtLibrary is built statically.
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The end result is that ``MathFunctions/CMakeLists.txt`` should look like:
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.. literalinclude:: Step11/MathFunctions/CMakeLists.txt
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:caption: MathFunctions/CMakeLists.txt
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:name: MathFunctions/CMakeLists.txt-add_library-STATIC
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:language: cmake
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:lines: 1-36,42-
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Next, update ``MathFunctions/mysqrt.cxx`` to use the ``mathfunctions`` and
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``detail`` namespaces:
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.. literalinclude:: Step11/MathFunctions/mysqrt.cxx
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:caption: MathFunctions/mysqrt.cxx
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:name: MathFunctions/mysqrt.cxx-namespace
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:language: c++
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We also need to make some changes in ``tutorial.cxx``, so that it no longer
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uses ``USE_MYMATH``:
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#. Always include ``MathFunctions.h``
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#. Always use ``mathfunctions::sqrt``
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#. Don't include ``cmath``
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Finally, update ``MathFunctions/MathFunctions.h`` to use dll export defines:
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.. literalinclude:: Step11/MathFunctions/MathFunctions.h
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:caption: MathFunctions/MathFunctions.h
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:name: MathFunctions/MathFunctions.h
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:language: c++
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At this point, if you build everything, you may notice that linking fails
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as we are combining a static library without position independent code with a
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library that has position independent code. The solution to this is to
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explicitly set the :prop_tgt:`POSITION_INDEPENDENT_CODE` target property of
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SqrtLibrary to be ``True`` when building shared libraries.
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.. literalinclude:: Step11/MathFunctions/CMakeLists.txt
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:caption: MathFunctions/CMakeLists.txt
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:name: MathFunctions/CMakeLists.txt-POSITION_INDEPENDENT_CODE
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:language: cmake
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:lines: 37-42
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**Exercise**: We modified ``MathFunctions.h`` to use dll export defines.
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Using CMake documentation can you find a helper module to simplify this?
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