49 lines
2.7 KiB
Markdown
49 lines
2.7 KiB
Markdown
# Adding Features
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There are lots of compiler and linker settings. When you need to add something special, you could check first to see if CMake supports it; if it does, you can avoid explicitly tying yourself to a compiler version. And, better yet, you explain what you mean in your CMakeLists, rather than spewing flags.
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The first and most common feature was C++ standards support, which got it's own chapter.
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## Position independent code
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[This](https://cmake.org/cmake/help/latest/variable/CMAKE_POSITION_INDEPENDENT_CODE.html) is best known as the `-fPIC` flag. Much of the time, you don't need to do anything. CMake will include the flag for `SHARED` or `MODULE` libraries. If you do explicitly need it:
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```cmake
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set(CMAKE_POSITION_INDEPENDENT_CODE ON)
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```
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will do it globally, or:
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```cmake
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set_target_properties(lib1 PROPERTIES POSITION_INDEPENDENT_CODE ON)
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```
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to explicitly turn it `ON` (or `OFF`) for a target.
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## Little libraries
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If you need to link to the `dl` library, with `-ldl` on Linux, just use the built-in CMake variable [`${CMAKE_DL_LIBS}`](https://cmake.org/cmake/help/latest/variable/CMAKE_DL_LIBS.html) in a `target_link_libraries` command. No module or `find_package` needed. (This adds whatever is needed to get `dlopen` and `dlclose`)
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Unfortunately, the math library is not so lucky. If you need to explicitly link to it, you can always do `target_link_libraries(MyTarget PUBLIC m)`, but it might be better to use CMake's generic [`find_library`](https://cmake.org/cmake/help/latest/command/find_library.html):
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```cmake
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find_library(MATH_LIBRARY m)
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if(MATH_LIBRARY)
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target_link_libraries(MyTarget PUBLIC ${MATH_LIBRARY})
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endif()
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```
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You can pretty easily find `Find*.cmake`'s for this and other libraries that you need with a quick search; most major packages have a helper library of CMake modules. See the chapter on existing package inclusion for more.
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## Interprocedural optimization
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«prop:tgt:INTERPROCEDURAL*OPTIMIZATION», best known as \_link time optimization* and the `-flto` flag, is available on very recent versions of CMake. You can turn this on with «variable:CMAKE_INTERPROCEDURAL_OPTIMIZATION» (CMake 3.9+ only) or the «prop:tgt:INTERPROCEDURAL_OPTIMIZATION» property on targets. Support for GCC and Clang was added in CMake 3.8. If you set `cmake_minimum_required(VERSION 3.9)` or better (see «policy:CMP0069»), setting this to `ON` on a target is an error if the compiler doesn't support it. You can use check_ipo_supported(), from the built-in «module:CheckIPOSupported» module, to see if support is available before hand. An example of 3.9 style usage:
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```cmake
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include(CheckIPOSupported)
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check_ipo_supported(RESULT result)
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if(result)
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set_target_properties(foo PROPERTIES INTERPROCEDURAL_OPTIMIZATION TRUE)
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endif()
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```
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