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Brad King authored
Since commit 07223c5c (Tutorial: Update Step 5 to work on Windows, 2020-02-18, v3.18.0-rc1~655^2) the logic does not work on non-Windows platforms when cmake is re-run on an existing build tree. It is also more complicated than we'd like for a tutorial example. Avoid the need to consider the `m` library case by performing the check as C++. Since `check_cxx_symbol_exists` cannot handle overloaded functions like `exp` and `log`, check with `check_cxx_source_compiles` instead. This also presents a more general-purpose example in the tutorial. Fixes: #23524
Brad King authoredSince commit 07223c5c (Tutorial: Update Step 5 to work on Windows, 2020-02-18, v3.18.0-rc1~655^2) the logic does not work on non-Windows platforms when cmake is re-run on an existing build tree. It is also more complicated than we'd like for a tutorial example. Avoid the need to consider the `m` library case by performing the check as C++. Since `check_cxx_symbol_exists` cannot handle overloaded functions like `exp` and `log`, check with `check_cxx_source_compiles` instead. This also presents a more general-purpose example in the tutorial. Fixes: #23524
Step 5: Adding System Introspection
Let us consider adding some code to our project that depends on features the
target platform may not have. For this example, we will add some code that
depends on whether or not the target platform has the log and exp
functions. Of course almost every platform has these functions but for this
tutorial assume that they are not common.
If the platform has log and exp then we will use them to compute the
square root in the mysqrt function. We first test for the availability of
these functions using the :module:`CheckCXXSourceCompiles` module in
MathFunctions/CMakeLists.txt.
Add the checks for log and exp to MathFunctions/CMakeLists.txt,
after the call to :command:`target_include_directories`:
If available, use :command:`target_compile_definitions` to specify
HAVE_LOG and HAVE_EXP as PRIVATE compile definitions.
If log and exp are available on the system, then we will use them to
compute the square root in the mysqrt function. Add the following code to
the mysqrt function in MathFunctions/mysqrt.cxx (don't forget the
#endif before returning the result!):
We will also need to modify mysqrt.cxx to include cmath.
Run the :manual:`cmake <cmake(1)>` executable or the :manual:`cmake-gui <cmake-gui(1)>` to configure the project and then build it with your chosen build tool and run the Tutorial executable.
Which function gives better results now, sqrt or mysqrt?