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+# 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 CheckSymbolExists.cmake macro in the top-level CMakeLists
+file as follows:
+
+  # does this system provide the log and exp functions?
+  include(CheckSymbolExists)
+  set(CMAKE_REQUIRED_LIBRARIES "m")
+  check_symbol_exists(log "math.h" HAVE_LOG)
+  check_symbol_exists(exp "math.h" HAVE_EXP)
+
+Now let's add these defines to TutorialConfig.h.in so that we can use them
+from mysqrt.cxx:
+
+  // does the platform provide exp and log functions?
+  #cmakedefine HAVE_LOG
+  #cmakedefine HAVE_EXP
+
+Modify mysqrt.cxx to include math.h. Next, in the mysqrt function we can
+provide an alternate implementation based on log and exp if they are available
+on the system using the following code:
+
+  // if we have both log and exp then use them
+  #if defined(HAVE_LOG) && defined (HAVE_EXP)
+    double result = exp(log(x)*0.5);
+    std::cout << "Computing sqrt of " << x << " to be " << result << " using log" << std::endl;
+  #else
+    ...
+
+Run cmake or cmake-gui to configure the project and then build it with your
+chosen build tool.
+
+You will notice that even though HAVE_LOG and HAVE_EXP are both defined mysqrt
+isn't using them. We should realize quickly that we have forgotten to include
+TutorialConfig.h in mysqrt.cxx. We will also need to update
+MathFunctions/CMakeLists.txt with where it is located.
+
+So let's go ahead and update MathFunctions/CMakeLists.txt to look like:
+
+  add_library(MathFunctions mysqrt.cxx)
+
+  target_include_directories(MathFunctions
+            INTERFACE ${CMAKE_CURRENT_SOURCE_DIR}
+            PRIVATE ${Tutorial_BINARY_DIR}
+            )
+
+  install(TARGETS MathFunctions DESTINATION lib)
+  install(FILES MathFunctions.h DESTINATION include)
+
+Now all we need to do is include TutorialConfig.h in mysqrt.cxx
+
+At this point you should go ahead and build the project again.
+
+Run the built Tutorial executable. Which function gives better results now,
+Step1’s sqrt or Step5’s mysqrt?
+
+Exercise: Why is it important that we configure TutorialConfig.h.in after the
+checks for HAVE_LOG and HAVE_EXP? What would happen if we inverted the two?
+
+Exercise: Is there a better place for us to save the HAVE_LOG and HAVE_EXP
+values other than in TutorialConfig.h?