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