Development/Tutorials/Debugging/Debugging symbols

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    Overview

    Debugging symbols allow you to debug your application better. They are added to your binary by the compiler. For KDE, you have to decide during the cmake step if you want debugging symbols or not. To compile your application with debugging symbols, use

    cmake . -DCMAKE_BUILD_TYPE=debugfull
    

    to compile it without debugging symbols, use

    cmake . -DCMAKE_BUILD_TYPE=release
    

    Depending on your decision, output generated with the command kDebug will also be (debugfull) or not be (release) added to your application.

    Example app

    As an example, let's write an application that crashes:

    main.cpp

        
    #include <KApplication>
    #include <KAboutData>
    #include <KCmdLineArgs>
    #include <KMessageBox>
    #include <iostream>
    
    using namespace std;
    
    int main (int argc, char *argv[])
    {
        KAboutData aboutData( "tutorial1", 0, ki18n("Tutorial 1"), "1.0",
                              ki18n("Displays a KMessageBox popup"),
                              KAboutData::License_GPL,
                              ki18n("(c) 2009"), ki18n("Some text..."),
                              "http://tutorial.com/",
                              "[email protected]");
    
        KCmdLineArgs::init( argc, argv, &aboutData );
        KApplication app;
    
        KMessageBox::questionYesNo( 0, i18n( "Hello World" ) );
        int* i;
        cout << "i is at " << i << " value " << *i << endl;
        i=(int*)0x0;
        cout << "i is at " << i << " value " << *i << endl;
    
        return 0;
    }
    

    CMakeLists.txt

    project (tutorial1)
    find_package(KDE4 REQUIRED)
    include (KDE4Defaults)
    include_directories(${KDE4_INCLUDES})
    set(tutorial1_SRCS main.cpp)
    kde4_add_executable(tutorial1 ${tutorial1_SRCS})
    target_link_libraries(tutorial1 ${KDE4_KDEUI_LIBS})
    install(TARGETS tutorial1  ${INSTALL_TARGETS_DEFAULT_ARGS})
    

    Now let's compile this without debugging symbols:

    cmake . -DCMAKE_BUILD_TYPE=release && make -j4
    

    We see that the resulting file is small:

    # ls -lh tutorial1
    -rwxr-xr-x 1 user user 15K Jul 11 18:07 tutorial1
    

    With debugging symbols, the file is bigger:

    cmake . -DCMAKE_BUILD_TYPE=debugfull && make
    # ls -lh tutorial1
    -rwxr-xr-x 1 root root 250K Jul 11 18:09 tutorial1
    

    Backtraces

    Now let's start the application and look at the backtrace:

    cmake . -DCMAKE_BUILD_TYPE=release && make -j4
    

    Gives you the backtrace

    Application: Tutorial 1 (tutorial1), signal SIGSEGV
    �[?1034h[Thread debugging using libthread_db enabled]
    0x00007f58abba4cb0 in nanosleep () from /lib64/libc.so.6
    [Current thread is 1 (Thread 0x7f58b0cfd750 (LWP 21264))]
    
    Thread 1 (Thread 0x7f58b0cfd750 (LWP 21264)):
    [KCrash Handler]
    #5  0x00000000004016aa in main ()
    

    The debugging build

    cmake . -DCMAKE_BUILD_TYPE=debugfull && make -j4
    

    Gives you the backtrace

    Application: Tutorial 1 (tutorial1), signal SIGSEGV
    �[?1034h[Thread debugging using libthread_db enabled]
    0x00007fd0b8161cb0 in nanosleep () from /lib64/libc.so.6
    [Current thread is 1 (Thread 0x7fd0bd2ba750 (LWP 21327))]
    
    Thread 1 (Thread 0x7fd0bd2ba750 (LWP 21327)):
    [KCrash Handler]
    #5  0x0000000000401625 in main (argc=1, argv=0x7fffc52f5138) at /root/kdehello/main.cpp:25
    

    So you see: with debugging symbols, you see the line number where the crash occurred. Without, you do not see this.

    Where are they?

    Where are the debugging symbols stored? Use objdump -g to find out:

    # objdump  -g tutorial1-release | wc -l
    511
    # objdump  -g tutorial1-debugfull | wc -l
    40943
    

    It is important to know that the code lines (in assembler) to be executed actually do not differ a lot. We see this when disassembling the code:

    # objdump -d tutorial1-debugfull | wc -l
    658
    # objdump -d tutorial1-release | wc -l
    697
    

    This gives us hope that there will be no major speed difference between a debug- and a release-version of a binary.

    Speed implications

    We remove the lines that cause the crash and the messagebox. Then we execute the program 100 times:

    cmake . -DCMAKE_BUILD_TYPE=debugfull && make -j4
    time for i in $(seq 1 1 100); do ./tutorial1; done
    
    real    0m6.201s
    user    0m4.368s
    sys     0m1.320s
    

    Lasts 6 seconds. Now with the release version:

    cmake . -DCMAKE_BUILD_TYPE=release && make -j4
    time for i in $(seq 1 1 100); do ./tutorial1; done
    
    real    0m6.259s
    user    0m4.368s
    sys     0m1.328s
    

    Also lasts 6 seconds. So the main difference is in the binaries size.

    And make?

    How does cmake propagate to make if a debug version is wanted? Do a

    cmake . -DCMAKE_BUILD_TYPE=debugfull && make VERBOSE=1 
    

    You will find a difference during the link step. The parameters

    -DNDEBUG -DQT_NO_DEBUG 
    

    are unique for the release-version. There are further differences like the O2 optimization.