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  3. Is main() a callback function?

Is main() a callback function?

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  • K k5054

    raddevus wrote:

    * Cannot have its address taken. * Cannot be called.

    Lies. You can call main, and you can take its address. This compiles and runs in VS2019:

    #include int main(int argc, char **argv)
    {
    if(argc <= 1) {
    auto main_ptr{main};
    std::cout << "pointer to main = " << main_ptr << std::endl;
    std::cout << "exiting ..." << std::endl;
    return 0;
    }
    std::cout << "argc = " << argc << std::endl;
    main(--argc, argv);
    }

    Interestingly, in linux you auto main_ptr{main} is 1, but for windows it looks like an address: 0008151E Update: I should also point out that the instances where you might need to call main from within you program are vanishingly small. In general, if you think you need to, you're almost certainly wrong.

    R Offline
    R Offline
    Rick York
    wrote on last edited by
    #31

    That has always been my thought.

    "They have a consciousness, they have a life, they have a soul! Damn you! Let the rabbits wear glasses! Save our brothers! Can I get an amen?"

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    • H honey the codewitch

      C is for wimps Real programmers use butterflies

      Real programmers use butterflies

      T Offline
      T Offline
      TheGreatAndPowerfulOz
      wrote on last edited by
      #32

      Real programmers use galaxy-sized gravitic lenses.

      #SupportHeForShe Government can give you nothing but what it takes from somebody else. A government big enough to give you everything you want is big enough to take everything you've got, including your freedom.-Ezra Taft Benson You must accept 1 of 2 basic premises: Either we are alone in the universe or we are not alone. Either way, the implications are staggering!-Wernher von Braun

      H 1 Reply Last reply
      0
      • K k5054

        raddevus wrote:

        * Cannot have its address taken. * Cannot be called.

        Lies. You can call main, and you can take its address. This compiles and runs in VS2019:

        #include int main(int argc, char **argv)
        {
        if(argc <= 1) {
        auto main_ptr{main};
        std::cout << "pointer to main = " << main_ptr << std::endl;
        std::cout << "exiting ..." << std::endl;
        return 0;
        }
        std::cout << "argc = " << argc << std::endl;
        main(--argc, argv);
        }

        Interestingly, in linux you auto main_ptr{main} is 1, but for windows it looks like an address: 0008151E Update: I should also point out that the instances where you might need to call main from within you program are vanishingly small. In general, if you think you need to, you're almost certainly wrong.

        P Offline
        P Offline
        PIEBALDconsult
        wrote on last edited by
        #33

        Hold my Jolt Cola...

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        0
        • T TheGreatAndPowerfulOz

          Real programmers use galaxy-sized gravitic lenses.

          #SupportHeForShe Government can give you nothing but what it takes from somebody else. A government big enough to give you everything you want is big enough to take everything you've got, including your freedom.-Ezra Taft Benson You must accept 1 of 2 basic premises: Either we are alone in the universe or we are not alone. Either way, the implications are staggering!-Wernher von Braun

          H Offline
          H Offline
          honey the codewitch
          wrote on last edited by
          #34

          Focused using butterflies

          Real programmers use butterflies

          T 1 Reply Last reply
          0
          • D DRHuff

            Well it never called me back - and I thought we had such a connection... :(( :(( :mad: :(( :((

            I, for one, like Roman Numerals.

            T Offline
            T Offline
            TheGreatAndPowerfulOz
            wrote on last edited by
            #35

            Did you give it your number, baby? ;P

            #SupportHeForShe Government can give you nothing but what it takes from somebody else. A government big enough to give you everything you want is big enough to take everything you've got, including your freedom.-Ezra Taft Benson You must accept 1 of 2 basic premises: Either we are alone in the universe or we are not alone. Either way, the implications are staggering!-Wernher von Braun

            1 Reply Last reply
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            • H honey the codewitch

              Focused using butterflies

              Real programmers use butterflies

              T Offline
              T Offline
              TheGreatAndPowerfulOz
              wrote on last edited by
              #36

              honey the codewitch wrote:

              Focused using butterflies

              that are manipulated using gravitic lenses

              #SupportHeForShe Government can give you nothing but what it takes from somebody else. A government big enough to give you everything you want is big enough to take everything you've got, including your freedom.-Ezra Taft Benson You must accept 1 of 2 basic premises: Either we are alone in the universe or we are not alone. Either way, the implications are staggering!-Wernher von Braun

              H 1 Reply Last reply
              0
              • T TheGreatAndPowerfulOz

                honey the codewitch wrote:

                Focused using butterflies

                that are manipulated using gravitic lenses

                #SupportHeForShe Government can give you nothing but what it takes from somebody else. A government big enough to give you everything you want is big enough to take everything you've got, including your freedom.-Ezra Taft Benson You must accept 1 of 2 basic premises: Either we are alone in the universe or we are not alone. Either way, the implications are staggering!-Wernher von Braun

                H Offline
                H Offline
                honey the codewitch
                wrote on last edited by
                #37

                created from the silk of butterfly cocoons :-D

                Real programmers use butterflies

                T 1 Reply Last reply
                0
                • OriginalGriffO OriginalGriff

                  Difficult to tell, it was yelling a lot and slurring it's words.

                  "I have no idea what I did, but I'm taking full credit for it." - ThisOldTony AntiTwitter: @DalekDave is now a follower!

                  D Offline
                  D Offline
                  DRHuff
                  wrote on last edited by
                  #38

                  So it was yaccing with a lisp?

                  I, for one, like Roman Numerals.

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                  • R Rob Philpott

                    That reminds me of a thing I read some time back, titled something along the lines of 'the 50 things Windows does before hitting main()'. Can't find it but it's out there somewhere, by one of the SysInternals lot I think. It was both interesting and really boring at the same time.

                    Regards, Rob Philpott.

                    R Offline
                    R Offline
                    Rick York
                    wrote on last edited by
                    #39

                    If you have Visual Studio you can see what the C run-time library does by looking at its source. I was checking that out just today.

                    "They have a consciousness, they have a life, they have a soul! Damn you! Let the rabbits wear glasses! Save our brothers! Can I get an amen?"

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                    • R Rick York

                      Actually things are a little different. In the Visual Studio source, VC/Tools/MSVC/**version**/crt/src/vcruntime/vcstartup_internal.h has the prototypes and the calls are in exe_common.inl. WinMain and main are two different calls along with their wide character versions.

                      "They have a consciousness, they have a life, they have a soul! Damn you! Let the rabbits wear glasses! Save our brothers! Can I get an amen?"

                      L Offline
                      L Offline
                      Lost User
                      wrote on last edited by
                      #40

                      I suspected this might be the case, but I have not had access to the source for a number of years.

                      R 1 Reply Last reply
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                      • R Rob Philpott

                        I can't make up my mind.

                        Regards, Rob Philpott.

                        S Offline
                        S Offline
                        Stefan_Lang
                        wrote on last edited by
                        #41

                        Only if used as such, e. g. in this code:

                        #include

                        using namespace std;

                        static int call_countdown = 3;
                        typedef int (*MyCallbackFun)();
                        int foo(MyCallbackFun cb, int count)
                        {
                        static int foo_counter = 0;
                        ++foo_counter;
                        cout << "enter foo[" << foo_counter <<"]: " << count << endl;
                        int result = cb();
                        cout << "exit foo[" << foo_counter << "]: " << result << endl;
                        return result;
                        }
                        int bar()
                        {
                        static int bar_counter = 0;
                        ++bar_counter;
                        cout << "bar[" << bar_counter << "]" << endl;
                        return -1;
                        }
                        int main()
                        {
                        static int call_counter = 0;
                        ++call_counter;
                        cout << "enter main[" << call_counter << "]" << endl;

                        int result = 0;
                        if (call\_counter < 5)
                        {
                            result = (call\_counter>2)
                                ? foo(bar, call\_counter)
                                : foo(main, call\_counter);
                        }
                        cout << "exit main\[" << call\_counter << "\]: " << result << endl;
                        
                        return call\_counter;
                        

                        }

                        You can test it here: https://www.onlinegdb.com/online_c++_compiler[^] or trust me that the output is:

                        enter main[1]
                        enter foo[1]: 1
                        enter main[2]
                        enter foo[2]: 2
                        enter main[3]
                        enter foo[3]: 3
                        bar[1]
                        exit foo[3]: -1
                        exit main[3]: -1
                        exit foo[3]: 3
                        exit main[3]: 3
                        exit foo[3]: 3
                        exit main[3]: 3

                        The tricky bit about this is that by using main() as a callback function, you're also using it recursively, which complicates matters considerably: it's easy to mess up the code and get an endless recursion. (that's why I added counters and output in every function) It's doable, but definitiely not a good idea.

                        GOTOs are a bit like wire coat hangers: they tend to breed in the darkness, such that where there once were few, eventually there are many, and the program's architecture collapses beneath them. (Fran Poretto)

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                        • H honey the codewitch

                          created from the silk of butterfly cocoons :-D

                          Real programmers use butterflies

                          T Offline
                          T Offline
                          TheGreatAndPowerfulOz
                          wrote on last edited by
                          #42

                          lol, you got me there.

                          #SupportHeForShe Government can give you nothing but what it takes from somebody else. A government big enough to give you everything you want is big enough to take everything you've got, including your freedom.-Ezra Taft Benson You must accept 1 of 2 basic premises: Either we are alone in the universe or we are not alone. Either way, the implications are staggering!-Wernher von Braun

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                          • L Lost User

                            I suspected this might be the case, but I have not had access to the source for a number of years.

                            R Offline
                            R Offline
                            Rick York
                            wrote on last edited by
                            #43

                            There were only minor changes. For the most part, what you wrote is correct.

                            "They have a consciousness, they have a life, they have a soul! Damn you! Let the rabbits wear glasses! Save our brothers! Can I get an amen?"

                            1 Reply Last reply
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                            • R Rob Philpott

                              I can't make up my mind.

                              Regards, Rob Philpott.

                              S Offline
                              S Offline
                              Slow Eddie
                              wrote on last edited by
                              #44

                              No. It's a state in the Northeast. :laugh:

                              Oh Well.....

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