I have the Memory Address of a function, how do I call it?

This is a discussion on I have the Memory Address of a function, how do I call it? within the C++ Programming forums, part of the General Programming Boards category; Say I have the Memory Address of a function, and I know the return type and the paramter types, how ...

  1. #1
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    I have the Memory Address of a function, how do I call it?

    Say I have the Memory Address of a function, and I know the return type and the paramter types, how can I call it?

    I tried something like this:
    Code:
    typedef void(*TESTFUNCTION)(void);
    
    ...
    
    TESTFUNCTION Test = (TESTFUNCTION)0x00400000 <--- not real address
    
    Test();
    Oh ya, the function would have to be called by a DLL that has been loaded by the process.

  2. #2
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    Type some parentheses after the function pointer name, and put the arguments between the parentheses:
    Code:
    #include <iostream>
    using namespace std;
    
    void display(int n, double d)
    {
    	cout<<n<<endl;
    	cout<<d<<endl;
    }
    
    int main()
    {
    	void (*myfunc)(int, double);
    	myfunc = display;
    	
    	myfunc(10, 30.5);
    
    	return 0;
    }
    Or, with a typedef:
    Code:
    #include <iostream>
    using namespace std;
    
    typedef void (*TESTFUNCTION)(int, double);
    
    void display(int n, double d)
    {
    	cout<<n<<endl;
    	cout<<d<<endl;
    }
    
    int main()
    {
    	TESTFUNCTION a = display;
    	a(100, 200.5);
    
    	return 0;
    }
    Last edited by 7stud; 05-14-2005 at 07:26 PM.

  3. #3
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    Oh sorry, I wasn't clear enough.
    I know how to do it like that. I'm saying if I only know the hexadecimal memory address of a function how can I get it to work.

  4. #4
    #include<xErath.h> xErath's Avatar
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    Do this
    Code:
    void display(int n, double d){....}
    cout << display <<endl;
    You'll get an hexadecimal value printed to cout, the function adress. The functions name is just a var that holds that adress.
    So explicitly writing the adress or typing the function name will do, but keep this in mind: You can never know the adress that the compiler will give to a function, therefore doing what you want can and will be erroneous.

  5. #5
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    No, you see it WON'T be erroneous because I'm using the debugger to find the memory address from the completed code.

    The function pointer code will be contained within a DLL that will be loaded into the memory, thus the address of the function will not change.

  6. #6
    #include<xErath.h> xErath's Avatar
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    hum, that way yes it may fail, because when loading a dll in memory you'll never know in which memory position it'll be. That's why there is a LoadLibrary(...) function in the win api. The same for the exe.
    Last edited by xErath; 05-21-2005 at 09:28 PM.

  7. #7
    vae victus! skorman00's Avatar
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    Code:
    void (*myfunc)(int, double);
    myfunc = ( void (*)(int, double) )0xADD12355;  //my leet version of address
    I think this would work, I don't have a compiler at ready to check it out for myself. However, like xErath said, if it's coming from a DLL, you might have a bit of trouble. Unless you figured that part out of course, in that case kudos!

    PS: If you're checking through the DLL using your debugger, what you're trying to do might be illegal. A DLL has a string table to associate a normal stringified function name with the address you're looking at so that you can call it like any normal function.

  8. #8
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    How can you know the exact memory address? It never stays the same...

    Hard coding a memory address will most likely not work

  9. #9
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    I think this would work, I don't have a compiler at ready to check it out for myself.
    I tried something like that earlier, but I couldn't get it to work.

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