Sending functions.. Possible?

[u8sand]

Ok so I've used function pointers for some time. I was trying to figure out if this was possible.

First. It IS possible to convert a function pointer into an array of bytes.

It is also possible to reconstruct that function with the bytes in that array.

I would like to save a function into an array of bytes, and lets say save it to a text file (func.dat). Then later read that text file and execute the particular function...
Is it possible? It seems it should be possible the only problem I run across is finding the size of the array that makes up the function.

Is there any way to do this?

int func()
{
    return 1+1;
}

int main()
{
    int (*foo)() = func;

    char* data = (char*)func;

    // write all the data
    char* copyFunc = new char[sizeof(func)];
    for(int i = 0; i < sizeof(func); i++)
        copyFunc[i] = data[i];

    int (*constructedFoo)() = (int (*)())copyFunc;

    return 0;
}

of course this code won't compile because sizeof does not work for functions, does anyone know how to get the size of a function? Or the size of the function header/footer.

I have tried things like

int func()
{
    1+1;
    new char('}');
}

Then searched for the } char (as the end of the function) but that size doesn't work.

If your wondering why I need it, it could be used for lets say, sending a function to a remote computer to execute (thinking of parallel processing) Or even saving a function in a file like in my first example to later be used, this can be helpful.

Any help is greatly appreciated.

[Ancient Dragon]

Not possible in the way you described it. But what you can do is to give functions a text string name that is associated with a function pointer.

struct fns
{
   std::string name;
   int (*fn)(); // function pointer
};

Now you will make an array of the above structures. Then when you need to execute a specific function just search the array by text string name. You can do similar with remote processes. Process1 (P1) sends Process2 (P2) the name of the function to be executed and P2 searches the array for the desired function pointer.

[mike_2000_17]

Uhmm... saving a function to a file to be able to call it later from anywhere else... if I'm not mistaken (rethorical), this is called a Dynamic Link Library (DLL) (in Windows) or a Shared Object (SO) (in *nix).

Any way you want to do what you want to do, it will either have to be based on or going through the use of a shared library (.dll or .so) file. So, I suggest you look into that topic.

[u8sand]

Ok. Well I'm guessing its not possible unless I compile it before hand basically. Thanks for the feedback.

[doug65536]

There is a lot more to it than blindly copying a block of code and expecting it to work. It depends very much on the processor and the instructions being used. For example, a switch statement might have a lookup table somewhere else. It might be using jump vectors for dynamic link library imports that won't necessarily be there. It might have string literals that it is expecting to be at a certain address in the const section. It might be referring to global variables that may not be there or may not be at the expected address.

The worst problem is there might be instructions that use absolute addresses - which will be wrong if you put it at a different address. It might call other functions, how will they work? How will it know what address those are at (assuming they exist at all).