Thread: Is it possible to copy function to a const function pointer by memcpy?

  1. #1
    Registered User
    Join Date
    Mar 2009
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    1

    Is it possible to copy function to a const function pointer by memcpy?

    I want to change an CONST function pointer after it's initialed, I tried the following, i printed the pointer address copyed by memcpy, the address is different from the function address. Also, I get a seg fault on executing it. How can i do this?

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>

    void function(void){
    }

    const void (*function_pointer)(void)=NULL;

    int main(){
    memcpy(&function_pointer, &function, sizeof(fuction_pointer));
    fprintf(stderr, "%p, %p\n", &function_pointer, &function);
    function_pointer();
    return 0;
    }

  2. #2
    Kernel hacker
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    Location
    Farncombe, Surrey, England
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    15,677
    &function_pointer is not the same as the content of function_pointer.

    memcpy() should complain that you are trying to modify a const variable, really, so it should not allow you to do what you want to do. The variable is either const, or it isn't! If you modify it, then it should not be const.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  3. #3
    Hurry Slowly vart's Avatar
    Join Date
    Oct 2006
    Location
    Rishon LeZion, Israel
    Posts
    6,788
    it is not const pointer - it is pointer to const - should be no problem to modify it

    Code:
    #include <stdio.h>
    
    void function(void)
    {
    	printf("Called\n");
    }
    
    const void (*function_pointer)(void)=NULL;
    
    int main()
    {
    	function_pointer = &function;
    	function_pointer();
    	return 0;
    }
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    – David J. Wheeler

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