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Comment by kragen

2 days ago

I'm not sure how you would override the function pointers repeatedly without getting a compile error. Maybe you could instead use functions with GCC's weak attribute?

10 comments

kragen

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Y_Y  2 days ago

I guess they mean to overwrite the pointers:

  void (*foo)(void)
  int main(){
    *foo();
  }
  void foo1(){...}
  foo = &foo1; // will be overwritten later
  void foo2(){...}
  foo = &foo2;

I'm on my phone so I haven't compiled it, but that's the rough idea I got, define lots of versions and then set the version to the latest one.

(You can skip the address-of for functions by punning, but reading the types is hard enough already.)

  • kragen  2 days ago

    That's not valid C; you can't put statements outside of functions, not even assignment statements or other expression statements. No C compiler I've ever used will accept it. Where would the compiled version of those statements go, in an init function? Linkers added support for those for C++ static initializers, where execution order isn't defined; C doesn't have anything similar. Are you expecting the compiler to execute the assignment statements at compile time, with a C interpreter?

    • Y_Y  2 days ago

      (What's the language lawyer equivalent of an ambulance chaser?)

      Fair point, I hadn't thought it all the way through. It's also all too easy to assume you can use C++ features or GNU extensions if you're not in the habit of enforcing the standard. For your trouble here are two partial solutions:

      ---

      1. CPP abuse

      If I was doing this myself I'd probably just use the preprocessor to make the last function my entry point like

        SOURCE=file.c  gcc -DLATEST_MAIN=$(grep -E "^[a-zA-Z_].*\(.*\)\s*\{" ${SOURCE} | tail -1 | grep -Po "[a-zA-Z_]\w*(?=\()") ${SOURCE}

      and then you can start with

        int main(void){LATEST_MAIN();};

      and append

        void foo(){...}
  ...
  void bar(){...}

      and execution will start from the last defined function (that matches the regex).

      ---

      2. Pre-defining non-static functions

      If that's cheating then you can redefine "external" functions before the linker has a chance to provide them. For example:

        #include <stdlib.h>
  int (*foo)(void);
  int main(void){exit(0); *foo();}

      is a valid program, and you can append

        void f1(){puts("oh");}
  int exit(){foo=&f1;}

      and foo will indeed be called. It does require some forward planning if you want to append multiple times, but it shouldn't be so difficult to generate a library full of dummy functions to use.

      5 replies →

Agraillo  2 days ago

I see no issue in setting it dynamically from the code, supposed to be set during start-up code. A global variable representing a function pointer and this variable either changed several times (if the latest addition allows execution of previous routines then setting its new value) or just once for the latest version if the logic somehow disables previous start-up routines. Can be called "dynamic inheritance"