Comment by Y_Y

(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:

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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).

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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.