Comment by 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?

6 comments

kragen

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.

kragen 1 day ago
This isn't a language-lawyer kind of thing, and, as I said, I don't think C++ features or GNU extensions will help. The pre-defining extern functions approach will get you a finite number of tries, but I suppose it could be a fairly large finite number.
If for some reason I had to solve this problem in real life I would do it with a postprocesing step like your #1 suggestion. But once you're putting code into your Makefile anything goes.
- Y_Y 1 day ago
  
  You can just append a function with __attribute__((constructor)) if GNU extensions are allowed and that will run before main and let you mess with the function pointers.
  
  3 replies →