Re: sizeof(function) functionality

[Ben Davis <bndavis at ntlworld dot com>]

On Monday 7 July 2003 4:55 pm, Alexandre Oliva wrote:
> On Jul  6, 2003, Kristis Makris <kristis.makris@asu.edu> wrote:
> > Other than instantiating a volatile variable right after a function and
> > evaluating variable - &funcname, is there another way of retrieving the
> > size a function occupies in memory using gcc ?
>
> First, this won't get what you want, because variable will likely be
> placed in a different section.  I can't think of any clean way to do
> this in C, especially if you consider functions-as-sections as a
> possibility.

Allegro <http://alleg.sf.net/> has used the following technique for a long 
time:

#define END_OF_FUNCTION(name) void name##_end(void) { }
#define END_OF_STATIC_FUNCTION(name) static void name##_end(void) { }

void func1(void) { ... } END_OF_FUNCTION(func1);
void func2(void) { ... } END_OF_FUNCTION(func2);

Then there are more macros for locking the functions in memory, and these 
macros merely find the difference between pointers to the two functions. The 
purpose of this is to prevent any disk access from occurring when the 
functions are called.

Note that this is only done on DJGPP; other operating systems supported by 
Allegro (like Windows and Linux) use threads and have re-entrant disk access 
functions, so the locking is unnecessary and the macros expand to nothing. So 
I have no idea how portable this technique would be. It could conceivably be 
broken by a compiler that set func2's entry point to be the 'return' 
instruction of func1, for instance ...

Assuming it doesn't exist already, I think sizeof(function) or something like 
it would be a very useful GNU Extension for future GCC versions :)

Ben