Re: Compiling C++ modules

[Avi Kivity <avi@argo.co.il>]

[de-cc'ed original poster, he's far away by now]

Kyle Moffett wrote:
> On Apr 24, 2006, at 17:03:46, Avi Kivity wrote:
> > Alan Cox wrote:
> > > There are a few anti C++ bigots around too, but the kernel choice of 
> > > C was based both on rational choices and experimentation early on 
> > > with the C++ compiler.
> >
> > Times have changed, though. The C++ compiler is much better now, and 
> > the recent slew of error handling bugs shows that C is a very unsafe 
> > language.
> >
> > I think it's easy to show that the equivalent C++ code would be 
> > shorter, faster, and safer.
>
> Really?  What features exactly does C++ have over C that you think 
> make that true?  Implicit memory allocation? Exceptions?  Operator 
> overloading?  Tendency to use StudlyCaps?  What else can C++ do that C 
> can not?
>
> For example, I could write the following:
>
> class Foo {
> public:
>     Foo() { /* ... init code ... */ }
>     ~Foo() { /* ... free code ... */ }
>     int do_thing(int arg) { /* ... code ... */ }
>
> private:
>     int data_member;
> };
>
> Or I could write it like this:
>
> struct foo {
>     int data_member;
> };
>
> int foo_init() { /* ... init code ... */ }
> int foo_destroy() { /* ... free code ... */ }
> int foo_do_thing(int arg) { /* ... code ... */ }
>
>
> The "advantages" of the former over the latter:
>
> (1)  Without exceptions (which are fragile in a kernel), the former 
> can't return an error instead of initializing the Foo.
Don't discount exceptions so fast. They're exactly what makes the code 
clearer and more robust.

A very large proportion of error handling consists of:
- detect the error
- undo local changes (freeing memory and unlocking spinlocks)
- propagate the error

Exceptions make that fully automatic. The kernel uses a mix of gotos and 
alternate returns which bloat the code and are incredibly error prone. 
See the recent 2.6.16.x for examples.
> (2)  You can't control when you initialize the Foo.  For example in 
> this code, the "Foo item;" declarations seem to be trivially 
> relocatable, even if they're not.
>     spin_lock(&foo_lock);
>     Foo item1;
>     Foo item2;
>     spin_unlock(&foo_lock);
They only seem relocatable with your C glasses on. Put on your C++ 
glasses (much thicker), and initialization no longer seems trivially 
movable.

On the other hand, you can replace the C code

{
   Foo item1, item2;
   int r;

   spin_lock(&foo_lock);
   if ((r = foo_init(&item1)) < 0) {
       spin_unlock(&foo_lock);
       return r;
   }
   if ((r = foo_init(&item2)) < 0) {
       foo_destroy(&item1);
       spin_unlock(&foo_lock);
       return r;
   }
   foo_destroy(&item2);
   foo_destroy(&item1);
   spin_unlock(&foo_lock);
   return 0;
}

with

{
   spinlock_t::guard foo_guard(foo_lock);
   Foo item1;
   Foo item2;
}

14 lines vs 3, one variable eliminated. How many potential security 
vulnerabilities? How much time freed to work on the algorithm/data 
structure, not on error handling?
> (3)  Foo could theoretically implement overloaded operators.  How 
> exactly is it helpful to do math on structs?
It isn't. It's nice for other application domains (matrix algebra, etc.) 
not for kernels.

This mailing list has a full complement of reviewers who can detect 
trailing whitespace in a dark room three miles away. Surely they can 
spot an attempt to sneak in the "operator" keyword.
> Does that actually make it any easier to understand the code?  How 
> does it make it more obvious to be able to write a "+" operator that 
> allocates memory?
Not all C++ features need to be used in the kernel. In fact, not all C++ 
features need to be used, period. Ever tried to understand code which 
uses overloaded operator,() (the comma operator)?

--
error compiling committee.c: too many arguments to function

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