Re: [TECH TOPIC] Rust for Linux

[Stephen Hemminger <stephen@networkplumber.org>]

On Tue, 6 Jul 2021 14:45:50 -0700
Bart Van Assche <bvanassche@acm.org> wrote:

> On 7/6/21 1:00 PM, Roland Dreier wrote:
> > One area I see where Rust could make a big improvement for drivers is
> > in using RAII for error paths.  Drivers often have a lot of code like
> > 
> >     if (something())
> >         return err;
> > 
> >     if (something_else())
> >         goto undo_something;
> > 
> >     if (a_third_thing())
> >         goto undo_two_things;
> > 
> > and so on, which is difficult to get right in the first place and even
> > harder to keep correct in the face of changes.  
> 
> Although cumbersome, it is possible to implement RAII by using the gcc
> extensions to the C language. An example:
> 
> #include <stdio.h>
> #include <stdlib.h>
> 
> #define CONCAT2(a, b) a##b
> #define CONCAT(a, b) CONCAT2(a, b)
> #define DECL_WITH_DTOR_NAME(declaration, initializer, cleanup_func_name,\
>                             arglist, cleanup_func_body)                 \
>     void cleanup_func_name(arglist) { cleanup_func_body; }              \
>     declaration __attribute__((cleanup(cleanup_func_name))) = initializer
> #define DECL_WITH_DTOR(declaration, initializer, arglist, cleanup_func_body) \
>     DECL_WITH_DTOR_NAME(declaration, initializer,                       \
>                         CONCAT(cleanup_func, __COUNTER__), arglist,     \
>                         cleanup_func_body)
> 
> int main(int argc, char** argv)
> {
>     DECL_WITH_DTOR(void *p, NULL, void **p,
>                    printf("freeing p = %p\n", *p); free(*p););
>     DECL_WITH_DTOR(void *q, NULL, void **q,
>                    printf("freeing q = %p\n", *q); free(*q););
>     p = malloc(7);
>     q = malloc(3);
>     printf("p = %p; q = %p\n", p, q);
>     return 0;
> }
> 
> The output of the above code:
> 
> p = 0xd952a0; q = 0xd952c0
> freeing q = 0xd952c0
> freeing p = 0xd952a0
> 

Why not use the GCC cleanup function attribute, it is a lot cleaner than
having ugly error prone macros.

#define _cleanup_free __atttribute__((__cleanup__(free)))

int main(int argc, char **argv)
{
          _cleanup_free_ void *p;
          _cleanup_free_  void *q;