Re: [devel] [SCM] packages/apt: heads/rework-dynamic-mmap

["Dmitry V. Levin" <ldv@altlinux.org>]

On Tue, Feb 11, 2020 at 08:05:38PM +0300, Ivan Zakharyaschev wrote:
> On Tue, 11 Feb 2020, Ivan Zakharyaschev wrote:
> 
> > On Tue, 11 Feb 2020, Dmitry V. Levin wrote:
> > 
> > > On Tue, Feb 11, 2020 at 04:47:41PM +0300, Ivan Zakharyaschev wrote:
> > > [...]
> > > > commit e785f0e8636e47a672445e70f2923a5eea566b33
> > > > Author: Ivan Zakharyaschev <imz@altlinux.org>
> > > > Date:   Wed Jan 29 04:41:13 2020 +0300
> > > > 
> > > >     use the safer C++-style static_cast instead of a C-style cast (from void*)
> > > >     
> > > >     What is happening here:
> > > >     
> > > >     Map->RawAllocate() returns the index in an array of bytes (i.e., of char;
> > > >     no matter whether they are (un)signed); therefore, we cast the base
> > > >     pointer to the corresponding type, so that the pointer arithmetic
> > > >     gives a pointer to the beginning of the allocated space.
> > > >     
> > > >     We do not want to rely on non-standard void*-arithmetic.
> > > 
> > > We - это кто, и почему они не хотят полагаться на то, что работает?
> > 
> > Да это расширение gcc, которое работает , когда язык C, а когда C++ -- 
> > запрещено и в gcc. (Я вроде пробовал, и gcc запретил, когда мы в прошлый 
> > раз обсуждали падение apt-а на e2k.)
> 
> -bash-4.3$ cat charp-arith.cxx
> #include <stddef.h>
> #include <stdio.h>
> 
> char buf[] = "foo";
> 
> int main(const int argc, char ** const argv)
> {
>   char * const begin = &buf[0];
>   char * const end = &buf[2];
>   const ptrdiff_t diff = end - begin;
>   printf("%td\n", diff);
> }
> -bash-4.3$ gcc -Wall -xc charp-arith.cxx && ./a.out; echo $?
> 2
> 0
> -bash-4.3$ gcc -Wall charp-arith.cxx && ./a.out; echo $?
> 2
> 0
> -bash-4.3$ gcc -Wall -std=gnu++17 charp-arith.cxx && ./a.out; echo $?
> 2
> 0
> -bash-4.3$ diff charp-arith.cxx voidp-arith.cxx
> 8,9c8,9
> <   char * const begin = &buf[0];
> <   char * const end = &buf[2];
> ---
> >   void * const begin = &buf[0];
> >   void * const end = &buf[2];
> -bash-4.3$ gcc -Wall -xc voidp-arith.cxx && ./a.out; echo $?
> 2
> 0
> -bash-4.3$ gcc -Wall voidp-arith.cxx && ./a.out; echo $?
> voidp-arith.cxx: In function ‘int main(int, char**)’:
> voidp-arith.cxx:10:32: error: invalid use of ‘void’
>    10 |   const ptrdiff_t diff = end - begin;
>       |                                ^~~~~
> 1
> -bash-4.3$ gcc -Wall -std=gnu++17 voidp-arith.cxx && ./a.out; echo $?
> voidp-arith.cxx: In function ‘int main(int, char**)’:
> voidp-arith.cxx:10:32: error: invalid use of ‘void’
>    10 |   const ptrdiff_t diff = end - begin;
>       |                                ^~~~~
> 1
> -bash-4.3$ gcc -Wall -std=gnu++2a voidp-arith.cxx && ./a.out; echo $?
> voidp-arith.cxx: In function ‘int main(int, char**)’:
> voidp-arith.cxx:10:32: error: invalid use of ‘void’
>    10 |   const ptrdiff_t diff = end - begin;
>       |                                ^~~~~
> 1
> -bash-4.3$ 

Если арифметика с void* в cxx не поддерживается, значит, она не
использовалась, и фраза "We do not want to rely on non-standard
void*-arithmetic" просто сбивает с толку.

C-style cast - это не арифметика с указателями.


-- 
ldv