Re: _Generic.3: EXAMPLES: C++'s static_cast() in C

[Alejandro Colomar <alx.manpages@gmail.com>]

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On 11/11/22 16:11, Alejandro Colomar wrote:
> On 11/11/22 15:53, Alejandro Colomar wrote:
>> Dear readers,
>>
>> I've been developing a cast macro for C that is similar to C++'s 
>> static_cast(), which allows to:
>>
>> - Cast between a limited set of types (actually, pointers to those types).
>> - Can add const, but not remove it.
>> - Easily grep for all casts.
>>
>> But forbids:
>>
>> - Cast between unrelated types.
>> - Removing const.
>>
>> This improves type safety in C, where casts allow programmers to commit all 
>> kinds of crimes by disabling most compiler warnings.
>>
>> The macro I originally wrote, allows to convert between u_char and char, for a 
>> code base that uses u_char internally but has to interface libc and syscalls. 
>> That one is unnecessary for most code bases which just use char (and that's 
>> how it should be for strings; if you want unsigned 'char's, you should use 
>> '-funsigned-char').
>>
>> But there are cases where the kernel or libc forces us to use casts, like for 
>> example in bind(2).  That's for what the following macro is, and I plan to add 
>> it to the EXAMPLES section in _Generic(3), replacing the previous program.
>>
>> The macro itself is quite huge, which might discourage some, but it is not so 
>> complex.  I'll release it to the public domain.  Could you please review it?
>>
>> Cheers,
>>
>> Alex
>>
> 
> With C2x's typeof_unqual(), it can be made a bit smaller, and better (it now 
> supports volatile).


And since it makes no sense to convert between some of the derivative types 
(e.g., sa_in to sa_un), it can be further simplified:

#define sockaddr_cast(t, p)                            \
     _Generic(&*(p),                                    \
     struct sockaddr *:                                 \
         _Generic((typeof_unqual(t)) NULL,              \
         struct sockaddr_in *: (t) (p),                 \
         struct sockaddr_in6 *: (t) (p),                \
         struct sockaddr_un *: (t) (p),                 \
         struct sockaddr_storage *: (t) (p),            \
         default: (p)),                                 \
     struct sockaddr **:                                \
         _Generic((t) NULL,                             \
         struct sockaddr_in **: (t) (p),                \
         struct sockaddr_in6 **: (t) (p),               \
         struct sockaddr_un **: (t) (p),                \
         struct sockaddr_storage **: (t) (p),           \
         default: (p)),                                 \
     const struct sockaddr *:                           \
         _Generic((t) NULL,                             \
         const struct sockaddr_in *: (t) (p),           \
         const struct sockaddr_in6 *: (t) (p),          \
         const struct sockaddr_un *: (t) (p),           \
         const struct sockaddr_storage *: (t) (p),      \
         default: (p)),                                 \
                                                        \
     struct sockaddr_in *:                              \
         _Generic((typeof_unqual(t)) NULL,              \
         struct sockaddr *: (t) (p),                    \
         struct sockaddr_storage *: (t) (p),            \
         default: (p)),                                 \
     struct sockaddr_in **:                             \
         _Generic((t) NULL,                             \
         struct sockaddr **: (t) (p),                   \
         struct sockaddr_storage **: (t) (p),           \
         default: (p)),                                 \
     const struct sockaddr_in *:                        \
         _Generic((t) NULL,                             \
         const struct sockaddr *: (t) (p),              \
         const struct sockaddr_storage *: (t) (p),      \
         default: (p)),                                 \
                                                        \
     struct sockaddr_in6 *:                             \
         _Generic((typeof_unqual(t)) NULL,              \
         struct sockaddr *: (t) (p),                    \
         struct sockaddr_storage *: (t) (p),            \
         default: (p)),                                 \
     struct sockaddr_in6 **:                            \
         _Generic((t) NULL,                             \
         struct sockaddr **: (t) (p),                   \
         struct sockaddr_storage **: (t) (p),           \
         default: (p)),                                 \
     const struct sockaddr_in6 *:                       \
         _Generic((t) NULL,                             \
         const struct sockaddr *: (t) (p),              \
         const struct sockaddr_storage *: (t) (p),      \
         default: (p)),                                 \
                                                        \
     struct sockaddr_un *:                              \
         _Generic((typeof_unqual(t)) NULL,              \
         struct sockaddr *: (t) (p),                    \
         struct sockaddr_storage *: (t) (p),            \
         default: (p)),                                 \
     struct sockaddr_un **:                             \
         _Generic((t) NULL,                             \
         struct sockaddr **: (t) (p),                   \
         struct sockaddr_storage **: (t) (p),           \
         default: (p)),                                 \
     const struct sockaddr_un *:                        \
         _Generic((t) NULL,                             \
         const struct sockaddr *: (t) (p),              \
         const struct sockaddr_storage *: (t) (p),      \
         default: (p)),                                 \
                                                        \
     struct sockaddr_storage *:                         \
         _Generic((typeof_unqual(t)) NULL,              \
         struct sockaddr *: (t) (p),                    \
         struct sockaddr_in *: (t) (p),                 \
         struct sockaddr_in6 *: (t) (p),                \
         struct sockaddr_un *: (t) (p),                 \
         default: (p)),                                 \
     struct sockaddr_storage **:                        \
         _Generic((t) NULL,                             \
         struct sockaddr **: (t) (p),                   \
         struct sockaddr_in **: (t) (p),                \
         struct sockaddr_in6 **: (t) (p),               \
         struct sockaddr_un **: (t) (p),                \
         default: (p)),                                 \
     const struct sockaddr_storage *:                   \
         _Generic((t) NULL,                             \
         const struct sockaddr *: (t) (p),              \
         const struct sockaddr_in *: (t) (p),           \
         const struct sockaddr_in6 *: (t) (p),          \
         const struct sockaddr_un *: (t) (p),           \
         default: (p)),                                 \
                                                        \
     default:                                           \
         (p)                                            \
     )


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