Re: [PATCH] Various pages: SYNOPSIS: Use VLA syntax in function parameters

Alejandro Colomar
2022-11-13T16:34:17Z

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On 11/13/22 17:31, Alejandro Colomar wrote:
>
>
> On 11/13/22 17:28, Alejandro Colomar wrote:
>> SYNOPSIS:
>>
>> unary-operator:  . identifier
>>
>>
>> DESCRIPTION:
>>
>> -  It is not an lvalue.
>>
>>     -  This means sizeof() and _Lengthof() cannot be applied to them.
>
> Sorry, the above is a thinko.
>
> I wanted to say that, like sizeof() and _Lengthof(), you can't assign to it.
>
>>     -  This prevents ambiguity with a designator in an initializer-list within
>> a nested braced-initializer.
>>
>> -  The type of a .identifier is always an incomplete type.

Or rather, more easily prohibit explicitly using typeof(), sizeof(), and
_Lengthof() to it.

>>
>>     -  This prevents circular dependencies involving sizeof() or _Lengthof().
>>
>> -  Shadowing rules apply.
>>
>>     -  This prevents ambiguity.
>>
>>
>> EXAMPLES:
>>
>>
>> -  Valid examples (libc):
>>
>>         int
>>         strncmp(const char s1[.n],
>>                 const char s2[.n],
>>                 size_t n);
>>
>>         int
>>         cacheflush(void addr[.nbytes],
>>                    int nbytes,
>>                    int cache);
>>
>>         long
>>         mbind(void addr[.len],
>>               unsigned long len,
>>               int mode,
>>               const unsigned long nodemask[(.maxnode + ULONG_WIDTH ‐ 1)
>>                                            / ULONG_WIDTH],
>>               unsigned long maxnode, unsigned int flags);
>>
>>         void *
>>         bsearch(const void key[.size],
>>                 const void base[.size * .nmemb],
>>                 size_t nmemb,
>>                 size_t size,
>>                 int (*compar)(const void [.size], const void [.size]));
>>
>> -  Valid examples (my own):
>>
>>         void
>>         ustr2str(char dst[restrict .len + 1],
>>                  const char src[restrict .len],
>>                  size_t len);
>>
>>         char *
>>         stpecpy(char dst[.end - .dst + 1],
>>                 char *restrict src,
>>                 char end[1]);
>>
>> -  Valid examples (from this thread):
>>
>>     -
>>         struct s { int a; };
>>         void f(int a, int b[((struct s) { .a = 1 }).a]);
>>
>>         Explanation:
>>         -  Because of shadowing rules, .a=1 refers to the struct member.
>>            -  Also, if .a referred to the parameter, it would be an rvalue, so
>> it wouldn't be valid to assign to it.
>>         -  (...).a refers to the struct member too, since otherwise an rvalue
>> is not expected there.
>>
>>     -
>>         void foo(struct bar { int x; char c[.x] } a, int x);
>>
>>         Explanation:
>>         -  Because of shadowing rules, [.x] refers to the struct member.
>>
>>     -
>>         struct bar { int y; };
>>         void foo(char p[((struct bar){ .y = .x }).y], int x);
>>
>>         Explanation:
>>         -  .x unambiguously refers to the parameter.
>>
>> -  Undefined behavior:
>>
>>     -
>>         struct bar { int y; };
>>         void foo(char p[((struct bar){ .y = .y }).y], int y);
>>
>>         Explanation:
>>         -  Because of shadowing rules, =.y refers to the struct member.
>>         -  .y=.y means initialize the member with itself (uninitialized use).
>>         -  (...).y refers to the struct member, since otherwise an rvalue is
>> not expected there.
>>
>> -  Constraint violations:
>>
>>     -
>>         void foo(char (*a)[sizeof *.b], char (*b)[sizeof *.a]);
>>
>>         Explanation:
>>         -  sizeof(*.b): Cannot get size of incomplete type.
>>         -  sizeof(*.a): Cannot get size of incomplete type.
>>
>>     -
>>         void f(size_t s, int a[sizeof(1) = 1]);
>>
>>         Explanation:
>>         -  Cannot assign to rvalue.
>>
>>     -
>>         void f(size_t s, int a[.s = 1]);
>>
>>         Explanation:
>>         -  Cannot assign to rvalue.
>>
>>     -
>>         void f(size_t s, int a[sizeof(.s)]);
>>
>>         Explanation:
>>         -  sizeof(.s): Cannot get size of incomplete type.
>>
>>
>> Does this idea make sense to you?
>>
>>
>> Cheers,
>> Alex