From: Jonny Grant <jg@jguk.org>
To: Jonathan Wakely <jwakely.gcc@gmail.com>, Xi Ruoyao <xry111@xry111.site>
Cc: gcc-help <gcc-help@gcc.gnu.org>
Subject: Re: std::string add nullptr attribute
Date: Sun, 12 Mar 2023 22:10:04 +0000 [thread overview]
Message-ID: <d7536cbb-ec12-f48a-0061-43aaa9462466@jguk.org> (raw)
In-Reply-To: <CAH6eHdR23=FG+a72JKdvFEd6ynR8XaR9kaNS5GouX5d=2J+s6Q@mail.gmail.com>
On 09/02/2023 17:52, Jonathan Wakely wrote:
> On Thu, 9 Feb 2023 at 16:30, Xi Ruoyao wrote:
>>
>> On Thu, 2023-02-09 at 14:56 +0000, Jonathan Wakely via Gcc-help wrote:
>>>> Note, my code isn't like this, it is just an example to suggest
>>>> adding the nullptr attribute, as its clearly already rejected at
>>>> runtime.
>>>
>>> I assume you mean the nonnull attribute. That was added in 2020 and
>>> then reverted because it broke some things:
>>
>> I remember I'd once made the same mistake when I suggested to add
>> nonnull for ostream::operator<<(const string &) and I was lectured:
>> nonnull is not only a diagnostic attribute, it also allows the compiler
>> to assume the parameter is never null and rendering std::string(nullptr)
>> an undefined behavior.
>
> Yes, I think that's what might have happened with the std::string change.
How about adding a method that is called by these operators that has the __attribute__ ((nonnull)) ?
example:
https://godbolt.org/z/bqW86PP34
>> Then the example may just silently continue to run, instead of throwing
>> an exception. It would be an ironic example: an attempt to improve
>> diagnostic finally made diagnostic more difficult.
>
> Indeed.
>
> Maybe we can add __attribute__((access(read, 1))) instead, which says
> that we will read from the pointer, which also implies it must be
> non-null.
>
> N.B. in C++23 string(nullptr) produces an error, although
> string((const char*)nullptr) doesn't, so in practice it only prevents
> the dumbest calls with a literal 'nullptr' token, and not the more
> realistic problems where you have a pointer that happens to be null.
There is a way to generate a build error for even string((const char*)nullptr)
I made another example that detects nullptr being passed around (should such stupid code occur) at build time providing optimizer is on. With -O0 it just gives the error always; so I put in an __OPTIMIZE__ check. This example doesn't need -fanalyzer.
https://godbolt.org/z/TdGnno4K5
#if __OPTIMIZE__
void nullptr_compile_abort() __attribute__((error("nullptr compile error")));
#endif
static void f2(const char * str)
{
#if __OPTIMIZE__
if (str == nullptr) nullptr_compile_abort();
#endif
}
int main()
{
f2((const char *)nullptr);
}
Regards, Jonny
next prev parent reply other threads:[~2023-03-12 22:10 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-09 13:26 Jonny Grant
2023-02-09 14:56 ` Jonathan Wakely
2023-02-09 16:30 ` Xi Ruoyao
2023-02-09 17:52 ` Jonathan Wakely
2023-02-10 21:30 ` Jonny Grant
2023-02-10 22:03 ` Jonathan Wakely
2023-02-10 22:38 ` Jonny Grant
2023-02-11 0:32 ` Jonathan Wakely
2023-02-13 22:02 ` Jonny Grant
2023-02-19 20:43 ` Jonny Grant
2023-02-19 21:33 ` Jonny Grant
2023-02-20 10:26 ` Xi Ruoyao
2023-02-20 10:37 ` Jonathan Wakely
2023-02-20 10:54 ` Xi Ruoyao
2023-02-20 11:10 ` Gabriel Ravier
2023-02-20 11:18 ` Marc Glisse
2023-02-20 11:28 ` Segher Boessenkool
2023-02-20 12:00 ` Jonny Grant
2023-02-20 14:50 ` Gabriel Ravier
2023-02-20 11:44 ` Jonny Grant
2023-02-21 15:02 ` Jonny Grant
2023-02-20 11:38 ` Jonny Grant
2023-02-20 11:30 ` Jonny Grant
2023-02-20 12:59 ` Xi Ruoyao
2023-02-20 13:44 ` Jonathan Wakely
2023-02-20 19:21 ` Jonny Grant
2023-02-20 19:35 ` Jonathan Wakely
2023-02-20 19:39 ` Jonny Grant
2023-02-22 20:27 ` Jonny Grant
2023-02-21 15:04 ` Jonny Grant
2023-02-21 22:48 ` Jonny Grant
2023-03-04 15:00 ` Jonny Grant
2023-02-20 11:25 ` Jonny Grant
2023-03-12 22:10 ` Jonny Grant [this message]
2023-03-13 10:10 ` Jonathan Wakely
2023-03-13 19:55 ` Jonny Grant
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