Re: std::string add nullptr attribute

[Jonny Grant <jg@jguk.org>]

On 13/03/2023 10:10, Jonathan Wakely wrote:
> On Sun, 12 Mar 2023 at 22:10, Jonny Grant wrote:
>>
>>
>>
>> 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);
>> }
> 
> This causes compilation to fail for code which is never executed at
> run-time, which is not permitted by the standard.
> 
> You can use __attribute__((warning(""))) instead, but that is broken^W
> inconvenient for inline functions. You need a non-inline definition of
> the function, which means exporting a new function from the shared
> library just for this diagnostic.
> 
> All these techniques you're rediscovering have been tried before :-)

Is there any link you can refer me to for these techniques?

Regards, Jonny