Path: ...!weretis.net!feeder8.news.weretis.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: Re: No warning at implicit removal of const. Was: relearning C: why
 does an in-place change to a char* segfault?
Date: Tue, 13 Aug 2024 13:08:57 +0200
Organization: A noiseless patient Spider
Lines: 73
Message-ID: <v9fes9$3rtc7$1@dont-email.me>
References: <IoGcndcJ1Zm83zb7nZ2dnZfqnPWdnZ2d@brightview.co.uk>
 <20240801174026.00002cda@yahoo.com> <v8gi7i$29iu1$1@dont-email.me>
 <slrnvaorkl.34j6.candycanearter07@candydeb.host.invalid>
 <87zfpvfdk4.fsf@nosuchdomain.example.com> <86ttfp2zpf.fsf@linuxsc.com>
 <871q2tiffa.fsf@nosuchdomain.example.com> <86jzgl1gw6.fsf@linuxsc.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Injection-Date: Tue, 13 Aug 2024 13:08:57 +0200 (CEST)
Injection-Info: dont-email.me; posting-host="c1a1041a93166a8aacca8d5e30b6a3ee";
	logging-data="4060551"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX194LNwqjKDuWfkZOCq6h5mz/DvJ2RUDc9I="
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.11.0
Cancel-Lock: sha1:kngDuHeuQeO1U5EbJRcS3tcW9QI=
Content-Language: en-GB
In-Reply-To: <86jzgl1gw6.fsf@linuxsc.com>
Bytes: 4138

On 13/08/2024 01:05, Tim Rentsch wrote:
> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
> 
>> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>>
>>> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>>>
>>>> candycanearter07 <candycanearter07@candycanearter07.nomail.afraid>
>>>> writes:
>>>>
>>>>> David Brown <david.brown@hesbynett.no> wrote at 17:56 this Thursday (GMT):
>>>>
>>>> [...]
>>>>
>>>>>> gcc has the option "-Wwrite-strings" that makes string literals in
>>>>>> C have "const char" array type, and thus give errors when you try
>>>>>> to assign to a non-const char * pointer.  But the option has to be
>>>>>> specified explicitly (it is not in -Wall) because it changes the
>>>>>> meaning of the code and can cause compatibility issues with
>>>>>> existing correct code.
>>>>>
>>>>> -Wwrite-strings is included in -Wpedantic.
>>>>
>>>> No it isn't, nor is it included in -Wall -- and it wouldn't make
>>>> sense to do so.
>>>>
>>>> The -Wpedantic option is intended to produce all required
>>>> diagnostics for the specified C standard.  -Wwrite-strings
>>>> gives string literals the type `const char[LENGTH]`, which
>>>> enables useful diagnostics but is *non-conforming*.
>>>
>>> As long as the -Wwrite-strings diagnostics are only warnings the
>>> result is still conforming.
>>
>> It's not just about diagnostics.  This program:
>>
>> #include <stdio.h>
>> int main(void) {
>>      puts(_Generic("hello",
>>                    char*:  "char*",
>>                    const char*:  "const char*",
>>                    default: "?"));
>> }
>>
>> must print "char*" in a conforming implementation.  With
>> (gcc|clang) -Wwrite-strings, it prints "const char*".
> 
> Good point.  I hadn't considered such cases.
> 
>> And something as simple as:
>>
>>      char *p = "hello";
>>
>> is rejected with a fatal error with "-Wwrite-strings -pedantic-errors".
> 
> That violates the "As long as the -Wwrite-strings diagnostics are
> only warnings" condition.

Indeed.

I personally think it is nice to have an option to make string literals 
"const" in C, even though it is non-conforming.  I also think it is very 
useful to have a warning on attempts to write to string literals.  But I 
think gcc has made a mistake here by conflating the two.  I'd rather see 
the warning being enabled by default (or at least in -Wall), while the 
"make string literals const" option should require an explicit flag and 
be a "-f" flag rather than a "-W" flag.  The current situation seems to 
be a quick-and-dirty way to get the warning.

Other people may have different opinions, of course :-)