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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
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: Mon, 12 Aug 2024 16:05:29 -0700
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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.