Re: No warning at implicit removal of const. Was: relearning C: why does an in-place change to a char* segfault?

David Brown <david.brown@hesbynett.no> writes:

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 :-)

I agree.  An alternative way to implement "-Wwrite-strings" might have
been to invent a new attribute that can be applied to string literal
objects.  With the current "-Wwrite-strings", gcc marks string literal
objects as const, with all the non-conforming consequences that implies.
Instead, they could have added an attribute like say, "unwritable" that
triggers warnings but no other changes in semantics and no fatal errors
(unless you use -Werror, but then you're literally asking for it).

--
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */