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From: candycanearter07 <candycanearter07@candycanearter07.nomail.afraid>
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: Fri, 2 Aug 2024 05:30:02 -0000 (UTC)
Organization: the-candyden-of-code
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David Brown <david.brown@hesbynett.no> wrote at 17:56 this Thursday (GMT):
> On 01/08/2024 16:40, Michael S wrote:
>> On Thu, 01 Aug 2024 08:06:57 +0000
>> Mark Summerfield <mark@qtrac.eu> wrote:
>> 
>>> This program segfaults at the commented line:
>>>
>>> #include <ctype.h>
>>> #include <stdio.h>
>>>
>>> void uppercase_ascii(char *s) {
>>>      while (*s) {
>>>          *s = toupper(*s); // SEGFAULT
>>>          s++;
>>>      }
>>> }
>>>
>>> int main() {
>>>      char* text = "this is a test";
>>>      printf("before [%s]\n", text);
>>>      uppercase_ascii(text);
>>>      printf("after  [%s]\n", text);
>>> }
>>>
>> 
>> The answers to your question are already given above, so I'd talk about
>> something else. Sorry about it.
>> 
>> To my surprise, none of the 3 major compilers that I tried issued the
>> warning at this line:
>>    char* text = "this is a test";
>> If implicit conversion of 'const char*' to 'char*' does not warrant
>> compiler warning than I don't know what does.
>> Is there something in the Standard that explicitly forbids diagnostic
>> for this sort of conversion?
>> 
>> BTW, all 3 compilers issue reasonable warnings when I write it slightly
>> differently:
>>    const char* ctext = "this is a test";
>>    char* text = ctext;
>> 
>> I am starting to suspect that compilers (and the Standard?) consider
>> string literals as being of type 'char*' rather than 'const char*'.
>> 
>
> Your suspicions are correct - in C, string literals are used to 
> initialise an array of char (or wide char, or other appropriate 
> character type).  Perhaps you are thinking of C++, where the type is 
> "const char" (or other const character type).
>
> So in C, when a string literal is used in an expression it is converted 
> to a "char *" pointer.  You can, of course, assign that to a "const char 
> *" pointer.  But it does not make sense to have a warning when assigning 
> it to a non-const "char *" pointer.  This is despite it being undefined 
> behaviour (explicitly stated in the standards) to attempt to write to a 
> string literal.
>
> The reason string literals are not const in C is backwards compatibility 
> - they existed before C had "const", and making string literals into 
> "const char" arrays would mean that existing code that assigned them to 
> non-const pointers would then be in error.  C++ was able to do the right 
> thing and make them arrays of const char because it had "const" from the 
> beginning.
>
> 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.
-- 
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