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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: Thu, 1 Aug 2024 19:56:00 +0200
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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.