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

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.