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.