Why char *s = "hello"; is allowed?

Question

char *s = "hello";

The code above allocates 6 bytes in read-only section of a program (I've forgot the name of the section) to store the string hello. Then, s is initialized to point to the first character of the string hello. Modifying the string "hello" is undefined behavior. Besides, "hello" itself is constant in nature. The program doesn't have permission to change read-only section.

I'm using MS VC++ 2010 Express. My question is, why does the compiler allows s, which is a char *, to point to the constant string? Shouldn't there be a compiler error? Shouldn't the compiler force us to use const char *s = "hello"; instead of char *s = "hello";?

Thanks.

Answer 1

In C, "hello" has type char[]. This is not C++. See this question.

Answer 2

This predates the times when the const qualifier was introduced to C. The C standards body is very conservative with respect to existing code. Any improvement of the language is required to be done in such a way that it wouldn't break any existing conforming code that is written for the previous version of the standard.

If such things then lead to undesirable complications, the feature is then deprecated and maybe changed years after that.

For the concrete feature of string literals being typed char[] instead of char const[], yes this is unfortunately a pitfall for beginners. Just get the habit from the start to use char const* whenever you reference such strings.

Edit: For the question if a compiler could or should warn about that, I think this is just difficult to trace. In this code

int main(void) {
  "hello"[0] = 'H';
  char * a = "hoho";
  a[0] = 'H';
}

gcc only gives me a warning on the first assignement, not on the second. clang doesn't capture it at all.