The difference between const char *str and char *str is the level of protection provided for the string stored at the memory location str points to.

const char *str is a pointer to a constant character. This means that the string stored at the memory location str points to cannot be modified through str. Attempting to modify the string will result in a compile-time error.

char *str is a pointer to a character that is not constant. This means that the string stored at the memory location str points to can be modified through str.

int main() {
  const char* str = "hello";
  *str = 'H';

  return 0;
}

$ gcc test.c
test.c:3:8: error: read-only variable is not assignable
  *str = 'H';
  ~~~~ ^
1 error generated.

In general, it is good practice to use const char *str when passing strings to functions to protect the original data from being modified.