Why no sanity checks in legacy strcpy()

Question

Following is the most popular implementation of strcpy in traditional systems. Why dest and src are not checked for NULL in the start? I heard once that in old days the memory was limited so short code was always preferred. Will you implement strcpy and other similar functions with NULL pointer checks at the start now days? Why not?

char *strcpy(char *dest, const char *src)
{
   char *save = dest;
   while(*dest++ = *src++);
   return save;
}

Answer 1

NULL is a bad pointer, but so is (char*)0x1. Should it also check for that? In my opinion (I don't know the definitive reason why), sanity checks in such a low-level operation are uncalled for. strcpy() is so fundamental that it should be treated something like as asm instruction, and you should do your own sanity checks in the caller if needed. Just my 2 cents :)

Answer 2

There are no sanity checks because one of the most important underlying ideologies of C is that the developer supplies the sanity. When you assume that the developer is sane, you end up with a language that can be used to do just about anything, anywhere.

This is not an explicitly stated goal — it's quite possible for someone to come up with an implementation that does check for this, and more. Maybe they have. But I doubt that many people used to C would clamour to use it, since they'd need to put the checks in anyway if there was any chance that their code would be ported to a more usual implementation.