Sort of. If rand() wasn't built into the standard, it would be impossible because you could override it at link-time with another function that connects to your

Sort of. If rand() wasn't built into the standard, it would be impossible because you could override it at link-time with another function that connects to your database or something. And the linker runs after the compiler. But because it's built-in, the compiler might be able to infer more about it, just like it reduces: printf("x"); to putchar('x');