This all started because I was complaining about some uninitialized pointer value causing me grief[^mmap] and someone (explicitly trolling) said they always check pointers using:
int fds[2] = { -1, -1};
pipe(fds);
if (write(fds[0], pointer_to_check, sizeof(intptr_t)) == -1) {
close(fds[0]);
close(fds[1]);
return not_valid;
} else {