Using original pointer after realloc?

I was reading Richard Reese's new (May 2013) O'Reilly book "Understanding and Using C Pointers", and I have a question about some code therein, on page 87. ``` if (++length > maximumLength) { char *newBuffer = realloc (buffer, maximumLength += sizeIncrement); if (newBuffer == NULL) { free (buffer); return NULL; } currentPosition = newBuffer + (currentPosition - buffer); buffer = newBuffer; } ``` I hope the names of the variables are self-explanatory; if context is needed, I will edit to provide the entire chunk of code and not just this excerpt. My question is about the line `currentPosition = newBuffer + (currentPosition - buffer);`. My understanding of `realloc()` is that when the new allocation succeeds, the originally allocated memory is freed. If that is correct, then the line in question is using dangling pointers, innit? Both `buffer` and `currentPosition` on the RHS of that expression are pointers to memory that has been freed. My instinct would be to rewrite this to avoid using the dangling pointers by using `length`, which after all is already around. I want to replace those last two lines with: ``` buffer = newBuffer; currentPosition = buffer + length; ``` However, presumably the code as written works because the two pointers still hold addresses (albeit of garbage), and the offset between those two addresses can still be calculated as a way of reassigning `currentPosition`. So am I being merely persnickety in feeling uneasy about this? To generalize the question: once a pointer is dangling, is it safe to use the address contained in the pointer for any purpose, such as calculating offsets? Thanks.