Is stdlib's qsort recursive?

Question

I've read that qsort is just a generic sort, with no promises about implementation. I don't know about how libraries vary from platform to plaform, but assuming the Mac OS X and Linux implementations are broadly similar, are the qsort implementations recursive and/or require a lot of stack?

I have a large array (hundreds of thousands of elements) and I want to sort it without blowing my stack to oblivion. Alternatively, any suggestions for an equivalent for large arrays?

Answer 1

Here's a version from BSD, copyright Apple, presumably used in OS X at some time or another:

http://www.opensource.apple.com/source/xnu/xnu-1456.1.26/bsd/kern/qsort.c

It is call-recursive, although the upper bound on the depth of recursion is small, as Blindy explains.

Here's a version from glibc, presumably used in Linux systems at some time or another:

http://www.umcs.maine.edu/~chaw/200801/capstone/n/qsort.c

It's not call recursive. For exactly the same reason that the limit on call-recursion is small, it can use a small fixed amount of stack to manage its loop-recursion.

Can I be bothered to look up the latest versions? Nope ;-)

For a few hundred thousand array elements, even the call-recursive implementation won't call more than 20 levels deep. In the grand scheme of things that is not deep, except on very limited embedded devices, which wouldn't have enough memory for you to have an array that big to sort in the first place. When N is bounded above, O(log N) obviously is a constant, but more than that it's normally quite a manageable constant. Usually 32 or 64 times "small" is "reasonable".

Answer 2

You know, the recursive part is logn deep. In 64 levels of recursion (which is ~64*4=~256 bytes of stack total) you can sort an array of size ~2^64, ie an array as large as you can address on a 64 bit cpu, which is 147573952589676412928 bytes for 64 bit integers. You can't even hold it in memory!

Worry about stuff that matters imo.