Generally speaking a chunk of memory you free is simply marked "free" for reuse but not given back to the OS. So in this regard, it is not a burden to the OS.
It looks like by default, the GNU libc won't return pages to the OS until 128 KiB are free at the top of the heap segment. The compiler has to make many small allocations, so even if it freed memory at every opportunity I doubt the heap trimming would happen very often.
