> Operating systems can grow the amount of real memory allocated to a thread, but never shrink it.

Operating systems can shrink the memory usage of a stack.

  madvise(page, size, MADV_DONTNEED);

Linux also supports mremap, which is essentially a kernel version of realloc. Supports growing and shrinking memory mappings.

  stack = mremap(stack, old_size, old_size / 2, MREMAP_MAYMOVE, 0);