cp_shared_mempool_free
returns a memory chunk obtained from
cp_shared_mempool_alloc(3)
or
cp_mempool_calloc(3)
to the memory pool for reuse. Note that this does not actually release the
memory block to the operating system. Memory allocated by cp_shared_mempool is
only returned to the operating system on pool destruction.
RETURN VALUE
zero on success or non-zero on locking failure if COLLECTION_MODE_NOSYNC is
not set.