todo

早期freebsd实现

Things that still need attention:

   *	Make implementation changes necessary to allow multiple processes
	to use the mmalloc managed region simultaneously.  This requires,
	at the minimum, some sort of cooperative locking that ensures that
	only one process at a time is changing any of the mmalloc managed
	data structures (its ok for the mmalloc managed data regions to be
	changed at any time since we don't care about their contents).

   *	In order to support multiple processes using the mmalloc managed
	region, the malloc descriptor needs to be broken into two parts,
	one part which is specific to the given process and is maintained
	separately on a per process basis, and another part which is common
	to all processes.  As an example, the file descriptor is specific
	to a given process, as are the morecore and abortfunc pointers.
	However magic[], the version number, the flags field, etc are
	common to all processes.