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`__malloc_lock', `__malloc_unlock'-lock malloc pool

     #include <malloc.h>
     void __malloc_lock (struct _reent *REENT);
     void __malloc_unlock (struct _reent *REENT);
The `malloc' family of routines call these functions when they need to
lock the memory pool.  The version of these routines supplied in the
library does not do anything.  If multiple threads of execution can
call `malloc', or if `malloc' can be called reentrantly, then you need
to define your own versions of these functions in order to safely lock
the memory pool during a call.  If you do not, the memory pool may
become corrupted.
   A call to `malloc' may call `__malloc_lock' recursively; that is,
the sequence of calls may go `__malloc_lock', `__malloc_lock',
`__malloc_unlock', `__malloc_unlock'.  Any implementation of these
routines must be careful to avoid causing a thread to wait for a lock
that it already holds.