"kernel" heap idea

DJ Delorie dj@delorie.com
Tue Jan 4 15:42:00 GMT 2000

Many unixes have a set of functions that function like
malloc/realloc/free, but use the kernel's memory heap instead of the
application's heap.  With Cygwin, such a feature could be used to
manage a chunk of shared memory instead of hard-coding items into the
existing shared memory area.  This would especially be useful for
cases where the number of items is variable (like delqueue).

While reading up on how to do this under Win32, I did notice
two shortcomings:

* NT doesn't guarantee that a given virtual memory range will be
  available for the next cygwin program, so the shared memory region
  may not be able to be mapped in the same place for each invocation
  of the dll.  I don't think this will be much of a problem for most
  apps, as the cygwin dll gets to initialize itself pretty early, but
  we may want a way to let the user pick a different address (perhaps
  through the tunable parameter idea I posted earlier) to avoid
  conflict with other allocations.

* Win32 doesn't let you expand a shared memory region once you've
  created it, so if we run out of memory in that region, we're out.
  Again, this may be a case where a tunable parameter is the right way
  to deal with this, and we'd need to be careful with functions that
  use the shared area to make sure they return the right errors if it
  runs out.

I don't think either of these are real show-stoppers as long as we
don't get carried away with what we put in the shared area.


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