[Not yet complete]
Cygwin has recently adopted something called the "cygwin heap".  This is
an internal heap that is inherited by forked/execed children.  It
consists of process specific information that should be inherited.  So
things like the file descriptor table, the current working directory,
and the chroot value live there.

The cygheap is also used to pass argv information to a child process.
There is a problem here, though.  If you allocate space for argv on the
heap and then exec a process the child process (1) will happily use the
space in the heap.  But what happens when that process execs another
process (2)?  The space used by child process (1) still is being used in
child process (2) but it is basically just a memory leak.

To rectify this problem, memory used by child process 1 is tagged in
such a way that child process 2 will know to delete it.  This is in
cygheap_fixup_in_child.

The cygheap memory allocation functions are adapted from memory
allocators developed by DJ Delorie.  They are similar to early BSD
malloc and are intended to be relatively lightweight and relatively
fast.