class TMapFile: public TObject

 Default ctor. Does not much except setting some basic values.

 Create a memory mapped file. This opens a file (to which the
 memory will be mapped) and attaches a memory region to it.
 Option can be either: "NEW", "CREATE", "RECREATE", "UPDATE" or
 "READ" (see TFile). The default open mode is "READ". The size
 argument specifies the maximum size of shared memory file in bytes.
 This protected ctor is called via the static Create() method.

 Private copy ctor. Used by the the ctor to create a new version
 of TMapFile in the memory mapped heap. It's main purpose is to
 correctly create the string data members.

 TMapFiles may not be deleted, since we want to keep the complete
 TMapFile object in the mapped file for later re-use. To enforce this
 the delete operator has been made private. Use Close() to properly
 terminate a TMapFile (also done via the TROOT dtor).

 Create the directory associated to this mapfile

 Add an object to the list of objects to be stored in shared memory.
 To place the object actually into shared memory call Update().

 Update an object (or all objects, if obj == 0) in shared memory.

 Remove object from shared memory. Returns pointer to removed
 object if successful, 0 otherwise.

 Remove object by name from shared memory. Returns pointer to removed
 object if successful, 0 otherwise.

 Remove all objects from shared memory.

 Return pointer to object retrieved from shared memory. The object must
 be deleted after use. If delObj is a pointer to a previously allocated
 object it will be deleted. Returns 0 in case object with the given
 name does not exist.

 Create semaphore used for synchronizing access to shared memory.

 Delete the semaphore.

 Acquire semaphore. Returns 0 if OK, -1 on error.

 Release semaphore. Returns 0 if OK, -1 on error.

 Close a mapped file. First detach mapped memory then close file.
 No member functions of a TMapFile that was opened in write mode
 may be called after Close() (this includes, of course, "delete" which
 would call the dtors). The option="dtor" is only used when called
 via the ~TMapFile.

 Returns shadow map file.

 Print some info about the mapped file.

 Returns kTRUE in case object is a folder (i.e. contains browsable lists).

 Browse contents of TMapFile.

 Cd to associated directory,

 List contents of TMapFile.

 Increment statistics for buffer sizes of objects in this file.

 Return the best buffer size for objects in this file.

 The best buffer size is estimated based on the current mean value
 and standard deviation of all objects written so far to this file.
 Returns mean value + one standard deviation.

 Create a memory mapped file. This opens a file (to which the
 memory will be mapped) and attaches a memory region to it.
 Option can be either: "NEW", "CREATE", "RECREATE", "UPDATE"
 or "READ" (see TFile). The default open mode is "READ". The size
 argument specifies the maximum size of shared memory file in bytes.
 TMapFile's can only be created via this method. Create() enforces that
 a TMapFile is always on the memory mapped heap (when "NEW", "CREATE"
 or "RECREATE" are used).

 Set preferred map address. Find out preferred map address as follows:
 1) Run consumer program to find the preferred map address:
       $ root
       root [0] m = TMapFile::Create("dummy.map", "recreate", 10000000);
       root [1] m.Print()
       Memory mapped file:   dummy.map
       Title:
       Option:               CREATE
       Mapped Memory region: 0x40b4c000 - 0x40d95f00 (2.29 MB)
       Current breakval:     0x40b53000
       root [2] .q
       $ rm dummy.map
    Remember begin of mapped region, i.e. 0x40b4c000

 2) Add to producer program, just before creating the TMapFile:
       TMapFile::SetMapAddress(0x40b4c000);

 Repeat this if more than one map file is being used.

 The above procedure allow programs using, e.g., different number of
 shared libraries (that cause the default mapping address to be
 different) to create shared memory regions in the same location
 without overwriting a shared library. The above assumes the consumer
 program is larger (i.e. has more shared memory occupied) than the
 producer. If this is not true inverse the procedure.

 Return the base address at which we would like the next TMapFile's
 mapped data to start.

 For now, we let the system decide (start address 0). There are
 a lot of issues to deal with here to make this work reasonably,
 including:

 - Avoid memory collisions with existing mapped address spaces

 - Reclaim address spaces when their mmalloc heaps are unmapped

 - When mmalloc heaps are shared between processes they have to be
   mapped at the same addresses in each

 Once created, a mmalloc heap that is to be mapped back in must be
 mapped at the original address.  I.E. each TMapFile will expect
 to be remapped at it's original address. This becomes a problem if
 the desired address is already in use.

 Need special "operator delete" in which we close the shared memory.
 This has to be done after the dtor chain has been finished.