MATLAB Programming

Accessing Files with Memory-Mapping
Deleting a Memory Map
It is not necessary to explicitly call a destructor method to clear a memmapfile
object from memory when you no longer need it. MATLAB calls the destructor
for you whenever you do any of the following:
• Reassign anothervaluetothememmapfile object’s variable
• Clear the object’s variable from memory
• Exit the function scope in which the object was created
The Effect of Shared Data Copies On Performance
When you assign the Data field of the memmapfile object to a variable,
MATLAB makes a shared data copy of the mapped data. This is very efficient
as no memory actually gets copied. In the following statement, memdat is a
shared data copy of the data mapped from the file:
memdat = m.Data;
When you finish using the mapped data, make sure to clear any variables
that shared data with the mapped file before clearing the object itself. If you
clear the object first, then the sharing of data between the file and dependent
variables is broken, and the data assigned to such variables must be copied
into memory before the object is destroyed. If access to the mapped file was
over a network, then copying this data to local memory can take considerable
time. So, if the statement shown above assigns data to the variable memdat,
you should be sure to clear memdat before clearing m when you are finished
with the object.
Note Keep in mind that the memmapfile object can be cleared in any of the
three ways described under “Deleting a Memory Map” on page 6-69.
Memory-Mapping Demo
In this demonstration, two separate MATLAB processes communicate with
each other by writing and reading from a shared file. They share the file by
mapping part of their memory space to a commonlocationinthefile.Awrite
operation to the memory map belonging to the first process can be read from
the map belonging to the second, and vice versa.
6-69