Creating a memory mapped file

I am trying to create a new memory-mapped 'big data' file by using sequential writes of structured binary data, where I write a vector of values for each variable separately. First, can I start with an empty file that I have opened for write? I cannot seem to write an array into mapped location unless data already exists at that location. See below

%%define the  memory map for the output binary file
  OutFile = 'testOutFile';
  fid = fopen('test.lls','w');
  mmLLS = memmapfile('test.lls',...
    'Format',{          ...
    'single' 1 'lat';   ...
    'single' 1 'lon'},...
    'Repeat',1,'Writable',true);

Lat = single(32.3);
Lon = single(-110.2);
mmLLS.Data.lat = Lat;
Error using memmapfile/subsasgn (line 732)
A subscripting operation on the Data field attempted to create a
comma-separated list. The memmapfile class does not support the use
of comma-separated lists when subscripting.

[per isakson]

"First, can I start with an empty file that I have opened for write?" &nbsp The short answer is NO!

Doc says: "Memory-mapping is a mechanism that maps a portion of a [existing] file, or an entire [existing] file, on disk to a range of addresses within an application's address space."

Answer 1

[Walter Roberson]

http://www.mathworks.com/help/matlab/ref/memmapfile.html

"memmapfile does not expand or append to a mapped file. Use instead standard file I/O functions like fopen and fwrite."

[Ken]

Thank you for this clarification, although it is disappointing, when combined with the fact that Matlab cannot write sequences of binary complex structures.

It would be helpful if this fact were stated clearly/explicitly when describing the capability to write to a memory mapped file.