At the command prompt, create a sample file in your current folder called records.dat, containing 10 uint8 values.

myData = uint8(1:10)';

fileID = fopen('records.dat','w');
fwrite(fileID, myData,'uint8');
fclose(fileID);

Create a map for records.dat. When using memmapfile, the default data format is uint8 so the file name is the only required input argument in this case.

m = memmapfile('records.dat')

m = 

    Filename: 'd:\matlab\records.dat'
    Writable: false
      Offset: 0
      Format: 'uint8'
      Repeat: Inf
        Data: 10x1 uint8 array

MATLAB maps the entire records.dat file to memory, setting all properties of the memory map to their default values. The memory map is assigned to the variable, m. In this example, the command maps the entire file as a sequence of unsigned 8-bit integers and gives the caller read-only access to its contents.

View the mapped data by accessing the Data property of m.

m.Data

ans =

    1
    2
    3
    4
    5
    6
    7
    8
    9
   10

At the command prompt, create a sample file in your current folder called records.dat, containing 10,000 int32 values.

myData = int32([1:10000]);

fileID = fopen('records.dat','w');
fwrite(fileID,myData,'int32');
fclose(fileID);

Create a memory map for records.dat, and set the Format property for the output to int32. Also, set the Offset property to disregard the first 9000 bytes in the file, and the Writable property to permit write access.

m = memmapfile('records.dat',...
               'Offset',9000,...
               'Format','int32',...
               'Writable',true);

An Offset value of 9000 indicates that the first 9000 bytes of records.dat are not mapped.

Type the name of the memory map to see the current settings for all properties.

m

m =
    Filename: 'd:\matlab\records.dat'
    Writable: true
      Offset: 9000
      Format: 'int32'
      Repeat: Inf
        Data: 7750x1 int32 array

The Format property indicates that any read or write operation made via the memory map reads and writes the file contents as a sequence of signed 32-bit integers. The Data property contains only 7750 elements because the first 9000 bytes of records.dat, representing the first 2250 values in the file, are not mapped.

View the first five elements of the mapped data by accessing the Data property of m.

m.Data(1:5)

ans =

        2251
        2252
        2253
        2254
        2255

At the command prompt, create a sample file in your current folder called mybinary.bin, containing 100 double-precision values.

rng('default')
randData = rand([100,1]);

fileID = fopen('mybinary.bin','w');
fwrite(fileID,randData,'double');
fclose(fileID);

Map the first 75 values in mybinary.bin to a 5-by-5-by-3 array of double-precision values that can be referenced in the structure of the memory map using the field name x. Specify these parameters with the Format name-value pair argument.

m = memmapfile('mybinary.bin',...
               'Format',{'double',[5 5 3],'x'})

m = 

    Filename: 'd:\matlab\mybinary.bin'
    Writable: false
      Offset: 0
      Format: {'double' [5 5 3] 'x'}
      Repeat: Inf
        Data: 1x1 struct array with fields:
                x

The Data property is a structure array that contains the mapped values in the field, x.

Assign the mapped data to a variable, A. Because the Data property is a structure array, you must index into the field, x, to access the data.

A = m.Data.x;

View information about A.

whos A

  Name      Size             Bytes  Class     Attributes

  A         5x5x3              600  double                       

At the command prompt, create a sample file in your current folder called mybinary.bin. Write uint16 data and double-precision data representing sample pressure, temperature, and volume values into the file. In this case, each of the uint16 arrays are 50-by-1 and the double-precision arrays are 5-by-10. k is a sample scaling factor.

rng('default')
k = 8.21;
pres1 = randi([1,300],[50,1],'uint16');
temp1 = randi([1,300],[50,1],'uint16');
vol1 = double(reshape(k*temp1./pres1,5,10));
pres2 = randi([5,500],[50,1],'uint16');
temp2 = randi([5,500],[50,1],'uint16');
vol2 = double(reshape(k*temp2./pres2,5,10));

fileID = fopen('mybinary.bin','w');
fwrite(fileID,pres1,'uint16');
fwrite(fileID,temp1,'uint16');
fwrite(fileID,vol1,'double');
fwrite(fileID,pres2,'uint16');
fwrite(fileID,temp2,'uint16');
fwrite(fileID,vol2,'double');
fclose(fileID);

Map the file to arrays accessible by unique names. Define a field, pressure, containing a 50-by-1 array of uint16 values, followed by a field, temperature, containing 50-by-1 uint16 values. Define a field, volume, containing a 5-by-10 array of double-precision values. Use a cell array to define the format of the mapped region and repeat the pattern twice.

m = memmapfile('mybinary.bin',...
'Format',{'uint16',[50 1],'pressure';...
'uint16',[50,1],'temperature';...
'double',[5,10],'volume'},'Repeat',2)

m = 

    Filename: 'd:\matlab\mybinary.bin'
    Writable: false
      Offset: 0
      Format: {'uint16' [50 1] 'pressure'
               'uint16' [50 1] 'temperature'
               'double' [5 10] 'volume'}
      Repeat: 2
        Data: 2x1 struct array with fields:
         pressure
      temperature
           volume

The Data property of the memory map, m, is a 2-by-1 structure array because the Format is applied twice.

Copy the Data property to a variable, A. Then, view the last block of double data, which you can access using the field name, volume.

A = m.Data;
myVolume = A(2).volume

myVolume =

     2    13    32     5     5    16     4    22     3     8
     2     9    53    38    13    19    23    85     2   120
    29    10     6     1     2     5     6    58    20    11
     7    15     4     1     5    18     1     4    14     8
     9     8     4     2     0     9     8     6     3     3