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The memmapfile Class

In this section...

“Setting Properties” on page 4-7

“Viewing Properties” on page 4-8

MATLAB implements memory-mapping using an object-oriented class called

memmapfile

.The

memmapfile

class has the properties and methods you need

to map to a file, read and write the file via the map, and remove the map from

memory when you are done.

Setting Properties

There are six properties defined for the

memmapfile

class. These are shown in

the table below. These properties control which file is being mapped, where in

the file the mapping is to begin and end, how the contents of the file are to be

formatted, and whether or not the file is writable. One property of the file

contains the file data itself.

Property

Description

Data Type

Default

Data

Contains the data read from the file or to be written

to the file. (See “Reading a Mapped File” on page

4-24 and “Writing to a Mapped File” on page 4-30)

Any of the

numeric

types

None

Filename

Path and name of the file to map into memory. (See

“Selecting the File to Map” on page 4-13)

char

array

None

Format

Format of the contents of the mapped region,

including class, array shape, and variable or field

name by which to access the data. (See “Identifying

the Contents of the Mapped Region” on page 4-14)

char

array

or N-by-3

cell array

uint8

Offset

Number of bytes from the start of the file to the start

of the mapped region. This number is zero-based.

That is, offset 0 represents the start of the file. Must

be a nonnegative integer value. (See “Setting the

Start of the Mapped Region” on page 4-14)

double

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Memory-Mapping Data Files

Property

Description

Data Type

Default

Repeat

Number of times to apply the specified format to the

mapped region of the file. Must be a positive integer

value or

Inf

.(See “Repeating a Format Scheme” on

page 4-21)

double

Inf

Writable

Type of access allowed to the mapped region. Must

be logical 1 or logical 0. (See “Setting the Type of

Access” on page 4-22)

logical

false

You can set the values for any property except for

Data

at the time you call

the

memmapfile

constructor, or at any time after that while the map is still

valid. Any properties that are not explicitly set when you construct the object

are given their default values as shown in the table above. For information on

calling the constructor, see “Constructing a memmapfile Object” on page 4-10.

Once a

memmapfile

object has been constructed, you can change the value of

any of its properties. Use the

objname.property

syntax in assigning the new

value. For example, to set a new

Offset

value for memory map object

,type

m.Offset = 2048;

Note Property names are not case sensitive. For example, MATLAB

considers

m.offset

to be the same as

m.Offset

Viewing Properties

To display the value of all properties of a

memmapfile

object, simply type the

object name. For a

memmapfile

object

,typing the variable name

displays

the following. Note that this example requires the file

records.dat

which

you will create at the beginning of the next section.

m =

Filename: 'records.dat'

Writable: true

Offset: 1024

Format: 'uint32'

Repeat: Inf

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The memmapfile Class

Data: 4778x1 uint32 array

To display the value of any individual property, for example the

Writable

property of object

,type

m.Writable

ans =

true

Alternatively, use the

disp (memmapfile)

get (memmapfile)

methods to

view properties.

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Memory-Mapping Data Files

Constructing a memmapfile Object

In this section...

“How to Run Examples in This Section” on page 4-10

“Constructing the Object with Default Property Values” on page 4-11

“Changing Property Values” on page 4-11

“Selecting the File to Map” on page 4-13

“Setting the Start of the Mapped Region” on page 4-14

“Identifying the Contents of the Mapped Region” on page 4-14

“Mapping of the Example File” on page 4-19

“Repeating a Format Scheme” on page 4-21

“Setting the Type of Access” on page 4-22

How to Run Examples in This Section

Most of the examples in this section use a file named

records.dat

that

contains a 5000-by-1 matrix of double-precision floating point numbers. Use

the following code to generate this file before going on to the next sections

of this documentation.

First, save this function in your current working directory:

function gendatafile(filename, count)

dmax32 = double(intmax('uint32'));

randData = gallery('uniformdata', [count, 1], 0) * dmax32;

fid = fopen(filename, 'w');

fwrite(fid, randData, 'double');

fclose(fid);

Now execute the

gendatafile

function to generate the

records.dat

file

that is referenced in this section. You can use this function at any time

to regenerate the file:

gendatafile('records.dat', 5000);

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Constructing a memmapfile Object

Constructing the Object with Default Property Values

The first step in mapping to any file is to construct an instance of the

memmapfile

class using the class constructor function. You can haveMATLAB

assign default values to each of the new object’s properties, or you can specify

property values yourself in the call to the

memmapfile

constructor.

The simplest and most general way to call the constructor is with one input

argument that specifies the name of the file you want to map. All other

properties are optional and are given their default values. Use the syntax

shown here:

objname = memmapfile(filename)

To construct a map for the file

records.dat

that resides in your current

working directory, type the following:

m = memmapfile('records.dat')

m =

Filename: 'd:\matlab\records.dat'

Writable: false

Offset: 0

Format: 'uint8'

Repeat: Inf

Data: 40000x1 uint8 array

MATLAB constructs an instance of the

memmapfile

class, assigns it to the

variable

,and maps the entire

records.dat

file to memory, setting all

object properties to their default values. 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.

Changing Property Values

You can make the memory map more specific to your needs by including

more information when calling the constructor. In addition to the

filename

argument, there are four other parameters that you can pass to the

constructor. Each of these parameters represents a property of the object, and

each requires an accompanying value to be passed, as well:

objname = memmapfile(filename, prop1, value1, prop2, value2, ...)

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Memory-Mapping Data Files

For example, to construct a map using nondefault values for the

Offset

Format

,and

Writable

properties, type the following, enclosing all property

names and string parameter values in quotes:

m = memmapfile('records.dat', ...

'Offset', 1024,

...

'Format', 'double',

...

'Writable', true);

Type the object name to see the current settings for all properties:

m =

Filename: 'd:\matlab\records.dat'

Writable: true

Offset: 1024

Format: 'double'

Repeat: Inf

Data: 4872x1 double array

You can also change the value of any property after the object has been

constructed. Use the syntax:

objname.property = newvalue;

For example, to set the format to

uint16

,type the following. (Property names,

Format

,are not case sensitive.)

m.format = 'uint16'

m =

Filename: 'd:\matlab\records.dat'

Writable: true

Offset: 1024

Format: 'uint16'

Repeat: Inf

Data: 19488x1 uint16 array

Further read and write operations to the region mapped by

now treat the

data in the file as a sequence of unsigned 16-bit integers. Whenever you

change the value of a

memmapfile

property, MATLAB remaps the file to

memory.

4-12

Constructing a memmapfile Object

Selecting the File to Map

filename

is the only required argument when you call the

memmapfile

constructor. When you call the

memmapfile

constructor, MATLAB assigns the

file name that you specify to the

Filename

property of the new object instance.

Specify the file name as a quoted string, (e.g.,

'records.dat'

). It must

be first in the argument list and not specified as a parameter-value pair.

filename

must include a file name extension if the name of the file being

mapped has an extension. The

filename

argument cannot include any

wildcard characters (e.g.,

), and is not case sensitive.

Note Unlike the other

memmapfile

constructor arguments, you must specify

filename

as a single string, and not as a parameter-value pair.

If the file to be mapped resides somewhere on the MATLAB path, you can use

apartial pathname. If the path to the file is not fully specified, MATLAB

searches for the file in your current working directory first, and then on the

MATLAB path.

Once

memmapfile

locates the file, MATLAB stores the absolute path name for

the file internally, and then uses this stored path to locate the file from that

point on. This enables you to work in other directories outside your current

work directory and retain access to the mapped file.

You can change the value of the

Filename

property at any time after

constructing the

memmapfile

object. You might want to do this if:

• You want to use the same

memmapfile

object on more than one file.

• You save your

memmapfile

object to a MAT-file, and then later load it back

into MATLAB in an environment where the mapped file has been moved to

adifferent location. This requires that you modify the path segment of the

Filename

string to represent the new location.

For example, save

memmapfile

object

to file

mymap.mat

disp(m.Filename)

d:\matlab\records.dat

4-13

Memory-Mapping Data Files

save mymat m

Now move the file to another location, load the object back into MATLAB, and

update the path in the

Filename

property:

load mymat m

m.Filename = 'f:\testfiles\oct1\records.dat'

Note You can only map an existing file. You cannot create a new file and map

that file to memory in one operation. Use the MATLAB file I/O functions to

create the file before attempting to map it to memory.

Setting the Start of the Mapped Region

By default, MATLAB begins a memory map at the start of the file. To begin

the mapped region at some point beyond the start of the file, specify an

Offset

parameter in the call to the

memmapfile

constructor:

objname = memmapfile(filename, 'Offset', bytecount)

The

bytecount

value is the number of bytes from the beginning of the file to

the point in the file where you want the memory map to start (a zero-based

offset). To map the file

records.dat

from a point 1024 bytes from the start

and extending to the end of the file, type

m = memmapfile('records.dat', 'Offset', 1024);

You can change the starting position of an existing memory map by setting

the

Offset

property for the associated object to a new value. The following

command sets the offset of

memmapfile

object

to be 2,048 bytes from the

start of the mapped file:

m.Offset = 2048;

Identifying the Contents of the Mapped Region

By default, MATLAB considers all the data in a mapped file to be a sequence

of unsigned 8-bit integers. To have the data interpreted otherwise as it is

read or written to in the mapped file, specify a

Format

parameter and value in

your call to the constructor:

4-14

Constructing a memmapfile Object

objname = memmapfile(filename, 'Format', formatspec)

The

formatspec

argument can either be a character string that identifies a

single class used throughout the mapped region, or a cell array that specifies

more than one class.

For example, say that you map a file that is 12 kilobytes in length. Data read

from this file could be treated as a sequence of 6,000 16-bit (2-byte) integers,

or as 1,500 8-byte double-precision floating-point numbers, to name just a

couple of possibilities. Or you could read this data in as a combination of

different types: for example, as 4,000 8-bit (1-byte) integers followed by 1,000

64-bit (8-byte) integers. You determine how MATLAB will interpret the

mapped data by setting the

Format

property of the

memmapfile

object when

you call its constructor.

MATLAB arrays are stored on disk in column-major order. (The sequence

of array elements is column 1, row 1; column 1, row 2; column 1, last row;

column 2, row 1, and so on.) You might need to transpose or rearrange the

order of array elements when reading or writing via a memory map.

For a list of data types supported for the

Format

property, see “Supported

Data Types for the Format Property” on page 4-20.

For more information on format options see these sections:

• “Mapping a Single Data Type” on page 4-15

• “Formatting the Mapped Data to an Array” on page 4-16

• “Mapping Multiple Data Types and Arrays” on page 4-17

Mapping a Single Data Type

If the file region being mapped contains data of only one type, specify the

Format

value as a character string identifying that type:

objname = memmapfile(filename, 'Format', datatype)

The following command constructs a

memmapfile

object for the entire file

records.dat

,and sets the

Format

property for that object to

uint64

. Any

read or write operations made via the memory map will read and write the

file contents as a sequence of unsigned 64-bit integers:

4-15

Memory-Mapping Data Files

m = memmapfile('records.dat', 'Format', 'uint64')

Filename: 'd:\matlab\records.dat'

Writable: false

Offset: 0

Format: 'uint64'

Repeat: Inf

Data: 5000x1 uint64 array

You can change the value of the

Format

property at any time after the

memmapfile

object is constructed. Use the

object.property

syntax shown

here in assigning the new value:

m.Format = 'int32';

Further read and write operations to the region mapped by

now treat the

data in the file as a sequence of signed 32-bit integers.

Property names, like

Format

,are not case sensitive.

Formatting the Mapped Data to an Array

You can also specify an array shape to be applied to the data read or written to

the mapped file, and a field name to be used in referencing this array. Use a

cell array to hold these values either when calling the

memmapfile

constructor

or when modifying

m.Format

after the object has been constructed. The cell

array contains three elements: the class to be applied to the mapped region,

the dimensions of the array shape that is applied to the region, and a field

name to use in referencing the data:

objname = memmapfile(filename, ...

'Format', {datatype, dimensions, varname})

The following command constructs a

memmapfile

object for a region of

records.dat

such that MATLAB handles the contents of the region as a

4-by-10-by-18 array of unsigned 32-bit integers, which you can reference in

the structure of the returned object using the field name

m = memmapfile('records.dat',

...

'Offset', 1024,

...

'Format', {'uint32' [4 10 18] 'x'})

m =

4-16

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