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Export Binary Data with Low-Level I/O

Low-Level Functions for Exporting Data

Low-level file I/O functions allow the most direct control over reading or writing data to a file. However, these functions require that you specify more detailed information about your file than the easier-to-use high-level functions. For a complete list of high-level functions and the file formats they support, see Supported File Formats for Import and Export.

If the high-level functions cannot export your data, use one of the following:

    Note:   The low-level file I/O functions are based on functions in the ANSI® Standard C Library. However, MATLAB® includes vectorized versions of the functions, to read and write data in an array with minimal control loops.

Write Binary Data to a File

This example shows how to use the fwrite function to export a stream of binary data to a file.

Create a file named nine.bin with the integers from 1 to 9. As with any of the low-level I/O functions, before writing, open or create a file with fopen and obtain a file identifier.

fileID = fopen('nine.bin','w');
fwrite(fileID, [1:9]);

By default, fwrite writes values from an array in column order as 8-bit unsigned integers (uint8).

When you finish processing a file, close it with fclose.


Create a file with double-precision values. You must specify the precision of the values if the values in your matrix are not 8-bit unsigned integers.

mydata = [pi 42 1/3];

fileID = fopen('double.bin','w');

Overwrite or Append to an Existing Binary File

This example shows how to overwrite a portion of an existing binary file and append values to the file.

By default, fopen opens files with read access. To change the type of file access, use the permission specifier in the call to fopen. Possible permission specifiers include:

  • 'r' for reading

  • 'w' for writing, discarding any existing contents of the file

  • 'a' for appending to the end of an existing file

To open a file for both reading and writing or appending, attach a plus sign to the permission, such as 'w+' or 'a+'. If you open a file for both reading and writing, you must call fseek or frewind between read and write operations.

Overwrite a Portion of an Existing File

Create a file named magic4.bin, specifying permission to write and read.

fileID = fopen('magic4.bin','w+');

The original magic(4) matrix is:

16     2     3    13
 5    11    10     8
 9     7     6    12
 4    14    15     1

The file contains 16 bytes, 1 for each value in the matrix.

Replace the values in the second column of the matrix with the vector, [44 44 44 44]. To do this, first seek to the fourth byte from the beginning of the file using fseek.


Write the vector [44 44 44 44] using fwrite.

fwrite(fileID,[44 44 44 44]);

Read the results from the file into a 4-by-4 matrix.

newdata = fread(fileID,[4,4])
newdata =

    16    44     3    13
     5    44    10     8
     9    44     6    12
     4    44    15     1

Close the file.


Append Binary Data to Existing File

Append the values [55 55 55 55] to magic4.bin. First. open the file with permission to append and read.

fileID = fopen('magic4.bin','a+');

Write values at end of file.

fwrite(fileID,[55 55 55 55]);

Read the results from the file into a 4-by-5 matrix.

appended = fread(fileID, [4,5])
appended =

    16    44     3    13    55
     5    44    10     8    55
     9    44     6    12    55
     4    44    15     1    55

Close the file.


Create a File for Use on a Different System

Different operating systems store information differently at the byte or bit level:

  • Big-endian systems store bytes starting with the largest address in memory (that is, they start with the big end).

  • Little-endian systems store bytes starting with the smallest address (the little end).

Windows® systems use little-endian byte ordering, and UNIX® systems use big-endian byte ordering.

To create a file for use on an opposite-endian system, specify the byte ordering for the target system. You can specify the ordering in the call to open the file, or in the call to write the file.

For example, to create a file named myfile.bin on a big-endian system for use on a little-endian system, use one (or both) of the following commands:

  • Open the file with

    fid = fopen('myfile.bin', 'w', 'l')
  • Write the file with

    fwrite(fid, mydata, precision, 'l')

where 'l' indicates little-endian ordering.

If you are not sure which byte ordering your system uses, call the computer function:

[cinfo, maxsize, ordering] = computer
The returned ordering is 'L' for little-endian systems, or 'B' for big-endian systems.

Open Files with Different Character Encodings

Encoding schemes support the characters required for particular alphabets, such as those for Japanese or European languages. Common encoding schemes include US-ASCII or UTF-8.

The encoding scheme determines the number of bytes required to read or write char values. For example, US-ASCII characters always use 1 byte, but UTF-8 characters use up to 4 bytes. MATLAB automatically processes the required number of bytes for each char value based on the specified encoding scheme. However, if you specify a uchar precision, MATLAB processes each byte as uint8, regardless of the specified encoding.

If you do not specify an encoding scheme, fopen opens files for processing using the default encoding for your system. To determine the default, open a file, and call fopen again with the syntax:

[filename, permission, machineformat, encoding] = fopen(fid);

If you specify an encoding scheme when you open a file, the following functions apply that scheme: fscanf, fprintf, fgetl, fgets, fread, and fwrite.

For a complete list of supported encoding schemes, and the syntax for specifying the encoding, see the fopen reference page.

Write and Read Complex Numbers

This example shows how to write and read complex numbers in binary files.

The available precision values for fwrite do not explicitly support complex numbers. To store complex numbers in a file, separate the real and imaginary components and write them separately to the file. There are two ways to do this:

  • Write all real components followed by all imaginary components

  • Interleave the components

Use the approach that allows you to read the data in your target application.

Separate Real and Imaginary Components

Create an array that contains complex values.

nrows = 5;
ncols = 5;
z = complex(rand(nrows, ncols), rand(nrows, ncols))
z =

  Columns 1 through 4

   0.8147 + 0.7577i   0.0975 + 0.7060i   0.1576 + 0.8235i   0.1419 + 0.4387i
   0.9058 + 0.7431i   0.2785 + 0.0318i   0.9706 + 0.6948i   0.4218 + 0.3816i
   0.1270 + 0.3922i   0.5469 + 0.2769i   0.9572 + 0.3171i   0.9157 + 0.7655i
   0.9134 + 0.6555i   0.9575 + 0.0462i   0.4854 + 0.9502i   0.7922 + 0.7952i
   0.6324 + 0.1712i   0.9649 + 0.0971i   0.8003 + 0.0344i   0.9595 + 0.1869i

  Column 5

   0.6557 + 0.4898i
   0.0357 + 0.4456i
   0.8491 + 0.6463i
   0.9340 + 0.7094i
   0.6787 + 0.7547i

Separate the complex values into real and imaginary components.

z_real = real(z);
z_imag = imag(z);

Write All Real Components Follwed By Imaginary Components

Write all the real components, z_real, followed by all the imaginary components, z_imag, to a file named complex_adj.bin.

adjacent = [z_real z_imag];

fileID = fopen('complex_adj.bin', 'w');

Read the values from the file using fread.

fileID = fopen('complex_adj.bin');
same_real = fread(fileID, [nrows, ncols], 'double');
same_imag = fread(fileID, [nrows, ncols], 'double');

same_z = complex(same_real, same_imag);

Interleave Real and Imaginary Components

An alternative approach is to interleave the real and imaginary components for each value. fwrite writes values in column order, so build an array that combines the real and imaginary parts by alternating rows.

First, preallocate the interleaved array.

interleaved = zeros(nrows*2, ncols);

Alternate real and imaginary data.

newrow = 1;
for row = 1:nrows
    interleaved(newrow,:) = z_real(row,:);
    interleaved(newrow + 1,:) = z_imag(row,:);
    newrow = newrow + 2;

Write the interleaved values to a file named complex_int.bin.

fileID = fopen('complex_int.bin','w');
fwrite(fileID, interleaved, 'double');

Open the file for reading and read the real values from the file. The fourth input to fread tells the function to skip the specified number of bytes after reading each value.

fileID = fopen('complex_int.bin');
same_real = fread(fileID, [nrows, ncols], 'double', 8);

Return to the first imaginary value in the file. Then, read all the imaginary data.

fseek(fileID, 8, 'bof');
same_imag = fread(fileID, [nrows, ncols], 'double', 8);

same_z = complex(same_real, same_imag);

See Also

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