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:
fprintf, which writes formatted data to a text or ASCII file; that is, a file you can view in a text editor or import into a spreadsheet. For more information, see Export to Text Data Files with Low-Level I/O.
fwrite, which writes a stream of binary data to a file. For more information, see Write Binary Data to a File.
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]);
fwrite writes values from an array in column order as 8-bit unsigned integers (
When you finish processing a file, close it with
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'); fwrite(fileID,mydata,'double'); fclose(fileID);
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.
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:
'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
'a+'. If you open a file for both reading and writing, you must call
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+'); fwrite(fileID,magic(4));
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
Write the vector
[44 44 44 44] using
fwrite(fileID,[44 44 44 44]);
Read the results from the file into a 4-by-4 matrix.
frewind(fileID); newdata = fread(fileID,[4,4])
newdata = 4×4 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.
frewind(fileID); appended = fread(fileID, [4,5])
appended = 4×5 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
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')
'l' indicates little-endian ordering.
If you are not sure which byte ordering your system uses, call
[cinfo, maxsize, ordering] = computer
'L'for little-endian systems, or
'B'for big-endian systems.
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 = 5×5 complex 0.8147 + 0.7577i 0.0975 + 0.7060i 0.1576 + 0.8235i 0.1419 + 0.4387i 0.6557 + 0.4898i 0.9058 + 0.7431i 0.2785 + 0.0318i 0.9706 + 0.6948i 0.4218 + 0.3816i 0.0357 + 0.4456i 0.1270 + 0.3922i 0.5469 + 0.2769i 0.9572 + 0.3171i 0.9157 + 0.7655i 0.8491 + 0.6463i 0.9134 + 0.6555i 0.9575 + 0.0462i 0.4854 + 0.9502i 0.7922 + 0.7952i 0.9340 + 0.7094i 0.6324 + 0.1712i 0.9649 + 0.0971i 0.8003 + 0.0344i 0.9595 + 0.1869i 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 Followed By Imaginary Components
Write all the real components,
z_real, followed by all the imaginary components,
z_imag, to a file named
adjacent = [z_real z_imag]; fileID = fopen('complex_adj.bin', 'w'); fwrite(fileID,adjacent,'double'); fclose(fileID);
Read the values from the file using
fileID = fopen('complex_adj.bin'); same_real = fread(fileID, [nrows, ncols], 'double'); same_imag = fread(fileID, [nrows, ncols], 'double'); fclose(fileID); 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; end
Write the interleaved values to a file named
fileID = fopen('complex_int.bin','w'); fwrite(fileID, interleaved, 'double'); fclose(fileID);
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); fclose(fileID); same_z = complex(same_real, same_imag);