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<h4 class="subsection">14.2.16 Binary I/O</h4>
<p>Octave can read and write binary data using the functions <code>fread</code>
and <code>fwrite</code>, which are patterned after the standard C functions
with the same names. They are able to automatically swap the byte order
of integer data and convert among the supported floating point formats
as the data are read.
<!-- file-io.cc -->
<p><a name="doc_002dfread"></a>
<div class="defun">
— Built-in Function: [<var>val</var>, <var>count</var>] = <b>fread</b> (<var>fid, size, precision, skip, arch</var>)<var><a name="index-fread-800"></a></var><br>
<blockquote><p>Read binary data of type <var>precision</var> from the specified file ID
<var>fid</var>.
<p>The optional argument <var>size</var> specifies the amount of data to read
and may be one of
<dl>
<dt><code>Inf</code><dd>Read as much as possible, returning a column vector.
<br><dt><var>nr</var><dd>Read up to <var>nr</var> elements, returning a column vector.
<br><dt><code>[</code><var>nr</var><code>, Inf]</code><dd>Read as much as possible, returning a matrix with <var>nr</var> rows. If the
number of elements read is not an exact multiple of <var>nr</var>, the last
column is padded with zeros.
<br><dt><code>[</code><var>nr</var><code>, </code><var>nc</var><code>]</code><dd>Read up to <var>nr</var><code> * </code><var>nc</var> elements, returning a matrix with
<var>nr</var> rows. If the number of elements read is not an exact multiple
of <var>nr</var>, the last column is padded with zeros.
</dl>
<p class="noindent">If <var>size</var> is omitted, a value of <code>Inf</code> is assumed.
<p>The optional argument <var>precision</var> is a string specifying the type of
data to read and may be one of
<dl>
<dt><code>"schar"</code><dt><code>"signed char"</code><dd>Signed character.
<br><dt><code>"uchar"</code><dt><code>"unsigned char"</code><dd>Unsigned character.
<br><dt><code>"int8"</code><dt><code>"integer*1"</code><dd>
8-bit signed integer.
<br><dt><code>"int16"</code><dt><code>"integer*2"</code><dd>16-bit signed integer.
<br><dt><code>"int32"</code><dt><code>"integer*4"</code><dd>32-bit signed integer.
<br><dt><code>"int64"</code><dt><code>"integer*8"</code><dd>64-bit signed integer.
<br><dt><code>"uint8"</code><dd>8-bit unsigned integer.
<br><dt><code>"uint16"</code><dd>16-bit unsigned integer.
<br><dt><code>"uint32"</code><dd>32-bit unsigned integer.
<br><dt><code>"uint64"</code><dd>64-bit unsigned integer.
<br><dt><code>"single"</code><dt><code>"float32"</code><dt><code>"real*4"</code><dd>32-bit floating point number.
<br><dt><code>"double"</code><dt><code>"float64"</code><dt><code>"real*8"</code><dd>64-bit floating point number.
<br><dt><code>"char"</code><dt><code>"char*1"</code><dd>Single character.
<br><dt><code>"short"</code><dd>Short integer (size is platform dependent).
<br><dt><code>"int"</code><dd>Integer (size is platform dependent).
<br><dt><code>"long"</code><dd>Long integer (size is platform dependent).
<br><dt><code>"ushort"</code><dt><code>"unsigned short"</code><dd>Unsigned short integer (size is platform dependent).
<br><dt><code>"uint"</code><dt><code>"unsigned int"</code><dd>Unsigned integer (size is platform dependent).
<br><dt><code>"ulong"</code><dt><code>"unsigned long"</code><dd>Unsigned long integer (size is platform dependent).
<br><dt><code>"float"</code><dd>Single precision floating point number (size is platform dependent).
</dl>
<p class="noindent">The default precision is <code>"uchar"</code>.
<p>The <var>precision</var> argument may also specify an optional repeat
count. For example, ‘<samp><span class="samp">32*single</span></samp>’ causes <code>fread</code> to read
a block of 32 single precision floating point numbers. Reading in
blocks is useful in combination with the <var>skip</var> argument.
<p>The <var>precision</var> argument may also specify a type conversion.
For example, ‘<samp><span class="samp">int16=>int32</span></samp>’ causes <code>fread</code> to read 16-bit
integer values and return an array of 32-bit integer values. By
default, <code>fread</code> returns a double precision array. The special
form ‘<samp><span class="samp">*TYPE</span></samp>’ is shorthand for ‘<samp><span class="samp">TYPE=>TYPE</span></samp>’.
<p>The conversion and repeat counts may be combined. For example, the
specification ‘<samp><span class="samp">32*single=>single</span></samp>’ causes <code>fread</code> to read
blocks of single precision floating point values and return an array
of single precision values instead of the default array of double
precision values.
<p>The optional argument <var>skip</var> specifies the number of bytes to skip
after each element (or block of elements) is read. If it is not
specified, a value of 0 is assumed. If the final block read is not
complete, the final skip is omitted. For example,
<pre class="example"> fread (f, 10, "3*single=>single", 8)
</pre>
<p class="noindent">will omit the final 8-byte skip because the last read will not be
a complete block of 3 values.
<p>The optional argument <var>arch</var> is a string specifying the data format
for the file. Valid values are
<dl>
<dt><code>"native"</code><dd>The format of the current machine.
<br><dt><code>"ieee-be"</code><dd>IEEE big endian.
<br><dt><code>"ieee-le"</code><dd>IEEE little endian.
<br><dt><code>"vaxd"</code><dd>VAX D floating format.
<br><dt><code>"vaxg"</code><dd>VAX G floating format.
<br><dt><code>"cray"</code><dd>Cray floating format.
</dl>
<p class="noindent">Conversions are currently only supported for <code>"ieee-be"</code> and
<code>"ieee-le"</code> formats.
<p>The data read from the file is returned in <var>val</var>, and the number of
values read is returned in <code>count</code>
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dfwrite.html#doc_002dfwrite">fwrite</a>, <a href="doc_002dfopen.html#doc_002dfopen">fopen</a>, <a href="doc_002dfclose.html#doc_002dfclose">fclose</a>.
</p></blockquote></div>
<!-- file-io.cc -->
<p><a name="doc_002dfwrite"></a>
<div class="defun">
— Built-in Function: <var>count</var> = <b>fwrite</b> (<var>fid, data, precision, skip, arch</var>)<var><a name="index-fwrite-801"></a></var><br>
<blockquote><p>Write data in binary form of type <var>precision</var> to the specified file
ID <var>fid</var>, returning the number of values successfully written to the
file.
<p>The argument <var>data</var> is a matrix of values that are to be written to
the file. The values are extracted in column-major order.
<p>The remaining arguments <var>precision</var>, <var>skip</var>, and <var>arch</var> are
optional, and are interpreted as described for <code>fread</code>.
<p>The behavior of <code>fwrite</code> is undefined if the values in <var>data</var>
are too large to fit in the specified precision.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dfread.html#doc_002dfread">fread</a>, <a href="doc_002dfopen.html#doc_002dfopen">fopen</a>, <a href="doc_002dfclose.html#doc_002dfclose">fclose</a>.
</p></blockquote></div>
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