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<a name="Opening-and-Closing-Files"></a>
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<p>
Next: <a href="Simple-Output.html#Simple-Output" accesskey="n" rel="next">Simple Output</a>, Up: <a href="C_002dStyle-I_002fO-Functions.html#C_002dStyle-I_002fO-Functions" accesskey="u" rel="up">C-Style I/O Functions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Opening-and-Closing-Files-1"></a>
<h4 class="subsection">14.2.1 Opening and Closing Files</h4>
<p>When reading data from a file it must be opened for reading first, and
likewise when writing to a file. The <code>fopen</code> function returns a
pointer to an open file that is ready to be read or written. Once all
data has been read from or written to the opened file it should be closed.
The <code>fclose</code> function does this. The following code illustrates
the basic pattern for writing to a file, but a very similar pattern is
used when reading a file.
</p>
<div class="example">
<pre class="example">filename = "myfile.txt";
fid = fopen (filename, "w");
# Do the actual I/O here…
fclose (fid);
</pre></div>
<a name="XREFfopen"></a><dl>
<dt><a name="index-fopen"></a><em><var>fid</var> =</em> <strong>fopen</strong> <em>(<var>name</var>)</em></dt>
<dt><a name="index-fopen-1"></a><em><var>fid</var> =</em> <strong>fopen</strong> <em>(<var>name</var>, <var>mode</var>)</em></dt>
<dt><a name="index-fopen-2"></a><em><var>fid</var> =</em> <strong>fopen</strong> <em>(<var>name</var>, <var>mode</var>, <var>arch</var>)</em></dt>
<dt><a name="index-fopen-3"></a><em>[<var>fid</var>, <var>msg</var>] =</em> <strong>fopen</strong> <em>(…)</em></dt>
<dt><a name="index-fopen-4"></a><em><var>fid_list</var> =</em> <strong>fopen</strong> <em>("all")</em></dt>
<dt><a name="index-fopen-5"></a><em>[<var>file</var>, <var>mode</var>, <var>arch</var>] =</em> <strong>fopen</strong> <em>(<var>fid</var>)</em></dt>
<dd><p>Open a file for low-level I/O or query open files and file descriptors.
</p>
<p>The first form of the <code>fopen</code> function opens the named file with
the specified mode (read-write, read-only, etc.) and architecture
interpretation (IEEE big endian, IEEE little endian, etc.), and returns
an integer value that may be used to refer to the file later. If an
error occurs, <var>fid</var> is set to -1 and <var>msg</var> contains the
corresponding system error message. The <var>mode</var> is a one or two
character string that specifies whether the file is to be opened for
reading, writing, or both.
</p>
<p>The second form of the <code>fopen</code> function returns a vector of file ids
corresponding to all the currently open files, excluding the
<code>stdin</code>, <code>stdout</code>, and <code>stderr</code> streams.
</p>
<p>The third form of the <code>fopen</code> function returns information about the
open file given its file id.
</p>
<p>For example,
</p>
<div class="example">
<pre class="example">myfile = fopen ("splat.dat", "r", "ieee-le");
</pre></div>
<p>opens the file <samp>splat.dat</samp> for reading. If necessary, binary
numeric values will be read assuming they are stored in IEEE format with
the least significant bit first, and then converted to the native
representation.
</p>
<p>Opening a file that is already open simply opens it again and returns a
separate file id. It is not an error to open a file several times,
though writing to the same file through several different file ids may
produce unexpected results.
</p>
<p>The possible values of <var>mode</var> are
</p>
<dl compact="compact">
<dt>‘<samp>r</samp>’ (default)</dt>
<dd><p>Open a file for reading.
</p>
</dd>
<dt>‘<samp>w</samp>’</dt>
<dd><p>Open a file for writing. The previous contents are discarded.
</p>
</dd>
<dt>‘<samp>a</samp>’</dt>
<dd><p>Open or create a file for writing at the end of the file.
</p>
</dd>
<dt>‘<samp>r+</samp>’</dt>
<dd><p>Open an existing file for reading and writing.
</p>
</dd>
<dt>‘<samp>w+</samp>’</dt>
<dd><p>Open a file for reading or writing. The previous contents are
discarded.
</p>
</dd>
<dt>‘<samp>a+</samp>’</dt>
<dd><p>Open or create a file for reading or writing at the end of the
file.
</p></dd>
</dl>
<p>Append a <code>"t"</code> to the mode string to open the file in text mode or a
<code>"b"</code> to open in binary mode. On Windows systems,
text mode reading and writing automatically converts linefeeds to the
appropriate line end character for the system (carriage-return linefeed on
Windows). The default when no mode is specified is binary.
</p>
<p>Additionally, you may append a <code>"z"</code> to the mode string to open a
gzipped file for reading or writing. For this to be successful, you
must also open the file in binary mode.
</p>
<p>The parameter <var>arch</var> is a string specifying the default data format
for the file. Valid values for <var>arch</var> are:
</p>
<dl compact="compact">
<dt><code>"native"</code> or <code>"n"</code> (default)</dt>
<dd><p>The format of the current machine.
</p>
</dd>
<dt><code>"ieee-be"</code> or <code>"b"</code></dt>
<dd><p>IEEE big endian format.
</p>
</dd>
<dt><code>"ieee-le"</code> or <code>"l"</code></dt>
<dd><p>IEEE little endian format.
</p></dd>
</dl>
<p>However, conversions are currently only supported for ‘<samp>native</samp>’,
‘<samp>ieee-be</samp>’, and ‘<samp>ieee-le</samp>’ formats.
</p>
<p>When opening a new file that does not yet exist, permissions will be set to
<code>0666 - <var>umask</var></code>.
</p>
<p>Compatibility Note: Octave opens files using buffered I/O. Small writes are
accumulated until an internal buffer is filled, and then everything is written
in a single operation. This is very efficient and improves performance.
<small>MATLAB</small>, however, opens files using flushed I/O where every write operation
is immediately performed. If the write operation must be performed immediately
after data has been written then the write should be followed by a call to
<code>fflush</code> to flush the internal buffer.
</p>
<p><strong>See also:</strong> <a href="#XREFfclose">fclose</a>, <a href="Line_002dOriented-Input.html#XREFfgets">fgets</a>, <a href="Line_002dOriented-Input.html#XREFfgetl">fgetl</a>, <a href="Formatted-Input.html#XREFfscanf">fscanf</a>, <a href="Binary-I_002fO.html#XREFfread">fread</a>, <a href="Simple-Output.html#XREFfputs">fputs</a>, <a href="Simple-File-I_002fO.html#XREFfdisp">fdisp</a>, <a href="Formatted-Output.html#XREFfprintf">fprintf</a>, <a href="Binary-I_002fO.html#XREFfwrite">fwrite</a>, <a href="Line_002dOriented-Input.html#XREFfskipl">fskipl</a>, <a href="File-Positioning.html#XREFfseek">fseek</a>, <a href="File-Positioning.html#XREFfrewind">frewind</a>, <a href="File-Positioning.html#XREFftell">ftell</a>, <a href="EOF-and-Errors.html#XREFfeof">feof</a>, <a href="EOF-and-Errors.html#XREFferror">ferror</a>, <a href="EOF-and-Errors.html#XREFfclear">fclear</a>, <a href="Paging-Screen-Output.html#XREFfflush">fflush</a>, <a href="EOF-and-Errors.html#XREFfreport">freport</a>, <a href="Filesystem-Utilities.html#XREFumask">umask</a>.
</p></dd></dl>
<a name="XREFfclose"></a><dl>
<dt><a name="index-fclose"></a><em></em> <strong>fclose</strong> <em>(<var>fid</var>)</em></dt>
<dt><a name="index-fclose-1"></a><em></em> <strong>fclose</strong> <em>("all")</em></dt>
<dt><a name="index-fclose-2"></a><em><var>status</var> =</em> <strong>fclose</strong> <em>("all")</em></dt>
<dd><p>Close the file specified by the file descriptor <var>fid</var>.
</p>
<p>If successful, <code>fclose</code> returns 0, otherwise, it returns -1. The
second form of the <code>fclose</code> call closes all open files except
<code>stdin</code>, <code>stdout</code>, <code>stderr</code>, and any FIDs associated
with gnuplot.
</p>
<p><strong>See also:</strong> <a href="#XREFfopen">fopen</a>, <a href="Paging-Screen-Output.html#XREFfflush">fflush</a>, <a href="EOF-and-Errors.html#XREFfreport">freport</a>.
</p></dd></dl>
<a name="XREFis_005fvalid_005ffile_005fid"></a><dl>
<dt><a name="index-is_005fvalid_005ffile_005fid"></a><em></em> <strong>is_valid_file_id</strong> <em>(<var>fid</var>)</em></dt>
<dd><p>Return true if <var>fid</var> refers to an open file.
</p>
<p><strong>See also:</strong> <a href="EOF-and-Errors.html#XREFfreport">freport</a>, <a href="#XREFfopen">fopen</a>.
</p></dd></dl>
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