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<a name="fopen_002d_002d_002dopen-a-file"></a>
<h3 class="section">4.17 <code>fopen</code>&mdash;open a file</h3>
<a name="index-fopen"></a>
<a name="index-_005ffopen_005fr"></a>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">#include &lt;stdio.h&gt;
FILE *fopen(const char *<var>file</var>, const char *<var>mode</var>);

FILE *_fopen_r(struct _reent *<var>reent</var>, 
    const char *<var>file</var>, const char *<var>mode</var>);

</pre></div>
<p><strong>Description</strong><br>
<code>fopen</code> initializes the data structures needed to read or write a
file.  Specify the file&rsquo;s name as the string at <var>file</var>, and the kind
of access you need to the file with the string at <var>mode</var>.
</p>
<p>The alternate function <code>_fopen_r</code> is a reentrant version.
The extra argument <var>reent</var> is a pointer to a reentrancy structure.
</p>
<p>Three fundamental kinds of access are available: read, write, and append.
<code>*<var>mode</var></code> must begin with one of the three characters &lsquo;<code>r</code>&rsquo;,
&lsquo;<code>w</code>&rsquo;, or &lsquo;<code>a</code>&rsquo;, to select one of these:
</p>
<dl compact="compact">
<dt><code>r</code></dt>
<dd><p>Open the file for reading; the operation will fail if the file does
not exist, or if the host system does not permit you to read it.
</p>
</dd>
<dt><code>w</code></dt>
<dd><p>Open the file for writing <em>from the beginning</em> of the file:
effectively, this always creates a new file.  If the file whose name you
specified already existed, its old contents are discarded.
</p>
</dd>
<dt><code>a</code></dt>
<dd><p>Open the file for appending data, that is writing from the end of
file.  When you open a file this way, all data always goes to the
current end of file; you cannot change this using <code>fseek</code>.
</p></dd>
</dl>


<p>Some host systems distinguish between &ldquo;binary&rdquo; and &ldquo;text&rdquo; files.
Such systems may perform data transformations on data written to, or
read from, files opened as &ldquo;text&rdquo;.
If your system is one of these, then you can append a &lsquo;<code>b</code>&rsquo; to any
of the three modes above, to specify that you are opening the file as
a binary file (the default is to open the file as a text file).
</p>
<p>&lsquo;<code>rb</code>&rsquo;, then, means &ldquo;read binary&rdquo;; &lsquo;<code>wb</code>&rsquo;, &ldquo;write binary&rdquo;; and
&lsquo;<code>ab</code>&rsquo;, &ldquo;append binary&rdquo;.
</p>
<p>To make C programs more portable, the &lsquo;<code>b</code>&rsquo; is accepted on all
systems, whether or not it makes a difference.
</p>
<p>Finally, you might need to both read and write from the same file.
You can also append a &lsquo;<code>+</code>&rsquo; to any of the three modes, to permit
this.  (If you want to append both &lsquo;<code>b</code>&rsquo; and &lsquo;<code>+</code>&rsquo;, you can do it
in either order: for example, <code>&quot;rb+&quot;</code> means the same thing as
<code>&quot;r+b&quot;</code> when used as a mode string.)
</p>
<p>Use <code>&quot;r+&quot;</code> (or <code>&quot;rb+&quot;</code>) to permit reading and writing anywhere in
an existing file, without discarding any data; <code>&quot;w+&quot;</code> (or <code>&quot;wb+&quot;</code>)
to create a new file (or begin by discarding all data from an old one)
that permits reading and writing anywhere in it; and <code>&quot;a+&quot;</code> (or
<code>&quot;ab+&quot;</code>) to permit reading anywhere in an existing file, but writing
only at the end.
</p>
<br>
<p><strong>Returns</strong><br>
<code>fopen</code> returns a file pointer which you can use for other file
operations, unless the file you requested could not be opened; in that
situation, the result is <code>NULL</code>.  If the reason for failure was an
invalid string at <var>mode</var>, <code>errno</code> is set to <code>EINVAL</code>.
</p>
<br>
<p><strong>Portability</strong><br>
<code>fopen</code> is required by ANSI C.
</p>
<p>Supporting OS subroutines required: <code>close</code>, <code>fstat</code>, <code>isatty</code>,
<code>lseek</code>, <code>open</code>, <code>read</code>, <code>sbrk</code>, <code>write</code>.
</p>
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