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<h4 class="subsection">16.3.3 Reading the Address of a Socket</h4>

<p><a name="index-sys_002fsocket_002eh-1664"></a>Use the function <code>getsockname</code> to examine the address of an
Internet socket.  The prototype for this function is in the header file
<samp><span class="file">sys/socket.h</span></samp>.

<!-- sys/socket.h -->
<!-- BSD -->
<div class="defun">
&mdash; Function: int <b>getsockname</b> (<var>int socket, struct sockaddr *addr, socklen_t *length-ptr</var>)<var><a name="index-getsockname-1665"></a></var><br>
<blockquote><p>The <code>getsockname</code> function returns information about the
address of the socket <var>socket</var> in the locations specified by the
<var>addr</var> and <var>length-ptr</var> arguments.  Note that the
<var>length-ptr</var> is a pointer; you should initialize it to be the
allocation size of <var>addr</var>, and on return it contains the actual
size of the address data.

        <p>The format of the address data depends on the socket namespace.  The
length of the information is usually fixed for a given namespace, so
normally you can know exactly how much space is needed and can provide
that much.  The usual practice is to allocate a place for the value
using the proper data type for the socket's namespace, then cast its
address to <code>struct sockaddr *</code> to pass it to <code>getsockname</code>.

        <p>The return value is <code>0</code> on success and <code>-1</code> on error.  The
following <code>errno</code> error conditions are defined for this function:

          <dl>
<dt><code>EBADF</code><dd>The <var>socket</var> argument is not a valid file descriptor.

          <br><dt><code>ENOTSOCK</code><dd>The descriptor <var>socket</var> is not a socket.

          <br><dt><code>ENOBUFS</code><dd>There are not enough internal buffers available for the operation. 
</dl>
        </p></blockquote></div>

   <p>You can't read the address of a socket in the file namespace.  This is
consistent with the rest of the system; in general, there's no way to
find a file's name from a descriptor for that file.

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