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<h3 class="section">29.11 Identifying Who Logged In</h3>

<p><a name="index-login-name_002c-determining-3329"></a><a name="index-user-ID_002c-determining-3330"></a>
You can use the functions listed in this section to determine the login
name of the user who is running a process, and the name of the user who
logged in the current session.  See also the function <code>getuid</code> and
friends (see <a href="Reading-Persona.html#Reading-Persona">Reading Persona</a>).  How this information is collected by
the system and how to control/add/remove information from the background
storage is described in <a href="User-Accounting-Database.html#User-Accounting-Database">User Accounting Database</a>.

   <p>The <code>getlogin</code> function is declared in <samp><span class="file">unistd.h</span></samp>, while
<code>cuserid</code> and <code>L_cuserid</code> are declared in <samp><span class="file">stdio.h</span></samp>. 
<a name="index-stdio_002eh-3331"></a><a name="index-unistd_002eh-3332"></a>
<!-- unistd.h -->
<!-- POSIX.1 -->

<div class="defun">
&mdash; Function: char * <b>getlogin</b> (<var>void</var>)<var><a name="index-getlogin-3333"></a></var><br>
<blockquote><p>The <code>getlogin</code> function returns a pointer to a string containing the
name of the user logged in on the controlling terminal of the process,
or a null pointer if this information cannot be determined.  The string
is statically allocated and might be overwritten on subsequent calls to
this function or to <code>cuserid</code>. 
</p></blockquote></div>

<!-- stdio.h -->
<!-- POSIX.1 -->
<div class="defun">
&mdash; Function: char * <b>cuserid</b> (<var>char *string</var>)<var><a name="index-cuserid-3334"></a></var><br>
<blockquote><p>The <code>cuserid</code> function returns a pointer to a string containing a
user name associated with the effective ID of the process.  If
<var>string</var> is not a null pointer, it should be an array that can hold
at least <code>L_cuserid</code> characters; the string is returned in this
array.  Otherwise, a pointer to a string in a static area is returned. 
This string is statically allocated and might be overwritten on
subsequent calls to this function or to <code>getlogin</code>.

        <p>The use of this function is deprecated since it is marked to be
withdrawn in XPG4.2 and has already been removed from newer revisions of
POSIX.1. 
</p></blockquote></div>

<!-- stdio.h -->
<!-- POSIX.1 -->
<div class="defun">
&mdash; Macro: int <b>L_cuserid</b><var><a name="index-L_005fcuserid-3335"></a></var><br>
<blockquote><p>An integer constant that indicates how long an array you might need to
store a user name. 
</p></blockquote></div>

   <p>These functions let your program identify positively the user who is
running or the user who logged in this session.  (These can differ when
setuid programs are involved; see <a href="Process-Persona.html#Process-Persona">Process Persona</a>.)  The user cannot
do anything to fool these functions.

   <p>For most purposes, it is more useful to use the environment variable
<code>LOGNAME</code> to find out who the user is.  This is more flexible
precisely because the user can set <code>LOGNAME</code> arbitrarily. 
See <a href="Standard-Environment.html#Standard-Environment">Standard Environment</a>.

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