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<h3 class="section">29.9 Setuid Program Example</h3>

<p>Here's an example showing how to set up a program that changes its
effective user ID.

   <p>This is part of a game program called <code>caber-toss</code> that manipulates
a file <samp><span class="file">scores</span></samp> that should be writable only by the game program
itself.  The program assumes that its executable file will be installed
with the setuid bit set and owned by the same user as the <samp><span class="file">scores</span></samp>
file.  Typically, a system administrator will set up an account like
<code>games</code> for this purpose.

   <p>The executable file is given mode <code>4755</code>, so that doing an
&lsquo;<samp><span class="samp">ls -l</span></samp>&rsquo; on it produces output like:

<pre class="smallexample">     -rwsr-xr-x   1 games    184422 Jul 30 15:17 caber-toss
</pre>
   <p class="noindent">The setuid bit shows up in the file modes as the &lsquo;<samp><span class="samp">s</span></samp>&rsquo;.

   <p>The scores file is given mode <code>644</code>, and doing an &lsquo;<samp><span class="samp">ls -l</span></samp>&rsquo; on
it shows:

<pre class="smallexample">     -rw-r--r--  1 games           0 Jul 31 15:33 scores
</pre>
   <p>Here are the parts of the program that show how to set up the changed
user ID.  This program is conditionalized so that it makes use of the
file IDs feature if it is supported, and otherwise uses <code>setreuid</code>
to swap the effective and real user IDs.

<pre class="smallexample">     #include &lt;stdio.h&gt;
     #include &lt;sys/types.h&gt;
     #include &lt;unistd.h&gt;
     #include &lt;stdlib.h&gt;
     
     
     /* <span class="roman">Remember the effective and real UIDs.</span> */
     
     static uid_t euid, ruid;
     
     
     /* <span class="roman">Restore the effective UID to its original value.</span> */
     
     void
     do_setuid (void)
     {
       int status;
     
     #ifdef _POSIX_SAVED_IDS
       status = seteuid (euid);
     #else
       status = setreuid (ruid, euid);
     #endif
       if (status &lt; 0) {
         fprintf (stderr, "Couldn't set uid.\n");
         exit (status);
         }
     }
     
     
     /* <span class="roman">Set the effective UID to the real UID.</span> */
     
     void
     undo_setuid (void)
     {
       int status;
     
     #ifdef _POSIX_SAVED_IDS
       status = seteuid (ruid);
     #else
       status = setreuid (euid, ruid);
     #endif
       if (status &lt; 0) {
         fprintf (stderr, "Couldn't set uid.\n");
         exit (status);
         }
     }
     
     /* <span class="roman">Main program.</span> */
     
     int
     main (void)
     {
       /* <span class="roman">Remember the real and effective user IDs.</span>  */
       ruid = getuid ();
       euid = geteuid ();
       undo_setuid ();
     
       /* <span class="roman">Do the game and record the score.</span>  */
       ...
     }
</pre>
   <p>Notice how the first thing the <code>main</code> function does is to set the
effective user ID back to the real user ID.  This is so that any other
file accesses that are performed while the user is playing the game use
the real user ID for determining permissions.  Only when the program
needs to open the scores file does it switch back to the file user ID,
like this:

<pre class="smallexample">     /* <span class="roman">Record the score.</span> */
     
     int
     record_score (int score)
     {
       FILE *stream;
       char *myname;
     
       /* <span class="roman">Open the scores file.</span> */
       do_setuid ();
       stream = fopen (SCORES_FILE, "a");
       undo_setuid ();
     
       /* <span class="roman">Write the score to the file.</span> */
       if (stream)
         {
           myname = cuserid (NULL);
           if (score &lt; 0)
             fprintf (stream, "%10s: Couldn't lift the caber.\n", myname);
           else
             fprintf (stream, "%10s: %d feet.\n", myname, score);
           fclose (stream);
           return 0;
         }
       else
         return -1;
     }
</pre>
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