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<h3 class="section">26.9 Process Creation Example</h3>

<p>Here is an example program showing how you might write a function
similar to the built-in <code>system</code>.  It executes its <var>command</var>
argument using the equivalent of &lsquo;<samp><span class="samp">sh -c </span><var>command</var></samp>&rsquo;.

<pre class="smallexample">     #include &lt;stddef.h&gt;
     #include &lt;stdlib.h&gt;
     #include &lt;unistd.h&gt;
     #include &lt;sys/types.h&gt;
     #include &lt;sys/wait.h&gt;
     
     /* <span class="roman">Execute the command using this shell program.</span>  */
     #define SHELL "/bin/sh"
     
     int
     my_system (const char *command)
     {
       int status;
       pid_t pid;
     
       pid = fork ();
       if (pid == 0)
         {
           /* <span class="roman">This is the child process.  Execute the shell command.</span> */
           execl (SHELL, SHELL, "-c", command, NULL);
           _exit (EXIT_FAILURE);
         }
       else if (pid &lt; 0)
         /* <span class="roman">The fork failed.  Report failure.</span>  */
         status = -1;
       else
         /* <span class="roman">This is the parent process.  Wait for the child to complete.</span>  */
         if (waitpid (pid, &amp;status, 0) != pid)
           status = -1;
       return status;
     }
</pre>
   <!-- Yes, this example has been tested. -->
   <p>There are a couple of things you should pay attention to in this
example.

   <p>Remember that the first <code>argv</code> argument supplied to the program
represents the name of the program being executed.  That is why, in the
call to <code>execl</code>, <code>SHELL</code> is supplied once to name the program
to execute and a second time to supply a value for <code>argv[0]</code>.

   <p>The <code>execl</code> call in the child process doesn't return if it is
successful.  If it fails, you must do something to make the child
process terminate.  Just returning a bad status code with <code>return</code>
would leave two processes running the original program.  Instead, the
right behavior is for the child process to report failure to its parent
process.

   <p>Call <code>_exit</code> to accomplish this.  The reason for using <code>_exit</code>
instead of <code>exit</code> is to avoid flushing fully buffered streams such
as <code>stdout</code>.  The buffers of these streams probably contain data
that was copied from the parent process by the <code>fork</code>, data that
will be output eventually by the parent process.  Calling <code>exit</code> in
the child would output the data twice.  See <a href="Termination-Internals.html#Termination-Internals">Termination Internals</a>.

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