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<h3 class="section">26.1 Running a Command</h3>

<p><a name="index-running-a-command-3150"></a>
The easy way to run another program is to use the <code>system</code>
function.  This function does all the work of running a subprogram, but
it doesn't give you much control over the details: you have to wait
until the subprogram terminates before you can do anything else.

<!-- stdlib.h -->
<!-- ISO -->
<div class="defun">
&mdash; Function: int <b>system</b> (<var>const char *command</var>)<var><a name="index-system-3151"></a></var><br>
<blockquote><p><a name="index-sh-3152"></a>This function executes <var>command</var> as a shell command.  In the GNU C
library, it always uses the default shell <code>sh</code> to run the command. 
In particular, it searches the directories in <code>PATH</code> to find
programs to execute.  The return value is <code>-1</code> if it wasn't
possible to create the shell process, and otherwise is the status of the
shell process.  See <a href="Process-Completion.html#Process-Completion">Process Completion</a>, for details on how this
status code can be interpreted.

        <p>If the <var>command</var> argument is a null pointer, a return value of zero
indicates that no command processor is available.

        <p>This function is a cancellation point in multi-threaded programs.  This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time <code>system</code> is
called.  If the thread gets canceled these resources stay allocated
until the program ends.  To avoid this calls to <code>system</code> should be
protected using cancellation handlers. 
<!-- ref pthread_cleanup_push / pthread_cleanup_pop -->

        <p><a name="index-stdlib_002eh-3153"></a>The <code>system</code> function is declared in the header file
<samp><span class="file">stdlib.h</span></samp>. 
</p></blockquote></div>

   <p><strong>Portability Note:</strong> Some C implementations may not have any
notion of a command processor that can execute other programs.  You can
determine whether a command processor exists by executing
<code>system&nbsp;(NULL)</code><!-- /@w -->; if the return value is nonzero, a command
processor is available.

   <p>The <code>popen</code> and <code>pclose</code> functions (see <a href="Pipe-to-a-Subprocess.html#Pipe-to-a-Subprocess">Pipe to a Subprocess</a>) are closely related to the <code>system</code> function.  They
allow the parent process to communicate with the standard input and
output channels of the command being executed.

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