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<h4 class="subsection">17.8.2 Opening a Pseudo-Terminal Pair</h4>

<p><a name="index-opening-a-pseudo_002dterminal-pair-2031"></a>
These functions, derived from BSD, are available in the separate
<samp><span class="file">libutil</span></samp> library, and declared in <samp><span class="file">pty.h</span></samp>.

<!-- pty.h -->
<!-- BSD -->
<div class="defun">
&mdash; Function: int <b>openpty</b> (<var>int *amaster, int *aslave, char *name, const struct termios *termp, const struct winsize *winp</var>)<var><a name="index-openpty-2032"></a></var><br>
<blockquote><p>This function allocates and opens a pseudo-terminal pair, returning the
file descriptor for the master in <var>*amaster</var>, and the file
descriptor for the slave in <var>*aslave</var>.  If the argument <var>name</var>
is not a null pointer, the file name of the slave pseudo-terminal
device is stored in <code>*name</code>.  If <var>termp</var> is not a null pointer,
the terminal attributes of the slave are set to the ones specified in
the structure that <var>termp</var> points to (see <a href="Terminal-Modes.html#Terminal-Modes">Terminal Modes</a>). 
Likewise, if the <var>winp</var> is not a null pointer, the screen size of
the slave is set to the values specified in the structure that
<var>winp</var> points to.

        <p>The normal return value from <code>openpty</code> is 0; a value of
-1 is returned in case of failure.  The following <code>errno</code>
conditions are defined for this function:

          <dl>
<dt><code>ENOENT</code><dd>There are no free pseudo-terminal pairs available. 
</dl>

        <p><strong>Warning:</strong> Using the <code>openpty</code> function with <var>name</var> not
set to <code>NULL</code> is <strong>very dangerous</strong> because it provides no
protection against overflowing the string <var>name</var>.  You should use
the <code>ttyname</code> function on the file descriptor returned in
<var>*slave</var> to find out the file name of the slave pseudo-terminal
device instead. 
</p></blockquote></div>

<!-- pty.h -->
<!-- BSD -->
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&mdash; Function: int <b>forkpty</b> (<var>int *amaster, char *name, const struct termios *termp, const struct winsize *winp</var>)<var><a name="index-forkpty-2033"></a></var><br>
<blockquote><p>This function is similar to the <code>openpty</code> function, but in
addition, forks a new process (see <a href="Creating-a-Process.html#Creating-a-Process">Creating a Process</a>) and makes the
newly opened slave pseudo-terminal device the controlling terminal
(see <a href="Controlling-Terminal.html#Controlling-Terminal">Controlling Terminal</a>) for the child process.

        <p>If the operation is successful, there are then both parent and child
processes and both see <code>forkpty</code> return, but with different values:
it returns a value of 0 in the child process and returns the child's
process ID in the parent process.

        <p>If the allocation of a pseudo-terminal pair or the process creation
failed, <code>forkpty</code> returns a value of -1 in the parent
process.

        <p><strong>Warning:</strong> The <code>forkpty</code> function has the same problems with
respect to the <var>name</var> argument as <code>openpty</code>. 
</p></blockquote></div>

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