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<head>
<title>needstack(3) - Plan 9 from User Space</title>
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<tr><td width=20><td><b>NEEDSTACK(3)</b><td align=right><b>NEEDSTACK(3)</b>
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<p><font size=+1><b>NAME </b></font><br>
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needstack &ndash; check for execution stack overflow<br>
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<p><font size=+1><b>SYNOPSIS </b></font><br>
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<tt><font size=+1>#include &lt;u.h&gt;
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</font></tt>
<tt><font size=+1>#include &lt;libc.h&gt;
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</font></tt>
<tt><font size=+1>int &nbsp;&nbsp;&nbsp;needstack(int n)<br>
</font></tt>
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<p><font size=+1><b>DESCRIPTION </b></font><br>
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Stack overflow in the thread library leads to bugs that are difficult
to diagnose. The Plan 9 libraries are careful about not allocating
large structures on the stack, so typically four or eight kilobytes
is plenty of stack for a thread. Other libraries are not always
as careful. Calling <i>needstack</i> indicates to the thread library
that an external routine is about to be called that will require
<i>n</i> bytes of stack space. If there is not enough space left on the
stack, the thread library prints an error and terminates the program.
The call <tt><font size=+1>needstack(0)</font></tt> can be used to check whether the stack is
currently overflowed.
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<i>Needstack</i> is defined in <tt><font size=+1>libc.h</font></tt> so that library functions used
in threaded and non-threaded contexts can call it. The implementation
of <i>needstack</i> in <tt><font size=+1>lib9</font></tt> is a no-op.
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<i>Needstack</i> should be thought of as a comment checked at run time,
like <a href="../man3/assert.html"><i>assert</i>(3)</a>.<br>
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<p><font size=+1><b>EXAMPLE </b></font><br>
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The X Window library implementation of <i>XLookupString</i> allocates
some very large buffers on the stack, so <tt><font size=+1>/usr/local/plan9/src/libdraw/x11&#8722;itrans.c</font></tt>
calls <tt><font size=+1>needstack(20*1024)</font></tt> before making calls to <i>XLookupString</i>.
If a thread (in this case, the keyboard-reading thread used inside
the <a href="../man3/draw.html"><i>draw</i>(3)</a>
library) does not allocate a large enough stack, the problem is
diagnosed immediately rather than left to corrupt memory.<br>
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<p><font size=+1><b>SOURCE </b></font><br>
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<tt><font size=+1>/usr/local/plan9/src/lib9/needstack.c<br>
/usr/local/plan9/src/libthread<br>
</font></tt>
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<p><font size=+1><b>SEE ALSO </b></font><br>
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<a href="../man3/thread.html"><i>thread</i>(3)</a><br>
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<!-- TRAILER -->
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<tr height=15><td width=10><td><td width=10>
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<center>
<a href="../../"><img src="../../dist/spaceglenda100.png" alt="Space Glenda" border=1></a>
</center>
</table>
<!-- TRAILER -->
</body></html>