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<html>
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<title>Context</title>
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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="context.context"></a><a class="link" href="context.html" title="Context">Context</a>
</h2></div></div></div>
<div class="toc"><dl class="toc"><dt><span class="section"><a href="context/boost_fcontext.html"><code class="computeroutput"><span class="identifier">fcontext_t</span></code>
and related functions</a></span></dt></dl></div>
<p>
Each instance of <span class="emphasis"><em>fcontext_t</em></span> represents a context (CPU
registers and stack space). Together with its related functions <span class="emphasis"><em>jump_fcontext()</em></span>
and <span class="emphasis"><em>make_fcontext()</em></span> it provides a execution control transfer
mechanism similar interface like <a href="http://www.kernel.org/doc/man-pages/online/pages/man2/getcontext.2.html" target="_top">ucontext_t</a>.
<span class="emphasis"><em>fcontext_t</em></span> and its functions are located in <span class="emphasis"><em>boost::context</em></span>
and the functions are declared as extern "C".
</p>
<div class="warning"><table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
<th align="left">Warning</th>
</tr>
<tr><td align="left" valign="top"><p>
If <span class="emphasis"><em>fcontext_t</em></span> is used in a multi threaded application,
it can migrated between threads, but must not reference <span class="emphasis"><em>thread-local
storage</em></span>.
</p></td></tr>
</table></div>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../doc/src/images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top"><p>
The low level API is the part to port to new platforms.
</p></td></tr>
</table></div>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
If <span class="emphasis"><em>fiber-local storage</em></span> is used on Windows, the user
is responsible for calling <span class="emphasis"><em>::FlsAlloc()</em></span>, <span class="emphasis"><em>::FlsFree()</em></span>.
</p></td></tr>
</table></div>
<h4>
<a name="context.context.h0"></a>
<span class="phrase"><a name="context.context.executing_a_context"></a></span><a class="link" href="context.html#context.context.executing_a_context">Executing
a context</a>
</h4>
<p>
A new context supposed to execute a <span class="emphasis"><em>context-function</em></span> (returning
void and accepting intptr_t as argument) will be created on top of the stack
(at 16 byte boundary) by function <span class="emphasis"><em>make_fcontext()</em></span>.
</p>
<pre class="programlisting"><span class="comment">// context-function</span>
<span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">intptr</span><span class="special">);</span>
<span class="comment">// creates a new stack</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span> <span class="special">=</span> <span class="number">8192</span><span class="special">;</span>
<span class="keyword">void</span><span class="special">*</span> <span class="identifier">sp</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">malloc</span><span class="special">(</span><span class="identifier">size</span><span class="special">));</span>
<span class="comment">// context fc uses f() as context function</span>
<span class="comment">// fcontext_t is placed on top of context stack</span>
<span class="comment">// a pointer to fcontext_t is returned</span>
<span class="identifier">fcontext_t</span> <span class="identifier">fc</span><span class="special">(</span><span class="identifier">make_fcontext</span><span class="special">(</span><span class="identifier">sp</span><span class="special">,</span><span class="identifier">size</span><span class="special">,</span><span class="identifier">f</span><span class="special">));</span>
</pre>
<p>
Calling <span class="emphasis"><em>jump_fcontext()</em></span> invokes the <span class="emphasis"><em>context-function</em></span>
in a newly created context complete with registers, flags, stack and instruction
pointers. When control should be returned to the original calling context,
call <span class="emphasis"><em>jump_fcontext()</em></span>. The current context information
(registers, flags, and stack and instruction pointers) is saved and the original
context information is restored. Calling <span class="emphasis"><em>jump_fcontext()</em></span>
again resumes execution in the second context after saving the new state of
the original context.
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">context</span><span class="special">::</span><span class="identifier">fcontext_t</span> <span class="identifier">fcm</span><span class="special">,</span><span class="identifier">fc1</span><span class="special">,</span><span class="identifier">fc2</span><span class="special">;</span>
<span class="keyword">void</span> <span class="identifier">f1</span><span class="special">(</span><span class="identifier">intptr_t</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="string">"f1: entered"</span><span class="special"><<</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="string">"f1: call jump_fcontext( & fc1, fc2, 0)"</span><span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">context</span><span class="special">::</span><span class="identifier">jump_fcontext</span><span class="special">(&</span><span class="identifier">fc1</span><span class="special">,</span><span class="identifier">fc2</span><span class="special">,</span><span class="number">0</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="string">"f1: return"</span><span class="special"><<</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">context</span><span class="special">::</span><span class="identifier">jump_fcontext</span><span class="special">(&</span><span class="identifier">fc1</span><span class="special">,</span><span class="identifier">fcm</span><span class="special">,</span><span class="number">0</span><span class="special">);</span>
<span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">f2</span><span class="special">(</span><span class="identifier">intptr_t</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="string">"f2: entered"</span><span class="special"><<</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="string">"f2: call jump_fcontext( & fc2, fc1, 0)"</span><span class="special"><<</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">context</span><span class="special">::</span><span class="identifier">jump_fcontext</span><span class="special">(&</span><span class="identifier">fc2</span><span class="special">,</span><span class="identifier">fc1</span><span class="special">,</span><span class="number">0</span><span class="special">);</span>
<span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="keyword">false</span><span class="special">&&!</span><span class="string">"f2: never returns"</span><span class="special">);</span>
<span class="special">}</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">context</span><span class="special">::</span><span class="identifier">guarded_stack_allocator</span> <span class="identifier">alloc</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span><span class="special">(</span><span class="number">8192</span><span class="special">);</span>
<span class="keyword">void</span><span class="special">*</span> <span class="identifier">sp1</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">malloc</span><span class="special">(</span><span class="identifier">size</span><span class="special">));</span>
<span class="keyword">void</span><span class="special">*</span> <span class="identifier">sp2</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">malloc</span><span class="special">(</span><span class="identifier">size</span><span class="special">));</span>
<span class="identifier">fc1</span><span class="special">=</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">context</span><span class="special">::</span><span class="identifier">make_fcontext</span><span class="special">(</span><span class="identifier">sp1</span><span class="special">,</span><span class="identifier">size</span><span class="special">,</span><span class="identifier">f1</span><span class="special">);</span>
<span class="identifier">fc2</span><span class="special">=</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">context</span><span class="special">::</span><span class="identifier">make_fcontext</span><span class="special">(</span><span class="identifier">sp2</span><span class="special">,</span><span class="identifier">size</span><span class="special">,</span><span class="identifier">f2</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="string">"main: call jump_fcontext( & fcm, fc1, 0)"</span><span class="special"><<</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">context</span><span class="special">::</span><span class="identifier">jump_fcontext</span><span class="special">(&</span><span class="identifier">fcm</span><span class="special">,</span><span class="identifier">fc1</span><span class="special">,</span><span class="number">0</span><span class="special">);</span>
<span class="identifier">output</span><span class="special">:</span>
<span class="identifier">main</span><span class="special">:</span> <span class="identifier">call</span> <span class="identifier">jump_fcontext</span><span class="special">(</span> <span class="special">&</span> <span class="identifier">fcm</span><span class="special">,</span> <span class="identifier">fc1</span><span class="special">,</span> <span class="number">0</span><span class="special">)</span>
<span class="identifier">f1</span><span class="special">:</span> <span class="identifier">entered</span>
<span class="identifier">f1</span><span class="special">:</span> <span class="identifier">call</span> <span class="identifier">jump_fcontext</span><span class="special">(</span> <span class="special">&</span> <span class="identifier">fc1</span><span class="special">,</span> <span class="identifier">fc2</span><span class="special">,</span> <span class="number">0</span><span class="special">)</span>
<span class="identifier">f2</span><span class="special">:</span> <span class="identifier">entered</span>
<span class="identifier">f2</span><span class="special">:</span> <span class="identifier">call</span> <span class="identifier">jump_fcontext</span><span class="special">(</span> <span class="special">&</span> <span class="identifier">fc2</span><span class="special">,</span> <span class="identifier">fc1</span><span class="special">,</span> <span class="number">0</span><span class="special">)</span>
<span class="identifier">f1</span><span class="special">:</span> <span class="keyword">return</span>
</pre>
<p>
First call of <span class="emphasis"><em>jump_fcontext()</em></span> enters the <span class="emphasis"><em>context-function</em></span>
<code class="computeroutput"><span class="identifier">f1</span><span class="special">()</span></code>
by starting context fc1 (context fcm saves the registers of <code class="computeroutput"><span class="identifier">main</span><span class="special">()</span></code>). For jumping between context's fc1 and fc2
<code class="computeroutput"><span class="identifier">jump_fcontext</span><span class="special">()</span></code>
is called. Because context fcm is chained to fc1, <code class="computeroutput"><span class="identifier">main</span><span class="special">()</span></code> is entered (returning from <span class="emphasis"><em>jump_fcontext()</em></span>)
after context fc1 becomes complete (return from <code class="computeroutput"><span class="identifier">f1</span><span class="special">()</span></code>).
</p>
<div class="warning"><table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
<th align="left">Warning</th>
</tr>
<tr><td align="left" valign="top"><p>
Calling <span class="emphasis"><em>jump_fcontext()</em></span> to the same context from inside
the same context results in undefined behaviour.
</p></td></tr>
</table></div>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../doc/src/images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top"><p>
The size of the stack is required to be larger than the size of fcontext_t.
</p></td></tr>
</table></div>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
In contrast to threads, which are preemtive, <span class="emphasis"><em>fcontext_t</em></span>
switches are cooperative (programmer controls when switch will happen). The
kernel is not involved in the context switches.
</p></td></tr>
</table></div>
<h4>
<a name="context.context.h1"></a>
<span class="phrase"><a name="context.context.transfer_of_data"></a></span><a class="link" href="context.html#context.context.transfer_of_data">Transfer
of data</a>
</h4>
<p>
The third argument passed to <span class="emphasis"><em>jump_fcontext()</em></span>, in one context,
is passed as the first argument of the <span class="emphasis"><em>context-function</em></span>
if the context is started for the first time. In all following invocations
of <span class="emphasis"><em>jump_fcontext()</em></span> the intptr_t passed to <span class="emphasis"><em>jump_fcontext()</em></span>,
in one context, is returned by <span class="emphasis"><em>jump_fcontext()</em></span> in the
other context.
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">context</span><span class="special">::</span><span class="identifier">fcontext_t</span> <span class="identifier">fcm</span><span class="special">,</span><span class="identifier">fc</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">pair_t</span><span class="special">;</span>
<span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">intptr_t</span> <span class="identifier">param</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">pair_t</span><span class="special">*</span> <span class="identifier">p</span><span class="special">=(</span><span class="identifier">pair_t</span><span class="special">*)</span><span class="identifier">param</span><span class="special">;</span>
<span class="identifier">p</span><span class="special">=(</span><span class="identifier">pair_t</span><span class="special">*)</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">context</span><span class="special">::</span><span class="identifier">jump_fcontext</span><span class="special">(&</span><span class="identifier">fc</span><span class="special">,</span><span class="identifier">fcm</span><span class="special">,(</span><span class="identifier">intptr_t</span><span class="special">)(</span><span class="identifier">p</span><span class="special">-></span><span class="identifier">first</span><span class="special">+</span><span class="identifier">p</span><span class="special">-></span><span class="identifier">second</span><span class="special">));</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">context</span><span class="special">::</span><span class="identifier">jump_fcontext</span><span class="special">(&</span><span class="identifier">fc</span><span class="special">,</span><span class="identifier">fcm</span><span class="special">,(</span><span class="identifier">intptr_t</span><span class="special">)(</span><span class="identifier">p</span><span class="special">-></span><span class="identifier">first</span><span class="special">+</span><span class="identifier">p</span><span class="special">-></span><span class="identifier">second</span><span class="special">));</span>
<span class="special">}</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">context</span><span class="special">::</span><span class="identifier">guarded_stack_allocator</span> <span class="identifier">alloc</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span><span class="special">(</span><span class="number">8192</span><span class="special">);</span>
<span class="keyword">void</span><span class="special">*</span> <span class="identifier">sp</span><span class="special">(</span><span class="identifier">size</span><span class="special">);</span>
<span class="identifier">pair_t</span> <span class="identifier">p</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">make_pair</span><span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="number">7</span><span class="special">));</span>
<span class="identifier">fc</span><span class="special">=</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">context</span><span class="special">::</span><span class="identifier">make_fcontext</span><span class="special">(</span><span class="identifier">sp</span><span class="special">,</span><span class="identifier">size</span><span class="special">,</span><span class="identifier">f</span><span class="special">);</span>
<span class="keyword">int</span> <span class="identifier">res</span><span class="special">=(</span><span class="keyword">int</span><span class="special">)</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">context</span><span class="special">::</span><span class="identifier">jump_fcontext</span><span class="special">(&</span><span class="identifier">fcm</span><span class="special">,</span><span class="identifier">fc</span><span class="special">,(</span><span class="identifier">intptr_t</span><span class="special">)&</span><span class="identifier">p</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special"><<</span><span class="string">" + "</span><span class="special"><<</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span><span class="special"><<</span><span class="string">" == "</span><span class="special"><<</span><span class="identifier">res</span><span class="special"><<</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">p</span><span class="special">=</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">make_pair</span><span class="special">(</span><span class="number">5</span><span class="special">,</span><span class="number">6</span><span class="special">);</span>
<span class="identifier">res</span><span class="special">=(</span><span class="keyword">int</span><span class="special">)</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">context</span><span class="special">::</span><span class="identifier">jump_fcontext</span><span class="special">(&</span><span class="identifier">fcm</span><span class="special">,</span><span class="identifier">fc</span><span class="special">,(</span><span class="identifier">intptr_t</span><span class="special">)&</span><span class="identifier">p</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special"><<</span><span class="string">" + "</span><span class="special"><<</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span><span class="special"><<</span><span class="string">" == "</span><span class="special"><<</span><span class="identifier">res</span><span class="special"><<</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">output</span><span class="special">:</span>
<span class="number">2</span> <span class="special">+</span> <span class="number">7</span> <span class="special">==</span> <span class="number">9</span>
<span class="number">5</span> <span class="special">+</span> <span class="number">6</span> <span class="special">==</span> <span class="number">11</span>
</pre>
<h4>
<a name="context.context.h2"></a>
<span class="phrase"><a name="context.context.exceptions_in__emphasis_context_function__emphasis_"></a></span><a class="link" href="context.html#context.context.exceptions_in__emphasis_context_function__emphasis_">Exceptions
in <span class="emphasis"><em>context-function</em></span></a>
</h4>
<p>
If the <span class="emphasis"><em>context-function</em></span> emits an exception, the behaviour
is undefined.
</p>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../doc/src/images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top"><p>
<span class="emphasis"><em>context-function</em></span> should wrap the code in a try/catch
block.
</p></td></tr>
</table></div>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../doc/src/images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top"><p>
Do not jump from inside a catch block and than re-throw the exception in
another execution context.
</p></td></tr>
</table></div>
<h4>
<a name="context.context.h3"></a>
<span class="phrase"><a name="context.context.preserving_floating_point_registers"></a></span><a class="link" href="context.html#context.context.preserving_floating_point_registers">Preserving
floating point registers</a>
</h4>
<p>
Preserving the floating point registers increases the cycle count for a context
switch (see performance tests). The fourth argument of <span class="emphasis"><em>jump_fcontext()</em></span>
controls if fpu registers should be preserved by the context jump.
</p>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../doc/src/images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top"><p>
The use of the fpu controlling argument of <span class="emphasis"><em>jump_fcontext()</em></span>
must be consistent in the application. Otherwise the behaviour is undefined.
</p></td></tr>
</table></div>
<h4>
<a name="context.context.h4"></a>
<span class="phrase"><a name="context.context.stack_unwinding"></a></span><a class="link" href="context.html#context.context.stack_unwinding">Stack
unwinding</a>
</h4>
<p>
Sometimes it is necessary to unwind the stack of an unfinished context to destroy
local stack variables so they can release allocated resources (RAII pattern).
The user is responsible for this task.
</p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2009 Oliver Kowalke<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
</div></td>
</tr></table>
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