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<title>Stack allocation</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="fiber.stack"></a><a name="stack"></a><a class="link" href="stack.html" title="Stack allocation">Stack allocation</a>
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</h2></div></div></div>
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<p>
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A <a class="link" href="fiber_mgmt/fiber.html#class_fiber"> <code class="computeroutput">fiber</code></a> uses internally a <a href="http://www.boost.org/doc/libs/release/libs/context/doc/html/context/econtext.html" target="_top"><span class="emphasis"><em>execution_context</em></span></a>
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which manages a set of registers and a stack. The memory used by the stack
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is allocated/deallocated via a <span class="emphasis"><em>stack_allocator</em></span> which is
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required to model a <a class="link" href="stack.html#stack_allocator_concept"><span class="emphasis"><em>stack-allocator
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concept</em></span></a>.
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</p>
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<p>
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A <span class="emphasis"><em>stack_allocator</em></span> can be passed to <a class="link" href="fiber_mgmt/fiber.html#fiber_fiber"><code class="computeroutput"><span class="identifier">fiber</span><span class="special">::</span><span class="identifier">fiber</span><span class="special">()</span></code></a> or to <a class="link" href="synchronization/futures/future.html#fibers_async"> <code class="computeroutput">fibers::async()</code></a>.
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</p>
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<a name="stack_allocator_concept"></a><h4>
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<a name="fiber.stack.h0"></a>
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<span><a name="fiber.stack._link_linkend__stack_allocator_concept___emphasis_stack_allocator_concept__emphasis___link_"></a></span><a class="link" href="stack.html#fiber.stack._link_linkend__stack_allocator_concept___emphasis_stack_allocator_concept__emphasis___link_"><a class="link" href="stack.html#stack_allocator_concept"><span class="emphasis"><em>stack-allocator concept</em></span></a></a>
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</h4>
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<p>
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A <span class="emphasis"><em>stack_allocator</em></span> must satisfy the <a class="link" href="stack.html#stack_allocator_concept"><span class="emphasis"><em>stack-allocator
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concept</em></span></a> requirements shown in the following table, in which
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<code class="computeroutput"><span class="identifier">a</span></code> is an object of a <span class="emphasis"><em>stack_allocator</em></span>
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type, <code class="computeroutput"><span class="identifier">sctx</span></code> is a <a href="http://www.boost.org/doc/libs/release/libs/context/doc/html/context/stack/stack_context.html" target="_top"><code class="computeroutput"><span class="identifier">stack_context</span></code></a>, and <code class="computeroutput"><span class="identifier">size</span></code> is a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span></code>:
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</p>
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<div class="informaltable"><table class="table">
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<colgroup>
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<col>
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<col>
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<col>
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<thead><tr>
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<p>
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expression
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</p>
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<th>
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<p>
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return type
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</p>
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</th>
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<th>
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<p>
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notes
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</p>
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</th>
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</tr></thead>
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<tbody>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">a</span><span class="special">(</span><span class="identifier">size</span><span class="special">)</span></code>
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</p>
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</td>
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<td>
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</td>
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<td>
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<p>
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creates a stack allocator
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">a</span><span class="special">.</span><span class="identifier">allocate</span><span class="special">()</span></code>
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</p>
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</td>
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<td>
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<p>
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<a href="http://www.boost.org/doc/libs/release/libs/context/doc/html/context/stack/stack_context.html" target="_top"><code class="computeroutput"><span class="identifier">stack_context</span></code></a>
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</p>
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</td>
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<td>
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<p>
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creates a stack
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">a</span><span class="special">.</span><span class="identifier">deallocate</span><span class="special">(</span>
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<span class="identifier">sctx</span><span class="special">)</span></code>
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</p>
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</td>
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<td>
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<p>
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<code class="computeroutput"><span class="keyword">void</span></code>
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</p>
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</td>
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<td>
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<p>
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deallocates the stack created by <code class="computeroutput"><span class="identifier">a</span><span class="special">.</span><span class="identifier">allocate</span><span class="special">()</span></code>
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</p>
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</td>
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</tr>
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</tbody>
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</table></div>
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<div class="important"><table border="0" summary="Important">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../doc/src/images/important.png"></td>
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<th align="left">Important</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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The implementation of <code class="computeroutput"><span class="identifier">allocate</span><span class="special">()</span></code> might include logic to protect against
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exceeding the context's available stack size rather than leaving it as undefined
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behaviour.
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</p></td></tr>
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</table></div>
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<div class="important"><table border="0" summary="Important">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../doc/src/images/important.png"></td>
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<th align="left">Important</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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Calling <code class="computeroutput"><span class="identifier">deallocate</span><span class="special">()</span></code>
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with a <a href="http://www.boost.org/doc/libs/release/libs/context/doc/html/context/stack/stack_context.html" target="_top"><code class="computeroutput"><span class="identifier">stack_context</span></code></a> not obtained from
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<code class="computeroutput"><span class="identifier">allocate</span><span class="special">()</span></code>
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results in undefined behaviour.
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</p></td></tr>
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</table></div>
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<div class="note"><table border="0" summary="Note">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
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<th align="left">Note</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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The memory for the stack is not required to be aligned; alignment takes place
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inside <a href="http://www.boost.org/doc/libs/release/libs/context/doc/html/context/econtext.html" target="_top"><span class="emphasis"><em>execution_context</em></span></a>.
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</p></td></tr>
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</table></div>
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<p>
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See also <a href="http://www.boost.org/doc/libs/release/libs/context/doc/html/context/stack.html" target="_top">Boost.Context
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stack allocation</a>.
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</p>
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<p>
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</p>
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<h5>
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<a name="class_protected_fixedsize_stack_bridgehead"></a>
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<span><a name="class_protected_fixedsize_stack"></a></span>
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<a class="link" href="stack.html#class_protected_fixedsize_stack">Class
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<code class="computeroutput">protected_fixedsize_stack</code></a>
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</h5>
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<p>
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</p>
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<p>
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<span class="bold"><strong>Boost.Fiber</strong></span> provides the class <a class="link" href="stack.html#class_protected_fixedsize_stack"> <code class="computeroutput">protected_fixedsize_stack</code></a> which
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models the <a class="link" href="stack.html#stack_allocator_concept"><span class="emphasis"><em>stack-allocator
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concept</em></span></a>. It appends a guard page at the end of each stack
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to protect against exceeding the stack. If the guard page is accessed (read
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or write operation) a segmentation fault/access violation is generated by the
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operating system.
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</p>
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<div class="important"><table border="0" summary="Important">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../doc/src/images/important.png"></td>
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<th align="left">Important</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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Using <a class="link" href="stack.html#class_protected_fixedsize_stack"> <code class="computeroutput">protected_fixedsize_stack</code></a> is expensive.
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Launching a new fiber with a stack of this type incurs the overhead of setting
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the memory protection; once allocated, this stack is just as efficient to
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use as <a class="link" href="stack.html#class_fixedsize_stack"> <code class="computeroutput">fixedsize_stack</code></a>.
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</p></td></tr>
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</table></div>
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<div class="note"><table border="0" summary="Note">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
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<th align="left">Note</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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The appended <code class="computeroutput"><span class="identifier">guard</span> <span class="identifier">page</span></code>
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is <span class="bold"><strong>not</strong></span> mapped to physical memory, only virtual
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addresses are used.
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</p></td></tr>
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</table></div>
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<p>
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</p>
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<h5>
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<a name="class_fixedsize_stack_bridgehead"></a>
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<span><a name="class_fixedsize_stack"></a></span>
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<a class="link" href="stack.html#class_fixedsize_stack">Class
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<code class="computeroutput">fixedsize_stack</code></a>
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</h5>
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<p>
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</p>
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<p>
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<span class="bold"><strong>Boost.Fiber</strong></span> provides the class <a class="link" href="stack.html#class_fixedsize_stack"> <code class="computeroutput">fixedsize_stack</code></a> which
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models the <a class="link" href="stack.html#stack_allocator_concept"><span class="emphasis"><em>stack-allocator
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concept</em></span></a>. In contrast to <a class="link" href="stack.html#class_protected_fixedsize_stack"> <code class="computeroutput">protected_fixedsize_stack</code></a> it
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does not append a guard page at the end of each stack. The memory is simply
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managed by <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">malloc</span><span class="special">()</span></code>
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and <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">free</span><span class="special">()</span></code>.
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</p>
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<p>
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</p>
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<h5>
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<a name="class_segmented_stack_bridgehead"></a>
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<span><a name="class_segmented_stack"></a></span>
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<a class="link" href="stack.html#class_segmented_stack">Class
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<code class="computeroutput">segmented_stack</code></a>
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</h5>
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<p>
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</p>
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<p>
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<span class="bold"><strong>Boost.Fiber</strong></span> supports usage of a <a class="link" href="stack.html#class_segmented_stack"> <code class="computeroutput">segmented_stack</code></a>,
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i.e. the stack grows on demand. The fiber is created with a minimal stack size
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which will be increased as required. Class <a class="link" href="stack.html#class_segmented_stack"> <code class="computeroutput">segmented_stack</code></a> models
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the <a class="link" href="stack.html#stack_allocator_concept"><span class="emphasis"><em>stack-allocator concept</em></span></a>.
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In contrast to <a class="link" href="stack.html#class_protected_fixedsize_stack"> <code class="computeroutput">protected_fixedsize_stack</code></a> and
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<a class="link" href="stack.html#class_fixedsize_stack"> <code class="computeroutput">fixedsize_stack</code></a> it creates a stack which grows on demand.
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</p>
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<div class="note"><table border="0" summary="Note">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
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<th align="left">Note</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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Segmented stacks are currently only supported by <span class="bold"><strong>gcc</strong></span>
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from version <span class="bold"><strong>4.7</strong></span> and <span class="bold"><strong>clang</strong></span>
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from version <span class="bold"><strong>3.4</strong></span> onwards. In order to use
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a <a class="link" href="stack.html#class_segmented_stack"> <code class="computeroutput">segmented_stack</code></a>, <span class="bold"><strong>Boost.Fiber</strong></span>
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must be built with property <code class="computeroutput"><span class="identifier">segmented</span><span class="special">-</span><span class="identifier">stacks</span></code>,
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e.g. <span class="bold"><strong>toolset=gcc segmented-stacks=on</strong></span> at
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b2/bjam command line.
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</p></td></tr>
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</table></div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2013 Oliver Kowalke<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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>)
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</p>
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</div></td>
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</tr></table>
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