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<tr class="t-nv"><td colspan="5"> <strong class="selflink"> <code>syncronized</code>, <code>atomic_commit</code></strong>, etc<span class="t-mark-rev t-since-tm-ts t-mark-ts">(TM TS)</span></td></tr>
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<td class="mbox-text" style=""> <b>Experimental Feature</b> The functionality described on this page is part of the Transactional Memory Technical Specification ISO/IEC TS 19841:2015 <span class="t-mark-rev t-since-tm-ts t-mark-ts">(TM TS)</span> </td>
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<p>Transactional memory is a concurrency synchronization mechanism that combines groups of statements in transactions, that are
</p>
<ul><li> atomic (either all statements occur, or nothing occurs)
</li><li> isolated (statements in a transaction may not observe half-written writes made by another transaction, even if they execute in parallel)
</li></ul>
<p>Typical implementations use hardware transactional memory where supported and to the limits that it is available (e.g. until the changeset is saturated) and fall back to software transactional memory, usually implemented with optimistic concurrency: if another transaction updated some of the variables used by a transaction, it is silently retried. For that reason, retriable transactions ("atomic blocks") can only call transaction-safe functions.
</p><p>Note that accessing a variable in a transaction and out of a transaction without other external synchronization is a data race.
</p><p>If feature testing is supported, the features described here are indicated by the macro constant <span class="t-c"><span class="mw-geshi cpp source-cpp">__cpp_transactional_memory</span></span> with a value equal or greater <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">201505</span></span></span>.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="transactional_memory.html#Synchronized_blocks"><span class="tocnumber">1</span> <span class="toctext">Synchronized blocks</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="transactional_memory.html#Atomic_blocks"><span class="tocnumber">2</span> <span class="toctext">Atomic blocks</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="transactional_memory.html#Transaction-safe_functions"><span class="tocnumber">3</span> <span class="toctext">Transaction-safe functions</span></a>
<ul>
<li class="toclevel-2 tocsection-4"><a href="transactional_memory.html#Transaction-safe_virtual_functions"><span class="tocnumber">3.1</span> <span class="toctext">Transaction-safe virtual functions</span></a></li>
</ul>
</li>
<li class="toclevel-1 tocsection-5"><a href="transactional_memory.html#Attributes"><span class="tocnumber">4</span> <span class="toctext">Attributes</span></a></li>
<li class="toclevel-1 tocsection-6"><a href="transactional_memory.html#Notes"><span class="tocnumber">5</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-7"><a href="transactional_memory.html#Compiler_support"><span class="tocnumber">6</span> <span class="toctext">Compiler support</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/transactional_memory&action=edit&section=1" title="Edit section: Synchronized blocks">edit</a>]</span> <span class="mw-headline" id="Synchronized_blocks">Synchronized blocks</span></h3>
<p><code><b>synchronized</b></code> <span class="t-spar">compound-statement</span>
</p>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete </td></tr></table>
<p>All outermost synchronized blocks execute in a single total order. The end of each synchronized block synchronizes with the beginning of the next synchronized block in that order.
</p><p>Synchronized blocks may call transaction-unsafe functions.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co1">// the following function is thread-safe, and guarantees consistent before-after pairs</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="kw4">static</span> <span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
synchronized <span class="br0">{</span> <span class="co1">// begin transaction</span>
printf<span class="br0">(</span><span class="st0">"before %d<span class="es1">\n</span>"</span>, i<span class="br0">)</span><span class="sy4">;</span>
<span class="sy2">++</span>i<span class="sy4">;</span>
printf<span class="br0">(</span><span class="st0">"after %d<span class="es1">\n</span>"</span>, i<span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">return</span> i<span class="sy4">;</span> <span class="co1">// commit transaction</span>
<span class="br0">}</span>
<span class="br0">}</span></pre></div></div>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete </td></tr></table>
<p>Leaving a synchronized block by any means (reaching the end, goto, break, continue, return, exception) commits the transaction.
</p><p>Entering a synchronized block by goto or switch is not allowed.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/transactional_memory&action=edit&section=2" title="Edit section: Atomic blocks">edit</a>]</span> <span class="mw-headline" id="Atomic_blocks">Atomic blocks</span></h3>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete </td></tr></table>
<p><code><b>atomic_noexcept</b></code> <span class="t-spar">compound-statement</span>
</p><p><code><b>atomic_cancel</b></code> <span class="t-spar">compound-statement</span>
</p><p><code><b>atomic_commit</b></code> <span class="t-spar">compound-statement</span>
</p>
<div class="t-li1"><span class="t-li">1)</span> If an exception is thrown, <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../utility/program/abort.html"><span class="kw844">std::<span class="me2">abort</span></span></a></span></span> is called</div>
<div class="t-li1"><span class="t-li">2)</span> If an exception is thrown, <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../utility/program/abort.html"><span class="kw844">std::<span class="me2">abort</span></span></a></span></span> is called, unless the exception is one of the exceptions uses for transaction cancellation (see below) in which case the transaction is <i>cancelled</i>: the values of all memory locations in the program that were modified by side effects of the operations of the atomic block are restored to the values they had at the time the start of the atomic block was executed, and the exception continues stack unwinding as usual.</div>
<div class="t-li1"><span class="t-li">3)</span> If an exception is thrown, the transaction is committed normally.</div>
<p>The exceptions used for transaction cancellation in <code>atomic_cancel</code> blocks are <span class="t-lc"><a href="../memory/new/bad_alloc.html" title="cpp/memory/new/bad alloc">std::bad_alloc</a></span>, <span class="t-lc"><a href="../memory/new/bad_array_length.html" title="cpp/memory/new/bad array length">std::bad_array_length</a></span>, <span class="t-lc"><a href="../memory/new/bad_array_new_length.html" title="cpp/memory/new/bad array new length">std::bad_array_new_length</a></span>, <span class="t-lc"><a href="../types/bad_cast.html" title="cpp/types/bad cast">std::bad_cast</a></span>, <span class="t-lc"><a href="../types/bad_typeid.html" title="cpp/types/bad typeid">std::bad_typeid</a></span>, <span class="t-lc"><a href="../error/bad_exception.html" title="cpp/error/bad exception">std::bad_exception</a></span>, <span class="t-lc"><a href="../error/exception.html" title="cpp/error/exception">std::exception</a></span> and all standard library exceptions derived from it, and the new exception <span class="t-lc">std::tx_exception<T></span>
</p><p>The <span class="t-spar">compound-statement</span> in an atomic block is not allowed to execute any expression or statement or call any function that isn't <code>transaction_safe</code> (this is a compile time error)
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co1">// each call to f() retrieves a unique value of i, even when done in parallel</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="kw4">static</span> <span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
atomic_noexcept <span class="br0">{</span> <span class="co1">// begin transaction</span>
<span class="co1">// printf("before %d\n", i); // error: cannot call a non transaction-safe function</span>
<span class="sy2">++</span>i<span class="sy4">;</span>
<span class="kw1">return</span> i<span class="sy4">;</span> <span class="co1">// commit transaction</span>
<span class="br0">}</span>
<span class="br0">}</span></pre></div></div>
<p>Leaving an atomic block by any means other than exception (reaching the end, goto, break, continue, return) commits the transaction.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/transactional_memory&action=edit&section=3" title="Edit section: Transaction-safe functions">edit</a>]</span> <span class="mw-headline" id="Transaction-safe_functions">Transaction-safe functions</span></h3>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete </td></tr></table>
<p>A function can be explicitly declared to be transaction-safe by using the keyword <span class="t-c"><span class="mw-geshi cpp source-cpp">transaction_safe</span></span> in its declaration.
</p>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete </td></tr></table>
<p>In a <a href="lambda.html" title="cpp/language/lambda">lambda</a> declaration, it appears either immediately after the capture list, or immediately after the (keyword <code>mutable</code> (if one is used).
</p>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete </td></tr></table>
<p><br />
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">extern</span> <span class="kw4">volatile</span> <span class="kw4">int</span> <span class="sy2">*</span> p <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="kw1">struct</span> S <span class="br0">{</span>
<span class="kw1">virtual</span> ~S<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="br0">)</span> transaction_safe <span class="br0">{</span>
<span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="co1">// ok: not volatile</span>
p <span class="sy1">=</span> <span class="sy3">&</span>x<span class="sy4">;</span> <span class="co1">// ok: the pointer is not volatile</span>
<span class="kw4">int</span> i <span class="sy1">=</span> <span class="sy2">*</span>p<span class="sy4">;</span> <span class="co1">// error: read through volatile glvalue</span>
S s<span class="sy4">;</span> <span class="co1">// error: invocation of unsafe destructor</span>
<span class="br0">}</span></pre></div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span> <span class="br0">{</span> <span class="co1">// implicitly transaction-safe</span>
<span class="kw1">if</span> <span class="br0">(</span>x <span class="sy1"><=</span> <span class="nu0">0</span><span class="br0">)</span>
<span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="kw1">return</span> x <span class="sy2">+</span> f<span class="br0">(</span>x<span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>If a function that is not transaction-safe is called through a reference or pointer to a non-transaction-safe function, the behavior is undefined.
</p><p><br />
</p><p><br />
</p><p>Pointers to transaction-safe functions and pointers to transaction-safe member functions are implicitly convertible to pointers to functions and pointers to member functions respectively. It is unspecified if the resulting pointer compares equal to the original.
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/transactional_memory&action=edit&section=4" title="Edit section: Transaction-safe virtual functions">edit</a>]</span> <span class="mw-headline" id="Transaction-safe_virtual_functions">Transaction-safe virtual functions</span></h4>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete </td></tr></table>
<p>If the final overrider of a <code>transaction_safe_dynamic</code> function is not declared <code>transaction_safe</code>, calling it in an atomic block is undefined behavior.
</p><p><br />
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/transactional_memory&action=edit&section=5" title="Edit section: Attributes">edit</a>]</span> <span class="mw-headline" id="Attributes">Attributes</span></h3>
<p>The attribute <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">[</span><span class="br0">[</span>optimize_for_synchronized<span class="br0">]</span><span class="br0">]</span></span></span> may be applied to a declarator in a function declaration and must appear on the first declaration of the function.
</p><p>If a function is declared <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">[</span><span class="br0">[</span>optimize_for_synchronized<span class="br0">]</span><span class="br0">]</span></span></span> in one translation unit and the same function is declared without <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">[</span><span class="br0">[</span>optimize_for_synchronized<span class="br0">]</span><span class="br0">]</span></span></span> in another translation unit, the program is ill-formed; no diagnostic required.
</p><p>It indicates that a the function definition should be optimized for invocation from a <span class="t-c"><span class="mw-geshi cpp source-cpp">synchronized</span></span> statement. In particular, it avoids serializing synchronized blocks that make a call to a function that is transaction-safe for the majority of calls, but not for all calls (e.g. hash table insertion that may have to rehash, allocator that may have to request a new block, a simple function that may rarely log)
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><a href="../atomic/atomic.html"><span class="kw1790">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span><span class="kw4">bool</span><span class="sy1">></span> rehash<span class="br0">{</span><span class="kw2">false</span><span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// maintenance thread runs this loop</span>
<span class="kw4">void</span> maintenance_thread<span class="br0">(</span><span class="kw4">void</span><span class="sy2">*</span><span class="br0">)</span> <span class="br0">{</span>
<span class="kw1">while</span> <span class="br0">(</span><span class="sy3">!</span>shutdown<span class="br0">)</span> <span class="br0">{</span>
synchronized <span class="br0">{</span>
<span class="kw1">if</span> <span class="br0">(</span>rehash<span class="br0">)</span> <span class="br0">{</span>
hash.<span class="me1">rehash</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
rehash <span class="sy1">=</span> <span class="kw2">false</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span>
<span class="br0">}</span>
<span class="br0">}</span>
 
<span class="co1">// worker threads execute hundreds of thousands of calls to this function </span>
<span class="co1">// every second. Calls to insert_key() from synchronized blocks in other</span>
<span class="co1">// translation units will cause those blocks to serialize, unless insert_key()</span>
<span class="co1">// is marked [[optimize_for_synchronized]]</span>
<span class="br0">[</span><span class="br0">[</span>optimize_for_synchronized<span class="br0">]</span><span class="br0">]</span> <span class="kw4">void</span> insert_key<span class="br0">(</span><span class="kw4">char</span><span class="sy2">*</span> key, <span class="kw4">char</span><span class="sy2">*</span> value<span class="br0">)</span> <span class="br0">{</span>
<span class="kw4">bool</span> concern <span class="sy1">=</span> hash.<span class="me1">insert</span><span class="br0">(</span>key, value<span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">if</span> <span class="br0">(</span>concern<span class="br0">)</span> rehash <span class="sy1">=</span> <span class="kw2">true</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="co1">// GCC assembly without the attribute: the entire function is serialized</span>
insert_key<span class="br0">(</span><span class="kw4">char</span><span class="sy2">*</span>, <span class="kw4">char</span><span class="sy2">*</span><span class="br0">)</span><span class="sy4">:</span>
subq $<span class="nu19">8</span>, <span class="sy2">%</span>rsp
movq <span class="sy2">%</span>rsi, <span class="sy2">%</span>rdx
movq <span class="sy2">%</span>rdi, <span class="sy2">%</span>rsi
movl $hash, <span class="sy2">%</span>edi
call Hash<span class="sy4">::</span><span class="me2">insert</span><span class="br0">(</span><span class="kw4">char</span><span class="sy2">*</span>, <span class="kw4">char</span><span class="sy2">*</span><span class="br0">)</span>
testb <span class="sy2">%</span>al, <span class="sy2">%</span>al
je .<span class="me1">L20</span>
movb $<span class="nu19">1</span>, rehash<span class="br0">(</span><span class="sy2">%</span>rip<span class="br0">)</span>
mfence
.<span class="me1">L20</span><span class="sy4">:</span>
addq $<span class="nu19">8</span>, <span class="sy2">%</span>rsp
ret
 
 
<span class="co1">// GCC assembly with the attribute:</span>
transaction clone <span class="kw1">for</span> insert_key<span class="br0">(</span><span class="kw4">char</span><span class="sy2">*</span>, <span class="kw4">char</span><span class="sy2">*</span><span class="br0">)</span><span class="sy4">:</span>
subq $<span class="nu19">8</span>, <span class="sy2">%</span>rsp
movq <span class="sy2">%</span>rsi, <span class="sy2">%</span>rdx
movq <span class="sy2">%</span>rdi, <span class="sy2">%</span>rsi
movl $hash, <span class="sy2">%</span>edi
call transaction clone <span class="kw1">for</span> Hash<span class="sy4">::</span><span class="me2">insert</span><span class="br0">(</span><span class="kw4">char</span><span class="sy2">*</span>, <span class="kw4">char</span><span class="sy2">*</span><span class="br0">)</span>
testb <span class="sy2">%</span>al, <span class="sy2">%</span>al
je .<span class="me1">L27</span>
xorl <span class="sy2">%</span>edi, <span class="sy2">%</span>edi
call _ITM_changeTransactionMode <span class="co1">// Note: this is the serialization point</span>
movb $<span class="nu19">1</span>, rehash<span class="br0">(</span><span class="sy2">%</span>rip<span class="br0">)</span>
mfence
.<span class="me1">L27</span><span class="sy4">:</span>
addq $<span class="nu19">8</span>, <span class="sy2">%</span>rsp
ret</pre></div></div>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete<br />Reason: check assembly with trunk, also show caller-side changes </td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/transactional_memory&action=edit&section=6" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete<br />Reason: experience notes from Wyatt paper/talk </td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/transactional_memory&action=edit&section=7" title="Edit section: Compiler support">edit</a>]</span> <span class="mw-headline" id="Compiler_support">Compiler support</span></h3>
<p>An older variant of this specification was <a rel="nofollow" class="external text" href="http://www-users.cs.umn.edu/~boutcher/stm/">supported in GCC</a> as of 4.7, current gcc trunk (gcc 6.0) supports the technical specification (requires <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span>fgnu<span class="sy2">-</span>tm</span></span> to enable)
</p>
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