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<title>GNU libitm: C/C++ Language Constructs for TM</title>
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<a name="C_002fC_002b_002b-Language-Constructs-for-TM"></a>
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<div class="header">
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<p>
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Next: <a href="The-libitm-ABI.html#The-libitm-ABI" accesskey="n" rel="next">The libitm ABI</a>, Previous: <a href="Enabling-libitm.html#Enabling-libitm" accesskey="p" rel="prev">Enabling libitm</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Library-Index.html#Library-Index" title="Index" rel="index">Index</a>]</p>
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<a name="C_002fC_002b_002b-Language-Constructs-for-TM-1"></a>
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<h2 class="chapter">2 C/C++ Language Constructs for TM</h2>
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<p>Transactions are supported in C++ and C in the form of transaction statements,
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transaction expressions, and function transactions. In the following example,
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both <code>a</code> and <code>b</code> will be read and the difference will be written to
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<code>c</code>, all atomically and isolated from other transactions:
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</p>
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<div class="example">
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<pre class="example">__transaction_atomic { c = a - b; }
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</pre></div>
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<p>Therefore, another thread can use the following code to concurrently update
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<code>b</code> without ever causing <code>c</code> to hold a negative value (and without
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having to use other synchronization constructs such as locks or C++11
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atomics):
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</p>
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<div class="example">
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<pre class="example">__transaction_atomic { if (a > b) b++; }
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</pre></div>
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<p>GCC follows the <a href="https://sites.google.com/site/tmforcplusplus/">Draft
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Specification of Transactional Language Constructs for C++ (v1.1)</a> in its
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implementation of transactions.
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</p>
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<p>The precise semantics of transactions are defined in terms of the C++11/C11
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memory model (see the specification). Roughly, transactions provide
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synchronization guarantees that are similar to what would be guaranteed when
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using a single global lock as a guard for all transactions. Note that like
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other synchronization constructs in C/C++, transactions rely on a
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data-race-free program (e.g., a nontransactional write that is concurrent
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with a transactional read to the same memory location is a data race).
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