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