libitm/doc/c-c++-language-constructs-for-tm.rst

   1 ..
   2   Copyright 1988-2022 Free Software Foundation, Inc.
   3   This is part of the GCC manual.
   4   For copying conditions, see the copyright.rst file.
   5
   6 .. _c-c++-language-constructs-for-tm:
   7
   8 C/C++ Language Constructs for TM
   9 --------------------------------
  10
  11 Transactions are supported in C++ and C in the form of transaction statements,
  12 transaction expressions, and function transactions. In the following example,
  13 both ``a`` and ``b`` will be read and the difference will be written to
  14 ``c``, all atomically and isolated from other transactions:
  15
  16 .. code-block:: c++
  17
  18   __transaction_atomic { c = a - b; }
  19
  20 Therefore, another thread can use the following code to concurrently update
  21 ``b`` without ever causing ``c`` to hold a negative value (and without
  22 having to use other synchronization constructs such as locks or C++11
  23 atomics):
  24
  25 .. code-block:: c++
  26
  27   __transaction_atomic { if (a > b) b++; }
  28
  29 GCC follows the `Draft
  30 Specification of Transactional Language Constructs for C++ (v1.1) <https://sites.google.com/site/tmforcplusplus/>`_ in its
  31 implementation of transactions.
  32
  33 The precise semantics of transactions are defined in terms of the C++11/C11
  34 memory model (see the specification). Roughly, transactions provide
  35 synchronization guarantees that are similar to what would be guaranteed when
  36 using a single global lock as a guard for all transactions. Note that like
  37 other synchronization constructs in C/C++, transactions rely on a
  38 data-race-free program (e.g., a nontransactional write that is concurrent
  39 with a transactional read to the same memory location is a data race).