[/
 / Copyright (c) 2009 Helge Bahmann
 /
 / Distributed under the Boost Software License, Version 1.0. (See accompanying
 / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 /]

[library Boost.Atomic
    [quickbook 1.4]
    [authors [Bahmann, Helge]]
    [copyright 2011 Helge Bahmann]
    [copyright 2012 Tim Blechmann]
    [id atomic]
    [dirname atomic]
    [purpose Atomic operations]
    [license
        Distributed under the Boost Software License, Version 1.0.
        (See accompanying file LICENSE_1_0.txt or copy at
        [@http://www.boost.org/LICENSE_1_0.txt])
    ]
]

[section:introduction Introduction]

[section:introduction_presenting Presenting Boost.Atomic]

[*Boost.Atomic] is a library that provides [^atomic]
data types and operations on these data types, as well as memory
ordering constraints required for coordinating multiple threads through
atomic variables. It implements the interface as defined by the C++11
standard, but makes this feature available for platforms lacking
system/compiler support for this particular C++11 feature.

Users of this library should already be familiar with concurrency
in general, as well as elementary concepts such as "mutual exclusion".

The implementation makes use of processor-specific instructions where
possible (via inline assembler, platform libraries or compiler
intrinsics), and falls back to "emulating" atomic operations through
locking.

[endsect]

[section:introduction_purpose Purpose]

Operations on "ordinary" variables are not guaranteed to be atomic.
This means that with [^int n=0] initially, two threads concurrently
executing

[c++]

  void function()
  {
    n ++;
  }

might result in [^n==1] instead of 2: Each thread will read the
old value into a processor register, increment it and write the result
back. Both threads may therefore write [^1], unaware that the other thread
is doing likewise.

Declaring [^atomic<int> n=0] instead, the same operation on
this variable will always result in [^n==2] as each operation on this
variable is ['atomic]: This means that each operation behaves as if it
were strictly sequentialized with respect to the other.

Atomic variables are useful for two purposes:

* as a means for coordinating multiple threads via custom
  coordination protocols
* as faster alternatives to "locked" access to simple variables

Take a look at the [link atomic.usage_examples examples] section
for common patterns.

[endsect]

[endsect]

[section:thread_coordination Thread coordination using Boost.Atomic]

The most common use of [*Boost.Atomic] is to realize custom
thread synchronization protocols: The goal is to coordinate
accesses of threads to shared variables in order to avoid
"conflicts". The
programmer must be aware of the fact that
compilers, CPUs and the cache
hierarchies may generally reorder memory references at will.
As a consequence a program such as:

[c++]
  int x = 0, int y = 0;

  thread1:
    x = 1;
    y = 1;

  thread2
    if (y == 1) {
      assert(x == 1);
    }

might indeed fail as there is no guarantee that the read of `x`
by thread2 "sees" the write by thread1.

[*Boost.Atomic] uses a synchronisation concept based on the
['happens-before] relation to describe the guarantees under
which situations such as the above one cannot occur.

The remainder of this section will discuss ['happens-before] in
a "hands-on" way instead of giving a fully formalized definition.
The reader is encouraged to additionally have a
look at the discussion of the correctness of a few of the
[link atomic.usage_examples examples] afterwards.

[section:mutex Enforcing ['happens-before] through mutual exclusion]

As an introductory example to understand how arguing using
['happens-before] works, consider two threads synchronizing
using a common mutex:

[c++]

  mutex m;

  thread1:
    m.lock();
    ... /* A */
    m.unlock();

  thread2:
    m.lock();
    ... /* B */
    m.unlock();

The "lockset-based intuition" would be to argue that A and B
cannot be executed concurrently as the code paths require a
common lock to be held.

One can however also arrive at the same conclusion using
['happens-before]: Either thread1 or thread2 will succeed first
at [^m.lock()]. If this is be thread1, then as a consequence,
thread2 cannot succeed at [^m.lock()] before thread1 has executed
[^m.unlock()], consequently A ['happens-before] B in this case.
By symmetry, if thread2 succeeds at [^m.unlock()] first, we can
conclude B ['happens-before] A.

Since this already exhausts all options, we can conclude that
either A ['happens-before] B or B ['happens-before] A must
always hold. Obviously cannot state ['which] of the two relationships
holds, but either one is sufficient to conclude that A and B
cannot conflict.

Compare the [link boost_atomic.usage_examples.example_spinlock spinlock]
implementation to see how the mutual exclusion concept can be
mapped to [*Boost.Atomic].

[endsect]

[section:release_acquire ['happens-before] through [^release] and [^acquire]]

The most basic pattern for coordinating threads via [*Boost.Atomic]
uses [^release] and [^acquire] on an atomic variable for coordination: If ...

* ... thread1 performs an operation A,
* ... thread1 subsequently writes (or atomically
  modifies) an atomic variable with [^release] semantic,
* ... thread2 reads (or atomically reads-and-modifies)
  the value this value from the same atomic variable with
  [^acquire] semantic and
* ... thread2 subsequently performs an operation B,

... then A ['happens-before] B.

Consider the following example

[c++]

  atomic<int> a(0);

  thread1:
    ... /* A */
    a.fetch_add(1, memory_order_release);

  thread2:
    int tmp = a.load(memory_order_acquire);
    if (tmp == 1) {
      ... /* B */
    } else {
      ... /* C */
    }

In this example, two avenues for execution are possible:

* The [^store] operation by thread1 precedes the [^load] by thread2:
  In this case thread2 will execute B and "A ['happens-before] B"
  holds as all of the criteria above are satisfied.
* The [^load] operation by thread2 precedes the [^store] by thread1:
  In this case, thread2 will execute C, but "A ['happens-before] C"
  does ['not] hold: thread2 does not read the value written by
  thread1 through [^a].

Therefore, A and B cannot conflict, but A and C ['can] conflict.

[endsect]

[section:fences Fences]

Ordering constraints are generally specified together with an access to
an atomic variable. It is however also possible to issue "fence"
operations in isolation, in this case the fence operates in
conjunction with preceding (for `acquire`, `consume` or `seq_cst`
operations) or succeeding (for `release` or `seq_cst`) atomic
operations.

The example from the previous section could also be written in
the following way:

[c++]

  atomic<int> a(0);

  thread1:
    ... /* A */
    atomic_thread_fence(memory_order_release);
    a.fetch_add(1, memory_order_relaxed);

  thread2:
    int tmp = a.load(memory_order_relaxed);
    if (tmp == 1) {
      atomic_thread_fence(memory_order_acquire);
      ... /* B */
    } else {
      ... /* C */
    }

This provides the same ordering guarantees as previously, but
elides a (possibly expensive) memory ordering operation in
the case C is executed.

[endsect]

[section:release_consume ['happens-before] through [^release] and [^consume]]

The second pattern for coordinating threads via [*Boost.Atomic]
uses [^release] and [^consume] on an atomic variable for coordination: If ...

* ... thread1 performs an operation A,
* ... thread1 subsequently writes (or atomically modifies) an
  atomic variable with [^release] semantic,
* ... thread2 reads (or atomically reads-and-modifies)
  the value this value from the same atomic variable with [^consume] semantic and
* ... thread2 subsequently performs an operation B that is ['computationally
  dependent on the value of the atomic variable],

... then A ['happens-before] B.

Consider the following example

[c++]

  atomic<int> a(0);
  complex_data_structure data[2];

  thread1:
    data[1] = ...; /* A */
    a.store(1, memory_order_release);

  thread2:
    int index = a.load(memory_order_consume);
    complex_data_structure tmp = data[index]; /* B */

In this example, two avenues for execution are possible:

* The [^store] operation by thread1 precedes the [^load] by thread2:
  In this case thread2 will read [^data\[1\]] and "A ['happens-before] B"
  holds as all of the criteria above are satisfied.
* The [^load] operation by thread2 precedes the [^store] by thread1:
  In this case thread2 will read [^data\[0\]] and "A ['happens-before] B"
  does ['not] hold: thread2 does not read the value written by
  thread1 through [^a].

Here, the ['happens-before] relationship helps ensure that any
accesses (presumable writes) to [^data\[1\]] by thread1 happen before
before the accesses (presumably reads) to [^data\[1\]] by thread2:
Lacking this relationship, thread2 might see stale/inconsistent
data.

Note that in this example, the fact that operation B is computationally
dependent on the atomic variable, therefore the following program would
be erroneous:

[c++]

  atomic<int> a(0);
  complex_data_structure data[2];

  thread1:
    data[1] = ...; /* A */
    a.store(1, memory_order_release);

  thread2:
    int index = a.load(memory_order_consume);
    complex_data_structure tmp;
    if (index == 0)
      tmp = data[0];
    else
      tmp = data[1];

[^consume] is most commonly (and most safely! see
[link atomic.limitations limitations]) used with
pointers, compare for example the
[link boost_atomic.usage_examples.singleton singleton with double-checked locking].

[endsect]

[section:seq_cst Sequential consistency]

The third pattern for coordinating threads via [*Boost.Atomic]
uses [^seq_cst] for coordination: If ...

* ... thread1 performs an operation A,
* ... thread1 subsequently performs any operation with [^seq_cst],
* ... thread1 subsequently performs an operation B,
* ... thread2 performs an operation C,
* ... thread2 subsequently performs any operation with [^seq_cst],
* ... thread2 subsequently performs an operation D,

then either "A ['happens-before] D" or "C ['happens-before] B" holds.

In this case it does not matter whether thread1 and thread2 operate
on the same or different atomic variables, or use a "stand-alone"
[^atomic_thread_fence] operation.

[endsect]

[endsect]

[section:interface Programming interfaces]

[section:interface_memory_order Memory order]

The enumeration [^boost::memory_order] defines the following
values to represent memory ordering constraints:

[table
    [[Constant] [Description]]
    [[`memory_order_relaxed`] [No ordering constraint.
      Informally speaking, following operations may be reordered before,
      preceding operations may be reordered after the atomic
      operation. This constraint is suitable only when
      either a) further operations do not depend on the outcome
      of the atomic operation or b) ordering is enforced through
      stand-alone `atomic_thread_fence` operations
    ]]
    [[`memory_order_release`] [
      Perform `release` operation. Informally speaking,
      prevents all preceding memory operations to be reordered
      past this point.
    ]]
    [[`memory_order_acquire`] [
      Perform `acquire` operation. Informally speaking,
      prevents succeeding memory operations to be reordered
      before this point.
    ]]
    [[`memory_order_consume`] [
      Perform `consume` operation. More restrictive (and
      usually more efficient) than `memory_order_acquire`
      as it only affects succeeding operations that are
      computationally-dependent on the value retrieved from
      an atomic variable.
    ]]
    [[`memory_order_acq_rel`] [Perform both `release` and `acquire` operation]]
    [[`memory_order_seq_cst`] [
      Enforce sequential consistency. Implies `memory_order_acq_rel`, but
      additional enforces total order for all operations such qualified.
    ]]
]

See section [link atomic.thread_coordination ['happens-before]] for explanation
of the various ordering constraints.

[endsect]

[section:interface_atomic_object Atomic objects]

[^boost::atomic<['T]>] provides methods for atomically accessing
variables of a suitable type [^['T]]. The type is suitable if
it satisfies one of the following constraints:

* it is an integer, boolean, enum or pointer type
* it is any other data-type ([^class] or [^struct]) that has
  a non-throwing default constructor, that is copyable via
  [^memcpy] and comparable via [^memcmp].

Note that all classes having a trivial default constructor,
no destructor and no virtual methods satisfy the second condition
according to C++98. On a given platform, other data-types ['may]
also satisfy this constraint, however you should exercise
caution as the behaviour becomes implementation-defined. Also be warned
that structures with "padding" between data members may compare
non-equal via [^memcmp] even though all members are equal.

[section:interface_atomic_generic [^boost::atomic<['T]>] template class]

All atomic objects supports the following operations:

[table
    [[Syntax] [Description]]
    [
      [`atomic()`]
      [Initialize to an unspecified value]
    ]
    [
      [`atomic(T initial_value)`]
      [Initialize to [^initial_value]]
    ]
    [
      [`bool is_lock_free()`]
      [Checks if the atomic object is lock-free]
    ]
    [
      [`T load(memory_order order)`]
      [Return current value]
    ]
    [
      [`void store(T value, memory_order order)`]
      [Write new value to atomic variable]
    ]
    [
      [`T exchange(T new_value, memory_order order)`]
      [Exchange current value with `new_value`, returning current value]
    ]
    [
      [`bool compare_exchange_weak(T & expected, T desired, memory_order order)`]
      [Compare current value with `expected`, change it to `desired` if matches.
      Returns `true` if an exchange has been performed, and always writes the
      previous value back in `expected`. May fail spuriously, so must generally be
      retried in a loop.]
    ]
    [
      [`bool compare_exchange_weak(T & expected, T desired, memory_order success_order, memory_order failure_order)`]
      [Compare current value with `expected`, change it to `desired` if matches.
      Returns `true` if an exchange has been performed, and always writes the
      previous value back in `expected`. May fail spuriously, so must generally be
      retried in a loop.]
    ]
    [
      [`bool compare_exchange_strong(T & expected, T desired, memory_order order)`]
      [Compare current value with `expected`, change it to `desired` if matches.
      Returns `true` if an exchange has been performed, and always writes the
      previous value back in `expected`.]
    ]
    [
      [`bool compare_exchange_strong(T & expected, T desired, memory_order success_order, memory_order failure_order))`]
      [Compare current value with `expected`, change it to `desired` if matches.
      Returns `true` if an exchange has been performed, and always writes the
      previous value back in `expected`.]
    ]
]

`order` always has `memory_order_seq_cst` as default parameter.

The `compare_exchange_weak`/`compare_exchange_strong` variants
taking four parameters differ from the three parameter variants
in that they allow a different memory ordering constraint to
be specified in case the operation fails.

In addition to these explicit operations, each
[^atomic<['T]>] object also supports
implicit [^store] and [^load] through the use of "assignment"
and "conversion to [^T]" operators. Avoid using these operators,
as they do not allow explicit specification of a memory ordering
constraint.

[endsect]

[section:interface_atomic_integral [^boost::atomic<['integral]>] template class]

In addition to the operations listed in the previous section,
[^boost::atomic<['I]>] for integral
types [^['I]] supports the following operations:

[table
    [[Syntax] [Description]]
    [
      [`T fetch_add(T v, memory_order order)`]
      [Add `v` to variable, returning previous value]
    ]
    [
      [`T fetch_sub(T v, memory_order order)`]
      [Subtract `v` from variable, returning previous value]
    ]
    [
      [`T fetch_and(T v, memory_order order)`]
      [Apply bit-wise "and" with `v` to variable, returning previous value]
    ]
    [
      [`T fetch_or(T v, memory_order order)`]
      [Apply bit-wise "or" with `v` to variable, returning previous value]
    ]
    [
      [`T fetch_xor(T v, memory_order order)`]
      [Apply bit-wise "xor" with `v` to variable, returning previous value]
    ]
]

`order` always has `memory_order_seq_cst` as default parameter.

In addition to these explicit operations, each
[^boost::atomic<['I]>] object also
supports implicit pre-/post- increment/decrement, as well
as the operators `+=`, `-=`, `&=`, `|=` and `^=`.
Avoid using these operators,
as they do not allow explicit specification of a memory ordering
constraint.

[endsect]

[section:interface_atomic_pointer [^boost::atomic<['pointer]>] template class]

In addition to the operations applicable to all atomic object,
[^boost::atomic<['P]>] for pointer
types [^['P]] (other than [^void] pointers) support the following operations:

[table
    [[Syntax] [Description]]
    [
      [`T fetch_add(ptrdiff_t v, memory_order order)`]
      [Add `v` to variable, returning previous value]
    ]
    [
      [`T fetch_sub(ptrdiff_t v, memory_order order)`]
      [Subtract `v` from variable, returning previous value]
    ]
]

`order` always has `memory_order_seq_cst` as default parameter.

In addition to these explicit operations, each
[^boost::atomic<['P]>] object also
supports implicit pre-/post- increment/decrement, as well
as the operators `+=`, `-=`. Avoid using these operators,
as they do not allow explicit specification of a memory ordering
constraint.

[endsect]

[endsect]

[section:interface_fences Fences]

[table
    [[Syntax] [Description]]
    [
      [`void atomic_thread_fence(memory_order order)`]
      [Issue fence for coordination with other threads.]
    ]
    [
      [`void atomic_signal_fence(memory_order order)`]
      [Issue fence for coordination with signal handler (only in same thread).]
    ]
]

[endsect]

[section:feature_macros Feature testing macros]

[*Boost.Atomic] defines a number of macros to allow compile-time
detection whether an atomic data type is implemented using
"true" atomic operations, or whether an internal "lock" is
used to provide atomicity. The following macros will be
defined to `0` if operations on the data type always
require a lock, to `1` if operations on the data type may
sometimes require a lock, and to `2` if they are always lock-free:

[table
    [[Macro] [Description]]
    [
      [`BOOST_ATOMIC_CHAR_LOCK_FREE`]
      [Indicate whether `atomic<char>` (including signed/unsigned variants) is lock-free]
    ]
    [
      [`BOOST_ATOMIC_SHORT_LOCK_FREE`]
      [Indicate whether `atomic<short>` (including signed/unsigned variants) is lock-free]
    ]
    [
      [`BOOST_ATOMIC_INT_LOCK_FREE`]
      [Indicate whether `atomic<int>` (including signed/unsigned variants) is lock-free]
    ]
    [
      [`BOOST_ATOMIC_LONG_LOCK_FREE`]
      [Indicate whether `atomic<long>` (including signed/unsigned variants) is lock-free]
    ]
    [
      [`BOOST_ATOMIC_LLONG_LOCK_FREE`]
      [Indicate whether `atomic<long long>` (including signed/unsigned variants) is lock-free]
    ]
    [
      [`BOOST_ATOMIC_ADDRESS_LOCK_FREE`]
      [Indicate whether `atomic<T *>` is lock-free]
    ]
]

[endsect]

[endsect]

[section:usage_examples Usage examples]

[include examples.qbk]

[endsect]

[/
[section:platform_support Implementing support for additional platforms]

[include platform.qbk]

[endsect]
]

[/ [xinclude autodoc.xml] ]

[section:limitations Limitations]

While [*Boost.Atomic] strives to implement the atomic operations
from C++11 as faithfully as possible, there are a few
limitations that cannot be lifted without compiler support:

* [*Using non-POD-classes as template parameter to `atomic<T>` results
  in undefined behavior]: This means that any class containing a
  constructor, destructor, virtual methods or access control
  specifications is not a valid argument in C++98. C++11 relaxes
  this slightly by allowing "trivial" classes containing only
  empty constructors. [*Advise]: Use only POD types.
* [*C++98 compilers may transform computation- to control-dependency]:
  Crucially, `memory_order_consume` only affects computationally-dependent
  operations, but in general there is nothing preventing a compiler
  from transforming a computation dependency into a control dependency.
  A C++11 compiler would be forbidden from such a transformation.
  [*Advise]: Use `memory_order_consume` only in conjunction with
  pointer values, as the compiler cannot speculate and transform
  these into control dependencies.
* [*Fence operations enforce "too strong" compiler ordering]:
  Semantically, `memory_order_acquire`/`memory_order_consume`
  and `memory_order_release` need to restrain reordering of
  memory operations only in one direction. Since there is no
  way to express this constraint to the compiler, these act
  as "full compiler barriers" in this implementation. In corner
  cases this may lead to worse code than a C++11 compiler
  could generate.
* [*No interprocess fallback]: using `atomic<T>` in shared memory only works
  correctly, if `atomic<T>::is_lock_free == true`

[endsect]

[section:porting Porting]

[section:unit_tests Unit tests]

[*Boost.Atomic] provides a unit test suite to verify that the
implementation behaves as expected:

* [*fallback_api.cpp] verifies that the fallback-to-locking aspect
  of [*Boost.Atomic] compiles and has correct value semantics.
* [*native_api.cpp] verifies that all atomic operations have correct
  value semantics (e.g. "fetch_add" really adds the desired value,
  returning the previous). It is a rough "smoke-test" to help weed
  out the most obvious mistakes (for example with overflow,
  signed/unsigned extension, ...).
* [*lockfree.cpp] verifies that the [*BOOST_ATOMIC_*_LOCKFREE] macros
  are set properly according to the expectations for a given
  platform, and that they match up with the [*is_lock_free] member
  functions of the [*atomic] object instances.
* [*atomicity.cpp] lets two threads race against each other modifying
  a shared variable, verifying that the operations behave atomic
  as appropriate. By nature, this test is necessarily stochastic, and
  the test self-calibrates to yield 99% confidence that a
  positive result indicates absence of an error. This test is
  very useful on uni-processor systems with preemption already.
* [*ordering.cpp] lets two threads race against each other accessing
  multiple shared variables, verifying that the operations
  exhibit the expected ordering behavior. By nature, this test is
  necessarily stochastic, and the test attempts to self-calibrate to
  yield 99% confidence that a positive result indicates absence
  of an error. This only works on true multi-processor (or multi-core)
  systems. It does not yield any result on uni-processor systems
  or emulators (due to there being no observable reordering even
  the order=relaxed case) and will report that fact.

[endsect]

[section:tested_compilers Tested compilers]

[*Boost.Atomic] has been tested on and is known to work on
the following compilers/platforms:

* gcc 4.x: i386, x86_64, ppc32, ppc64, armv5, armv6, alpha
* Visual Studio Express 2008/Windows XP, i386

If you have an unsupported platform, contact me and I will
work to add support for it.

[endsect]

[endsect]
