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## Programming interfaces

### Configuration and building

The library contains header-only and compiled parts. The library is header-only for lock-free cases but requires a separate binary to implement the lock-based emulation. Users are able to detect whether linking to the compiled part is required by checking the feature macros.

The following macros affect library behavior:

Macro | Description
--- | ---

`BOOST_ATOMIC_NO_CMPXCHG16B`

Affects 64-bit x86 MSVC builds. When defined, the library assumes the target CPU does not support `cmpxchg16b` instruction used to support 128-bit atomic operations. This is the case with some early 64-bit AMD CPUs, all Intel CPUs and current AMD CPUs support this instruction. The library does not perform runtime detection of this instruction, so running the code that uses 128-bit atomics on such CPUs will result in crashes, unless this macro is defined. Note that the macro does not affect GCC and compatible compilers because the library infers this information from the compiler-defined macros.

`BOOST_ATOMIC_FORCE_FALLBACK` | When defined, all operations are implemented with locks. This is mostly used for testing and should not be used in real world projects.
`BOOST_ATOMIC_DYN_LINK` and `BOOST_ALL_DYN_LINK` | Control library linking. If defined, the library assumes dynamic linking, otherwise static. The latter macro affects all Boost libraries, not just Boost.Atomic.
`BOOST_ATOMIC_NO_LIB` and `BOOST_ALL_NO_LIB` | Control library auto-linking on Windows. When defined, disables auto-linking. The latter macro affects all Boost libraries, not just Boost.Atomic.

Besides macros, it is important to specify the correct compiler options for the target CPU. With GCC and compatible compilers this affects whether particular atomic operations are lock-free or not.

Boost building process is described in the Getting Started guide. For example, you can build Boost.Atomic with the following command line:

    bjam --with-atomic variant=release instruction-set=core2 stage

### Memory order

    #include <boost/memory_order.hpp>

The enumeration `boost::memory_order` defines the following values to represent memory ordering constraints:

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. The operation on the atomic value itself is still atomic though.

`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 relaxed (and on some architectures 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 additionally enforces total order for all operations such qualified.

See section happens-before for explanation of the various ordering constraints.

### Atomic objects

    #include <boost/atomic/atomic.hpp>

`boost::atomic<T>` provides methods for atomically accessing variables of a suitable type `T`. The type is suitable if it is trivially copyable (3.9/9 [basic.types]). Following are examples of the types compatible with this requirement:

- a scalar type (e.g. integer, boolean, enum or pointer type)
- a `class` or `struct` that has no non-trivial copy or move constructors or assignment operators, has a trivial destructor, and that is comparable via `memcmp`.

Note that classes with virtual functions or virtual base classes do not satisfy the requirements. Also be warned that structures with "padding" between data members may compare non-equal via `memcmp` even though all members are equal.

#### `boost::atomic<T>` template class

All atomic objects supports the following operations:

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.

#### `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:

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.

#### `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:

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.

### Fences

    #include <boost/atomic/fences.hpp>

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).

### Feature testing macros

    #include <boost/atomic/capabilities.hpp>

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:

Macro | Description
--- | ---
`BOOST_ATOMIC_FLAG_LOCK_FREE` | Indicate whether `atomic_flag` is lock-free
`BOOST_ATOMIC_BOOL_LOCK_FREE` | Indicate whether `atomic<bool>` is lock-free
`BOOST_ATOMIC_CHAR_LOCK_FREE` | Indicate whether `atomic<char>` (including signed/unsigned variants) is lock-free
`BOOST_ATOMIC_CHAR16_T_LOCK_FREE` | Indicate whether `atomic<char16_t>` (including signed/unsigned variants) is lock-free
`BOOST_ATOMIC_CHAR32_T_LOCK_FREE` | Indicate whether `atomic<char32_t>` (including signed/unsigned variants) is lock-free
`BOOST_ATOMIC_WCHAR_T_LOCK_FREE` | Indicate whether `atomic<wchar_t>` (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` or `BOOST_ATOMIC_POINTER_LOCK_FREE` | Indicate whether `atomic<T *>` is lock-free
`BOOST_ATOMIC_THREAD_FENCE` | Indicate whether `atomic_thread_fence` function is lock-free
`BOOST_ATOMIC_SIGNAL_FENCE` | Indicate whether `atomic_signal_fence` function is lock-free

In addition to these standard macros, Boost.Atomic also defines a number of extension macros, which can also be useful. Like the standard ones, these macros are defined to values `0`, `1` and `2` to indicate whether the corresponding operations are lock-free or not.

Macro | Description
--- | ---
`BOOST_ATOMIC_INT8_LOCK_FREE` | Indicate whether `atomic<int8_type>` is lock-free.
`BOOST_ATOMIC_INT16_LOCK_FREE` | Indicate whether `atomic<int16_type>` is lock-free.
`BOOST_ATOMIC_INT32_LOCK_FREE` | Indicate whether `atomic<int32_type>` is lock-free.
`BOOST_ATOMIC_INT64_LOCK_FREE` | Indicate whether `atomic<int64_type>` is lock-free.
`BOOST_ATOMIC_INT128_LOCK_FREE` | Indicate whether `atomic<int128_type>` is lock-free.
`BOOST_ATOMIC_NO_ATOMIC_FLAG_INIT` | Defined after including `atomic_flag.hpp`, if the implementation does not support the `BOOST_ATOMIC_FLAG_INIT` macro for static initialization of `atomic_flag`. This macro is typically defined for pre-C++11 compilers.

In the table above, `intN_type` is a type that fits storage of contiguous `N` bits, suitably aligned for atomic operations.