Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

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Programming interfaces

Memory order
Atomic objects
Fences
Feature testing macros

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

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 happens-before for explanation of the various ordering constraints.

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.

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.

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.

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.

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).

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_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


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