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

Configuration and building
Memory order
Atomic flags
Atomic objects
Fences
Feature testing macros

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_CMPXCHG8B

Affects 32-bit x86 Oracle Studio builds. When defined, the library assumes the target CPU does not support cmpxchg8b instruction used to support 64-bit atomic operations. This is the case with very old CPUs (pre-Pentium). The library does not perform runtime detection of this instruction, so running the code that uses 64-bit atomics on such CPUs will result in crashes, unless this macro is defined. Note that the macro does not affect MSVC, GCC and compatible compilers because the library infers this information from the compiler-defined macros.

BOOST_ATOMIC_NO_CMPXCHG16B

Affects 64-bit x86 MSVC and Oracle Studio 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_NO_MFENCE

Affects 32-bit x86 Oracle Studio builds. When defined, the library assumes the target CPU does not support mfence instruction used to implement thread fences. This instruction was added with SSE2 instruction set extension, which was available in CPUs since Intel Pentium 4. The library does not perform runtime detection of this instruction, so running the library code on older CPUs will result in crashes, unless this macro is defined. Note that the macro does not affect MSVC, GCC and compatible compilers because the library infers this information from the compiler-defined macros.

BOOST_ATOMIC_NO_FLOATING_POINT

When defined, support for floating point operations is disabled. Floating point types shall be treated similar to trivially copyable structs and no capability macros will be defined.

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
#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.

For compilers that support C++11 scoped enums, the library also defines scoped synonyms that are preferred in modern programs:

Pre-C++11 constant

C++11 equivalent

memory_order_relaxed

memory_order::relaxed

memory_order_release

memory_order::release

memory_order_acquire

memory_order::acquire

memory_order_consume

memory_order::consume

memory_order_acq_rel

memory_order::acq_rel

memory_order_seq_cst

memory_order::seq_cst

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

#include <boost/atomic/atomic_flag.hpp>

The boost::atomic_flag type provides the most basic set of atomic operations suitable for implementing mutually exclusive access to thread-shared data. The flag can have one of the two possible states: set and clear. The class implements the following operations:

Syntax

Description

atomic_flag()

Initialize to the clear state. See the discussion below.

bool test_and_set(memory_order order)

Sets the atomic flag to the set state; returns true if the flag had been set prior to the operation

void clear(memory_order order)

Sets the atomic flag to the clear state

order always has memory_order_seq_cst as default parameter.

Note that the default constructor atomic_flag() is unlike std::atomic_flag, which leaves the default-constructed object uninitialized. This potentially requires dynamic initialization during the program startup to perform the object initialization, which makes it unsafe to create global boost::atomic_flag objects that can be used before entring main(). Some compilers though (especially those supporting C++11 constexpr) may be smart enough to perform flag initialization statically (which is, in C++11 terms, a constant initialization).

This difference is deliberate and is done to support C++03 compilers. C++11 defines the ATOMIC_FLAG_INIT macro which can be used to statically initialize std::atomic_flag to a clear state like this:

std::atomic_flag flag = ATOMIC_FLAG_INIT; // constant initialization

This macro cannot be implemented in C++03 because for that atomic_flag would have to be an aggregate type, which it cannot be because it has to prohibit copying and consequently define the default constructor. Thus the closest equivalent C++03 code using Boost.Atomic would be:

boost::atomic_flag flag; // possibly, dynamic initialization in C++03;
                         // constant initialization in C++11

The same code is also valid in C++11, so this code can be used universally. However, for interface parity with std::atomic_flag, if possible, the library also defines the BOOST_ATOMIC_FLAG_INIT macro, which is equivalent to ATOMIC_FLAG_INIT:

boost::atomic_flag flag = BOOST_ATOMIC_FLAG_INIT; // constant initialization

This macro will only be implemented on a C++11 compiler. When this macro is not available, the library defines BOOST_ATOMIC_NO_ATOMIC_FLAG_INIT.

#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. This may also be the case with some floating point types, which include padding bits themselves.

All atomic objects support the following operations and properties:

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; the returned value is consistent with the is_always_lock_free static constant, see below

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.

static bool is_always_lock_free

This static boolean constant indicates if any atomic object of this type is lock-free

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 to specify a memory ordering constraint which always defaults to memory_order_seq_cst.

In addition to the operations listed in the previous section, boost::atomic<I> for integral types I, except bool, supports the following operations, which correspond to std::atomic<I>:

Syntax

Description

I fetch_add(I v, memory_order order)

Add v to variable, returning previous value

I fetch_sub(I v, memory_order order)

Subtract v from variable, returning previous value

I fetch_and(I v, memory_order order)

Apply bit-wise "and" with v to variable, returning previous value

I fetch_or(I v, memory_order order)

Apply bit-wise "or" with v to variable, returning previous value

I fetch_xor(I v, memory_order order)

Apply bit-wise "xor" with v to variable, returning previous value

Additionally, as a Boost.Atomic extension, the following operations are also provided:

Syntax

Description

I fetch_negate(memory_order order)

Change the sign of the value stored in the variable, returning previous value

I fetch_complement(memory_order order)

Set the variable to the one's complement of the current value, returning previous value

I negate(memory_order order)

Change the sign of the value stored in the variable, returning the result

I add(I v, memory_order order)

Add v to variable, returning the result

I sub(I v, memory_order order)

Subtract v from variable, returning the result

I bitwise_and(I v, memory_order order)

Apply bit-wise "and" with v to variable, returning the result

I bitwise_or(I v, memory_order order)

Apply bit-wise "or" with v to variable, returning the result

I bitwise_xor(I v, memory_order order)

Apply bit-wise "xor" with v to variable, returning the result

I bitwise_complement(memory_order order)

Set the variable to the one's complement of the current value, returning the result

void opaque_negate(memory_order order)

Change the sign of the value stored in the variable, returning nothing

void opaque_add(I v, memory_order order)

Add v to variable, returning nothing

void opaque_sub(I v, memory_order order)

Subtract v from variable, returning nothing

void opaque_and(I v, memory_order order)

Apply bit-wise "and" with v to variable, returning nothing

void opaque_or(I v, memory_order order)

Apply bit-wise "or" with v to variable, returning nothing

void opaque_xor(I v, memory_order order)

Apply bit-wise "xor" with v to variable, returning nothing

void opaque_complement(memory_order order)

Set the variable to the one's complement of the current value, returning nothing

bool negate_and_test(memory_order order)

Change the sign of the value stored in the variable, returning true if the result is non-zero and false otherwise

bool add_and_test(I v, memory_order order)

Add v to variable, returning true if the result is non-zero and false otherwise

bool sub_and_test(I v, memory_order order)

Subtract v from variable, returning true if the result is non-zero and false otherwise

bool and_and_test(I v, memory_order order)

Apply bit-wise "and" with v to variable, returning true if the result is non-zero and false otherwise

bool or_and_test(I v, memory_order order)

Apply bit-wise "or" with v to variable, returning true if the result is non-zero and false otherwise

bool xor_and_test(I v, memory_order order)

Apply bit-wise "xor" with v to variable, returning true if the result is non-zero and false otherwise

bool complement_and_test(memory_order order)

Set the variable to the one's complement of the current value, returning true if the result is non-zero and false otherwise

bool bit_test_and_set(unsigned int n, memory_order order)

Set bit number n in the variable to 1, returning true if the bit was previously set to 1 and false otherwise

bool bit_test_and_reset(unsigned int n, memory_order order)

Set bit number n in the variable to 0, returning true if the bit was previously set to 1 and false otherwise

bool bit_test_and_complement(unsigned int n, memory_order order)

Change bit number n in the variable to the opposite value, returning true if the bit was previously set to 1 and false otherwise

[Note] Note

In Boost.Atomic 1.66 the op_and_test operations returned the opposite value (i.e. true if the result is zero). This was changed to the current behavior in 1.67 for consistency with other operations in Boost.Atomic, as well as with conventions taken in the C++ standard library. Boost.Atomic 1.66 was the only release shipped with the old behavior. Users upgrading from Boost 1.66 to a later release can define BOOST_ATOMIC_HIGHLIGHT_OP_AND_TEST macro when building their code to generate deprecation warnings on the op_and_test function calls (the functions are not actually deprecated though; this is just a way to highlight their use).

order always has memory_order_seq_cst as default parameter.

The opaque_op and op_and_test variants of the operations may result in a more efficient code on some architectures because the original value of the atomic variable is not preserved. In the bit_test_and_op operations, the bit number n starts from 0, which means the least significand bit, and must not exceed std::numeric_limits<I>::digits - 1.

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 to specify a memory ordering constraint which always defaults to memory_order_seq_cst.

[Note] Note

The support for floating point types is optional and can be disabled by defining BOOST_ATOMIC_NO_FLOATING_POINT.

In addition to the operations applicable to all atomic objects, boost::atomic<F> for floating point types F supports the following operations, which correspond to std::atomic<F>:

Syntax

Description

F fetch_add(F v, memory_order order)

Add v to variable, returning previous value

F fetch_sub(F v, memory_order order)

Subtract v from variable, returning previous value

Additionally, as a Boost.Atomic extension, the following operations are also provided:

Syntax

Description

F fetch_negate(memory_order order)

Change the sign of the value stored in the variable, returning previous value

F negate(memory_order order)

Change the sign of the value stored in the variable, returning the result

F add(F v, memory_order order)

Add v to variable, returning the result

F sub(F v, memory_order order)

Subtract v from variable, returning the result

void opaque_negate(memory_order order)

Change the sign of the value stored in the variable, returning nothing

void opaque_add(F v, memory_order order)

Add v to variable, returning nothing

void opaque_sub(F v, memory_order order)

Subtract v from variable, returning nothing

order always has memory_order_seq_cst as default parameter.

The opaque_op variants of the operations may result in a more efficient code on some architectures because the original value of the atomic variable is not preserved.

In addition to these explicit operations, each boost::atomic<F> object also supports operators += and -=. Avoid using these operators, as they do not allow to specify a memory ordering constraint which always defaults to memory_order_seq_cst.

When using atomic operations with floating point types, bear in mind that Boost.Atomic always performs bitwise comparison of the stored values. This means that operations like compare_exchange* may fail if the stored value and comparand have different binary representation, even if they would normally compare equal. This is typically the case when either of the numbers is denormalized. This also means that the behavior with regard to special floating point values like NaN and signed zero is also different from normal C++.

Another source of the problem is padding bits that are added to some floating point types for alignment. One widespread example of that is Intel x87 extended double format, which is typically stored as 80 bits of value padded with 16 or 48 unused bits. These padding bits are often uninitialized and contain garbage, which makes two equal numbers have different binary representation. The library attempts to account for the known such cases, but in general it is possible that some platforms are not covered. Note that the C++ standard makes no guarantees about reliability of compare_exchange* operations in the face of padding or trap bits.

In addition to the operations applicable to all atomic objects, boost::atomic<P> for pointer types P (other than pointers to void, function or member pointers) support the following operations, which correspond to std::atomic<P>:

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

Similarly to integers, the following Boost.Atomic extensions are also provided:

Syntax

Description

void add(ptrdiff_t v, memory_order order)

Add v to variable, returning the result

void sub(ptrdiff_t v, memory_order order)

Subtract v from variable, returning the result

void opaque_add(ptrdiff_t v, memory_order order)

Add v to variable, returning nothing

void opaque_sub(ptrdiff_t v, memory_order order)

Subtract v from variable, returning nothing

bool add_and_test(ptrdiff_t v, memory_order order)

Add v to variable, returning true if the result is non-null and false otherwise

bool sub_and_test(ptrdiff_t v, memory_order order)

Subtract v from variable, returning true if the result is non-null and false otherwise

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 which always defaults to memory_order_seq_cst.

For convenience, several shorthand typedefs of boost::atomic<T> are provided:

typedef atomic< char > atomic_char;
typedef atomic< unsigned char > atomic_uchar;
typedef atomic< signed char > atomic_schar;
typedef atomic< unsigned short > atomic_ushort;
typedef atomic< short > atomic_short;
typedef atomic< unsigned int > atomic_uint;
typedef atomic< int > atomic_int;
typedef atomic< unsigned long > atomic_ulong;
typedef atomic< long > atomic_long;
typedef atomic< unsigned long long > atomic_ullong;
typedef atomic< long long > atomic_llong;

typedef atomic< void* > atomic_address;
typedef atomic< bool > atomic_bool;
typedef atomic< wchar_t > atomic_wchar_t;
typedef atomic< char16_t > atomic_char16_t;
typedef atomic< char32_t > atomic_char32_t;

typedef atomic< uint8_t > atomic_uint8_t;
typedef atomic< int8_t > atomic_int8_t;
typedef atomic< uint16_t > atomic_uint16_t;
typedef atomic< int16_t > atomic_int16_t;
typedef atomic< uint32_t > atomic_uint32_t;
typedef atomic< int32_t > atomic_int32_t;
typedef atomic< uint64_t > atomic_uint64_t;
typedef atomic< int64_t > atomic_int64_t;

typedef atomic< int_least8_t > atomic_int_least8_t;
typedef atomic< uint_least8_t > atomic_uint_least8_t;
typedef atomic< int_least16_t > atomic_int_least16_t;
typedef atomic< uint_least16_t > atomic_uint_least16_t;
typedef atomic< int_least32_t > atomic_int_least32_t;
typedef atomic< uint_least32_t > atomic_uint_least32_t;
typedef atomic< int_least64_t > atomic_int_least64_t;
typedef atomic< uint_least64_t > atomic_uint_least64_t;
typedef atomic< int_fast8_t > atomic_int_fast8_t;
typedef atomic< uint_fast8_t > atomic_uint_fast8_t;
typedef atomic< int_fast16_t > atomic_int_fast16_t;
typedef atomic< uint_fast16_t > atomic_uint_fast16_t;
typedef atomic< int_fast32_t > atomic_int_fast32_t;
typedef atomic< uint_fast32_t > atomic_uint_fast32_t;
typedef atomic< int_fast64_t > atomic_int_fast64_t;
typedef atomic< uint_fast64_t > atomic_uint_fast64_t;
typedef atomic< intmax_t > atomic_intmax_t;
typedef atomic< uintmax_t > atomic_uintmax_t;

typedef atomic< std::size_t > atomic_size_t;
typedef atomic< std::ptrdiff_t > atomic_ptrdiff_t;

typedef atomic< intptr_t > atomic_intptr_t;
typedef atomic< uintptr_t > atomic_uintptr_t;

The typedefs are provided only if the corresponding type is available.

#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).

#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.

For floating-point types the following macros are similarly defined:

Macro

Description

BOOST_ATOMIC_FLOAT_LOCK_FREE

Indicate whether atomic<float> is lock-free.

BOOST_ATOMIC_DOUBLE_LOCK_FREE

Indicate whether atomic<double> is lock-free.

BOOST_ATOMIC_LONG_DOUBLE_LOCK_FREE

Indicate whether atomic<long double> is lock-free.

These macros are not defined when support for floating point types is disabled by user.


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