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Difference between revisions of "cpp/atomic/atomic is lock free"

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< cpp‎ | atomic
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{{title|{{dsc small|std::}}atomic_is_lock_free, ATOMIC_xxx_LOCK_FREE}}
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{{cpp/title|atomic_is_lock_free, ATOMIC_xxx_LOCK_FREE}}
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{{cpp/atomic/navbar}}
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{{cpp/thread/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl header | atomic }}
+
{{dcl header|atomic}}
−
{{dcl | num=1 | notes={{mark since c++11}} | 1=
+
{{dcl|num=1|since=c++11|
−
template< class Atomic >
+
template< class T >
−
bool atomic_is_lock_free(const volatile Atomic* obj)
+
bool atomic_is_lock_free( const volatile std::atomic<T>* obj ) noexcept;
 
}}
 
}}
−
{{dcl | num=2 | notes={{mark since c++11}} | 1=
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{{dcl|num=2|since=c++11|
−
template< class Atomic >
+
template< class T >
−
bool atomic_is_lock_free(const Atomic* obj)
+
bool atomic_is_lock_free( const std::atomic<T>* obj ) noexcept;
 
}}
 
}}
−
{{dcl | num=3 | notes={{mark since c++11}} | 1=
+
{{dcl|num=3|since=c++11|
 +
#define ATOMIC_BOOL_LOCK_FREE    /* unspecified */
 
#define ATOMIC_CHAR_LOCK_FREE    /* unspecified */
 
#define ATOMIC_CHAR_LOCK_FREE    /* unspecified */
 
#define ATOMIC_CHAR16_T_LOCK_FREE /* unspecified */
 
#define ATOMIC_CHAR16_T_LOCK_FREE /* unspecified */
Line 21: Line 22:
 
#define ATOMIC_LLONG_LOCK_FREE    /* unspecified */
 
#define ATOMIC_LLONG_LOCK_FREE    /* unspecified */
 
#define ATOMIC_POINTER_LOCK_FREE  /* unspecified */
 
#define ATOMIC_POINTER_LOCK_FREE  /* unspecified */
 +
}}
 +
{{dcl|num=4|since=c++20|
 +
#define ATOMIC_CHAR8_T_LOCK_FREE  /* unspecified */
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
  
−
@1-2@ Determines if the atomic object pointed to by {{tt|obj}} is implemented lock-free, as if by calling {{c|obj->is_lock_free()}}
+
@1,2@ Determines if the atomic object pointed to by {{c|obj}} is implemented lock-free, as if by calling {{c|obj->is_lock_free()}}. In any given program execution, the result of the lock-free query is the same for all atomic objects of the same type.
  
−
@3@ Expands to an integer constant expression with value {{c|0}} for the built-in atomic types that are never lock-free, to {{c|1}} for the built-in atomic types that are sometimes lock-free, and to {{c|2}} for the built-in atomic types that are always lock-free.
+
@3,4@ Expands to an integer constant expression with value
−
 
+
* {{c|0}} for the built-in atomic types that are never lock-free,
−
===Notes===
+
* {{c|1}} for the built-in atomic types that are ''sometimes'' lock-free,
−
All atomic types except for {{lc|std::atomic_flag}} may be implemented using mutexes or other locking operations, rather than using the lock-free atomic CPU instructions. Atomic types are also allowed to be ''sometimes'' lock-free, e.g. if only aligned memory accesses are naturally atomic on a given architecture, misaligned objects of the same type have to use locks. If the type is sometimes lock-free, then the function (1-2) or its member function equivalent has to be used to determine if the particular instance is lock-free.
+
* {{c|2}} for the built-in atomic types that are always lock-free.
  
 
===Parameters===
 
===Parameters===
−
 
 
{{par begin}}
 
{{par begin}}
−
{{par | obj | pointer to the atomic object to examine}}
+
{{par|obj|pointer to the atomic object to examine}}
 
{{par end}}  
 
{{par end}}  
  
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{{c|true}} if {{c|*obj}} is a lock-free atomic, {{c|false}} otherwise.
 
{{c|true}} if {{c|*obj}} is a lock-free atomic, {{c|false}} otherwise.
  
−
===Exceptions===
+
===Notes===
−
{{noexcept}}
+
All atomic types except for {{lc|std::atomic_flag}} may be implemented using mutexes or other locking operations, rather than using the lock-free atomic CPU instructions. Atomic types are also allowed to be ''sometimes'' lock-free: for example, if only some subarchitectures support lock-free atomic access for a given type (such as the CMPXCHG16B instruction on x86-64), whether atomics are lock-free may not be known until runtime.
 +
 
 +
The C++ standard recommends (but does not require) that lock-free atomic operations are also address-free, that is, suitable for communication between processes using shared memory.
  
 
===Example===
 
===Example===
−
{{example
+
{{example|code=
−
|
+
#include <atomic>
−
| code=
+
#include <iostream>
−
| output=
+
#include <utility>
 +
 
 +
struct A { int a[100]; };
 +
struct B { int x, y; };
 +
 
 +
int main()
 +
{
 +
    std::atomic<A> a;
 +
    std::atomic<B> b;
 +
    std::cout << std::boolalpha
 +
              << "std::atomic<A> is lock free? "
 +
              << std::atomic_is_lock_free(&a) << '\n'
 +
              << "std::atomic<B> is lock free? "
 +
              << std::atomic_is_lock_free(&b) << '\n';
 +
}
 +
|p=true
 +
|output=
 +
std::atomic<A> is lock free? false
 +
std::atomic<B> is lock free? true
 
}}
 
}}
 +
 +
===Defect reports===
 +
{{dr list begin}}
 +
{{dr list item|wg=lwg|dr=3249|std=C++11|before={{tt|atomic_is_lock_free}} was specified via pointers, which<br>was ambiguous and might accept invalid pointer values|after=specified via<br>atomic objects}}
 +
{{dr list end}}
  
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/atomic/atomic/dsc is_lock_free | mem=std::atomic<T>}}
+
{{dsc inc|cpp/atomic/atomic/dsc is_lock_free}}
−
{{dsc tfun | cpp/memory/shared_ptr/atomic | title=std::atomic_is_lock_free{{dsc small|(std::shared_ptr)}} | specializes atomic operations for {{lc|std::shared_ptr}} }}
+
{{dsc inc|cpp/atomic/dsc atomic_flag}}
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{{dsc inc | cpp/atomic/dsc atomic_flag}}
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{{dsc inc|cpp/atomic/atomic/dsc is_always_lock_free}}
 +
{{dsc break}}
 +
{{dsc tfun|cpp/memory/shared_ptr/atomic|notes={{mark life|deprecated=c++20|removed=c++26|br=yes}}|title=std::atomic_is_lock_free{{dsc small|(std::shared_ptr)}}|specializes atomic operations for {{lc|std::shared_ptr}}}}
 +
{{dsc see c|c/atomic/atomic_is_lock_free}}
 +
{{dsc see c|c/atomic/ATOMIC_LOCK_FREE_consts|ATOMIC_*_LOCK_FREE}}
 
{{dsc end}}
 
{{dsc end}}
  
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[[de:cpp/atomic/atomic is lock free]]
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{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
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[[es:cpp/atomic/atomic is lock free]]
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[[fr:cpp/atomic/atomic is lock free]]
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[[it:cpp/atomic/atomic is lock free]]
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[[ja:cpp/atomic/atomic is lock free]]
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[[pt:cpp/atomic/atomic is lock free]]
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[[ru:cpp/atomic/atomic is lock free]]
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[[zh:cpp/atomic/atomic is lock free]]
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Latest revision as of 04:44, 24 April 2024

 
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Defined in header <atomic>
template< class T >
bool atomic_is_lock_free( const volatile std::atomic<T>* obj ) noexcept;
 (1) (since C++11)
template< class T >
bool atomic_is_lock_free( const std::atomic<T>* obj ) noexcept;
 (2) (since C++11)
#define ATOMIC_BOOL_LOCK_FREE     /* unspecified */

#define ATOMIC_CHAR_LOCK_FREE     /* unspecified */
#define ATOMIC_CHAR16_T_LOCK_FREE /* unspecified */
#define ATOMIC_CHAR32_T_LOCK_FREE /* unspecified */
#define ATOMIC_WCHAR_T_LOCK_FREE  /* unspecified */
#define ATOMIC_SHORT_LOCK_FREE    /* unspecified */
#define ATOMIC_INT_LOCK_FREE      /* unspecified */
#define ATOMIC_LONG_LOCK_FREE     /* unspecified */
#define ATOMIC_LLONG_LOCK_FREE    /* unspecified */

#define ATOMIC_POINTER_LOCK_FREE  /* unspecified */
 (3) (since C++11)
#define ATOMIC_CHAR8_T_LOCK_FREE  /* unspecified */
 (4) (since C++20)
1,2) Determines if the atomic object pointed to by obj is implemented lock-free, as if by calling obj->is_lock_free(). In any given program execution, the result of the lock-free query is the same for all atomic objects of the same type.
3,4) Expands to an integer constant expression with value
  • ​0​ for the built-in atomic types that are never lock-free,
  • 1 for the built-in atomic types that are sometimes lock-free,
  • 2 for the built-in atomic types that are always lock-free.

Contents

[edit] Parameters

obj - pointer to the atomic object to examine

[edit] Return value

true if *obj is a lock-free atomic, false otherwise.

[edit] Notes

All atomic types except for std::atomic_flag may be implemented using mutexes or other locking operations, rather than using the lock-free atomic CPU instructions. Atomic types are also allowed to be sometimes lock-free: for example, if only some subarchitectures support lock-free atomic access for a given type (such as the CMPXCHG16B instruction on x86-64), whether atomics are lock-free may not be known until runtime.

The C++ standard recommends (but does not require) that lock-free atomic operations are also address-free, that is, suitable for communication between processes using shared memory.

[edit] Example

Run this code
#include <atomic>
#include <iostream>
#include <utility>
 
struct A { int a[100]; };
struct B { int x, y; };
 
int main()
{
    std::atomic<A> a;
    std::atomic<B> b;
    std::cout << std::boolalpha
              << "std::atomic<A> is lock free? "
              << std::atomic_is_lock_free(&a) << '\n'
              << "std::atomic<B> is lock free? "
              << std::atomic_is_lock_free(&b) << '\n';
}

Possible output: 

std::atomic<A> is lock free? false
std::atomic<B> is lock free? true

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3249 C++11 atomic_is_lock_free was specified via pointers, which
was ambiguous and might accept invalid pointer values 		specified via
atomic objects

[edit] See also

 checks if the atomic object is lock-free
(public member function of std::atomic<T>) [edit]
(C++11)
 the lock-free boolean atomic type
(class) [edit]
[static] (C++17)
 indicates that the type is always lock-free
(public static member constant of std::atomic<T>) [edit]
(deprecated in C++20)(removed in C++26)
 specializes atomic operations for std::shared_ptr
(function template)
C documentation for atomic_is_lock_free
C documentation for ATOMIC_*_LOCK_FREE
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