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Difference between revisions of "cpp/atomic/atomic flag test and set"

From cppreference.com
< cpp‎ | atomic
m (fmt, {{c}}, headers sorted)
(Splitted the volatile overloads.)
 
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{{dcl begin}}
 
{{dcl begin}}
 
{{dcl header|atomic}}
 
{{dcl header|atomic}}
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{{dcl rev begin|num=1|since=c++11}}
+
{{dcl|num=1|since=c++11|
−
{{dcl|
+
bool atomic_flag_test_and_set( volatile std::atomic_flag* obj ) noexcept;
−
bool atomic_flag_test_and_set( volatile std::atomic_flag* p ) noexcept;
+
 
}}
 
}}
−
{{dcl|
+
{{dcl|num=2|since=c++11|
−
bool atomic_flag_test_and_set( std::atomic_flag* p ) noexcept;
+
bool atomic_flag_test_and_set( std::atomic_flag* obj ) noexcept;
 
}}
 
}}
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{{dcl rev end}}
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{{dcl|num=3|since=c++11|
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{{dcl rev begin|num=2|since=c++11}}
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bool atomic_flag_test_and_set_explicit( volatile std::atomic_flag* obj,  
−
{{dcl|
+
−
bool atomic_flag_test_and_set_explicit( volatile std::atomic_flag* p,  
+
 
                                         std::memory_order order ) noexcept;
 
                                         std::memory_order order ) noexcept;
 
}}
 
}}
−
{{dcl|
+
{{dcl|num=4|since=c++11|
−
bool atomic_flag_test_and_set_explicit( std::atomic_flag* p,  
+
bool atomic_flag_test_and_set_explicit( std::atomic_flag* obj,  
 
                                         std::memory_order order ) noexcept;
 
                                         std::memory_order order ) noexcept;
 
}}
 
}}
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{{dcl rev end}}
 
 
{{dcl end}}
 
{{dcl end}}
  
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Atomically changes the state of a {{lc|std::atomic_flag}} pointed to by {{c|p}} to set ({{c|true}}) and returns the value it held before.  
+
Atomically changes the state of a {{lc|std::atomic_flag}} pointed to by {{c|obj}} to set ({{c|true}}) and returns the value it held before.
 +
@1,2@ The memory synchronization order is {{c|std::memory_order_seq_cst}}.
 +
@3,4@ The memory synchronization order is {{c|order}}.
  
 
===Parameters===
 
===Parameters===
 
{{par begin}}
 
{{par begin}}
−
{{par|p|pointer to {{lc|std::atomic_flag}} to access}}
+
{{par|obj|pointer to {{lc|std::atomic_flag}} to access}}
−
{{par|order|the memory synchronization order for this operation}}
+
{{par|order|the memory synchronization order}}
 
{{par end}}  
 
{{par end}}  
  
 
===Return value===
 
===Return value===
−
The value previously held by the flag pointed to by {{c|p}}.
+
The value previously held by the flag pointed to by {{c|obj}}.
  
−
===Possible implementation===
+
===Notes===
−
{{eq fun
+
{{tt|std::atomic_flag_test_and_set}} and {{tt|std::atomic_flag_test_and_set_explicit}} can be implemented as {{c|obj->test_and_set()}} and {{c|obj->test_and_set(order)}} respectively.
−
|1=
+
−
bool atomic_flag_test_and_set(volatile std::atomic_flag* p)
+
−
{
+
−
    return p->test_and_set();
+
−
}
+
−
|2=
+
−
bool atomic_flag_test_and_set(std::atomic_flag* p)
+
−
{
+
−
    return p->test_and_set();
+
−
}
+
−
|3=
+
−
bool atomic_flag_test_and_set_explicit(volatile std::atomic_flag* p,
+
−
                                      std::memory_order order)
+
−
{
+
−
    return p->test_and_set(order);
+
−
}
+
−
|4=
+
−
bool atomic_flag_test_and_set_explicit(std::atomic_flag* p,
+
−
                                      std::memory_order order)
+
−
{
+
−
    return p->test_and_set(order);
+
−
}
+
−
}}
+
  
 
===Example===
 
===Example===
 
{{example
 
{{example
−
|A spinlock mutex can be implemented in userspace using an {{tt|atomic_flag}}.
+
|A spinlock mutex can be implemented in userspace using an {{tt|std::atomic_flag}}.
 
|code=
 
|code=
 
#include <atomic>
 
#include <atomic>

Latest revision as of 22:05, 24 September 2023

 
C++
 
Concurrency support library
Threads
(C++11)
(C++20)
(C++17)(C++17)
this_thread namespace
(C++11)
(C++11)
(C++11)
(C++11)
Cooperative cancellation
Mutual exclusion
(C++11)
(C++11)
(C++17)
Generic lock management
(C++11)
(C++11)
(C++17)
(C++11)
(C++14)
(C++11)
(C++11)
(C++11)
(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)
Condition variables
(C++11)
(C++11)
(C++11)
(C++11)
Semaphores
(C++20)(C++20)
Latches and Barriers
(C++20)
(C++20)
Futures
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
Safe Reclamation
(C++26)
(C++26)
(C++26)
Hazard Pointers
(C++26)
(C++26)
(C++26)
Atomic types
(C++11)
(C++20)
(C++11)
Initialization of atomic types
(C++11)(deprecated in C++20)
(C++11)(deprecated in C++20)
(C++11)
Memory ordering
(C++11)
(C++11)
(C++11)
(C++11)
Free functions for atomic operations
(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)
(C++26)(C++26)
(C++26)(C++26)
(C++11)
(C++20)(C++20)
(C++20)
(C++20)
Free functions for atomic flags
atomic_flag_test_and_setatomic_flag_test_and_set_explicit
(C++11)(C++11)
(C++11)(C++11)
(C++20)(C++20)
(C++20)(C++20)
(C++20)
(C++20)
 
Defined in header <atomic>
bool atomic_flag_test_and_set( volatile std::atomic_flag* obj ) noexcept;
 (1) (since C++11)
bool atomic_flag_test_and_set( std::atomic_flag* obj ) noexcept;
 (2) (since C++11)
bool atomic_flag_test_and_set_explicit( volatile std::atomic_flag* obj,
                                        std::memory_order order ) noexcept;
 (3) (since C++11)
bool atomic_flag_test_and_set_explicit( std::atomic_flag* obj,
                                        std::memory_order order ) noexcept;
 (4) (since C++11)

Atomically changes the state of a std::atomic_flag pointed to by obj to set (true) and returns the value it held before.

1,2) The memory synchronization order is std::memory_order_seq_cst.
3,4) The memory synchronization order is order.

Contents

[edit] Parameters

obj - pointer to std::atomic_flag to access
order - the memory synchronization order

[edit] Return value

The value previously held by the flag pointed to by obj.

[edit] Notes

std::atomic_flag_test_and_set and std::atomic_flag_test_and_set_explicit can be implemented as obj->test_and_set() and obj->test_and_set(order) respectively.

[edit] Example

A spinlock mutex can be implemented in userspace using an std::atomic_flag.

Run this code
#include <atomic>
#include <iostream>
#include <thread>
#include <vector>
 
std::atomic_flag lock = ATOMIC_FLAG_INIT;
 
void f(int n)
{
    for (int cnt = 0; cnt < 100; ++cnt)
    {
        while (std::atomic_flag_test_and_set_explicit(&lock, std::memory_order_acquire))
            ; // spin until the lock is acquired
        std::cout << "Output from thread " << n << '\n';
        std::atomic_flag_clear_explicit(&lock, std::memory_order_release);
    }
}
 
int main()
{
    std::vector<std::thread> v;
    for (int n = 0; n < 10; ++n)
        v.emplace_back(f, n);
    for (auto& t : v)
        t.join();
}

Output: 

Output from thread 2
Output from thread 6
Output from thread 7
...<exactly 1000 lines>...

[edit] See also

(C++11)
 the lock-free boolean atomic type
(class) [edit]
(C++11)(C++11)
 atomically sets the value of the flag to false
(function) [edit]
(C++11)
 defines memory ordering constraints for the given atomic operation
(enum) [edit]
C documentation for atomic_flag_test_and_set, atomic_flag_test_and_set_explicit
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