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std::atomic_fetch_add, std::atomic_fetch_add_explicit

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< cpp‎ | atomic

 
 
Atomique opérations de la bibliothèque
Types
Original:
Types
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atomic (C++11)
atomic_is_lock_free (C++11)
Fonctions
Original:
Functions
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atomic_store
atomic_store_explicit
(C++11)
(C++11)
atomic_load
atomic_load_explicit
(C++11)
(C++11)
atomic_exchange
atomic_exchange_explicit
(C++11)
(C++11)
atomic_compare_exchange_weak
atomic_compare_exchange_weak_explicit
atomic_compare_exchange_strong
atomic_compare_exchange_strong_explicit
(C++11)
(C++11)
(C++11)
(C++11)
atomic_fetch_add
atomic_fetch_add_explicit
(C++11)
(C++11)
atomic_fetch_sub
atomic_fetch_sub_explicit
(C++11)
(C++11)
atomic_fetch_and
atomic_fetch_and_explicit
(C++11)
(C++11)
atomic_fetch_or
atomic_fetch_or_explicit
(C++11)
(C++11)
atomic_fetch_xor
atomic_fetch_xor_explicit
(C++11)
(C++11)
Drapeaux atomiques
Original:
Atomic flags
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atomic_flag (C++11)
atomic_flag_test_and_set
atomic_flag_test_and_set_explicit
(C++11)
(C++11)
atomic_flag_clear
atomic_flag_clear_explicit
(C++11)
(C++11)
Initialisation
Original:
Initialization
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atomic_init (C++11)
ATOMIC_VAR_INIT (C++11)
ATOMIC_FLAG_INIT (C++11)
Mémoire de la commande
Original:
Memory ordering
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memory_order (C++11)
kill_dependency (C++11)
atomic_thread_fence (C++11)
atomic_signal_fence (C++11)
 
Defined in header <atomic>
template< class Integral >
Integral atomic_fetch_add( std::atomic<Integral>* obj, Integral arg );
(1) (depuis C++11)
template< class Integral >
Integral atomic_fetch_add( volatile std::atomic<Integral>* obj, Integral arg );
(2) (depuis C++11)
template< class Integral >

Integral atomic_fetch_add_explicit( std::atomic<Integral>* obj, Integral arg,

                                    std::memory_order order );
(3) (depuis C++11)
template< class Integral >

Integral atomic_fetch_add_explicit( volatile std::atomic<Integral>* obj, Integral arg,

                                    std::memory_order order );
(4) (depuis C++11)
template< class T >
T* atomic_fetch_add( std::atomic<T*>* obj, std::ptrdiff_t arg );
(5) (depuis C++11)
template< class T >
T* atomic_fetch_add( volatile std::atomic<T*>* obj, std::ptrdiff_t arg );
(6) (depuis C++11)
template< class T >

T* atomic_fetch_add_explicit( std::atomic<T*>* obj, std::ptrdiff_t arg,

                              std::memory_order order );
(7) (depuis C++11)
template< class T >

T* atomic_fetch_add_explicit( volatile std::atomic<T*>* obj, std::ptrdiff_t arg,

                              std::memory_order order );
(8) (depuis C++11)

1-2) Atomically adds arg to the value pointed to by obj and returns the value obj held previously, as if by obj->fetch_add(arg)

3-4) Atomically adds arg to the value pointed to by obj and returns the value obj held previously, as if by obj->fetch_add(arg, order)

5-6) Atomically increments the pointer value, pointed to by obj, by arg, and returns the value obj held previously, as if by obj->fetch_add(arg)

7-8) Atomically increments the pointer value, pointed to by obj, by arg, and returns the value obj held previously, as if by obj->fetch_add(arg, order)

Sommaire

[modifier] Paramètres

obj -
pointeur vers l'objet à modifier atomique
Original:
pointer to the atomic object to modify
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arg -
la valeur à ajouter à la valeur stockée dans l'objet atomique
Original:
the value to add to the value stored in the atomic object
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order -
sycnhronization la mémoire de commande pour cette opération: toutes les valeurs sont autorisées .
Original:
the memory sycnhronization ordering for this operation: all values are permitted.
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[modifier] Retourne la valeur

La valeur détenue précédemment par l'objet atomique pointée par obj
Original:
The value held previously by the atomic object pointed to by obj
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[modifier] Exceptions

noexcept specification:  
noexcept
   (depuis C++11)

[modifier] Mise en œuvre possible

First version
template< class T >
typename std::enable_if<std::is_integral<T>::value && !std::is_same<T, bool>::value, T>::type
atomic_fetch_add( std::atomic<T>* obj, T arg );
{
    return obj->fetch_add(arg);
}
Second version
template< class T >
T* atomic_fetch_add( std::atomic<T*>* obj, std::ptrdiff_t arg)
{
    return obj->fetch_add(arg);
}

[modifier] Exemple

Single-writer/multiple-reader lock can be made with fetch_add. Note that this simplistic implementation is not lockout-free

#include <string>
#include <thread>
#include <vector>
#include <iostream>
#include <atomic>
#include <chrono>
 
// meaning of cnt:
// 10: there are no active readers or writers.
// 1...9: there are 9...1 readers active, The writer is blocked
// 0: temporary value between fetch_sub and fetch_add in reader lock
// -1: there is a writer active. The readers are blocked.
const int N = 10; // nine concurrent readers are allowed
std::atomic<int> cnt = ATOMIC_VAR_INIT(N);
 
std::vector<int> data;
 
void reader(int id)
{
    for(;;)
    {
        // lock
        while(std::atomic_fetch_sub(&cnt, 1) <= 0)
            std::atomic_fetch_add(&cnt, 1);
        // read
        if(!data.empty())
            std::cout << (  "reader " + std::to_string(id)
                          + " sees " + std::to_string(*data.rbegin()) + '\n');
        if(data.size() == 100)
            break;
        // unlock
        std::atomic_fetch_add(&cnt, 1);
        // pause
        std::this_thread::sleep_for(std::chrono::milliseconds(1));
    }
}
 
void writer()
{
    for(int n = 0; n < 100; ++n)
    {
        // lock
        while(std::atomic_fetch_sub(&cnt, N+1) != N)
            std::atomic_fetch_add(&cnt, N+1);
        // write
        data.push_back(n);
        std::cout << "writer pushed back " << n << '\n';
        // unlock
        std::atomic_fetch_add(&cnt, N+1);
        // pause
        std::this_thread::sleep_for(std::chrono::milliseconds(1));
    }
}
 
int main()
{
    std::vector<std::thread> v;
    for (int n = 0; n < N; ++n) {
        v.emplace_back(reader, n);
    }
    v.emplace_back(writer);
    for (auto& t : v) {
        t.join();
    }
}

Résultat :

writer pushed back 0
reader 8 sees 0
reader 3 sees 0
reader 1 sees 0
<...>
reader 2 sees 99
reader 6 sees 99
reader 1 sees 99

[modifier] Voir aussi

(C++11)
ajoute l'argument atomique à la valeur stockée dans l'objet atomique et obtient la valeur détenue précédemment
Original:
atomically adds the argument to the value stored in the atomic object and obtains the value held previously
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(fonction membre publique de std::atomic) [edit]
soustrait une valeur non atomique à partir d'un objet atomique et obtient la valeur précédente de l'atome
Original:
subtracts a non-atomic value from an atomic object and obtains the previous value of the atomic
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(fonction générique) [edit]
C documentation for atomic_fetch_add, atomic_fetch_add_explicit