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std::atomic_fetch_sub, std::atomic_fetch_sub_explicit

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< cpp‎ | atomic

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C++
 
Atomic operazioni di biblioteca
Tipi
Original:
Types
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atomic(C++11)
atomic_is_lock_free(C++11)
Funzioni
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)
Bandiere Atomic
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)
Inizializzazione
Original:
Initialization
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atomic_init(C++11)
ATOMIC_VAR_INIT(C++11)
ATOMIC_FLAG_INIT(C++11)
Memoria di ordinazione
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)
 
Elemento definito nell'header <atomic>
template< class Integral >
Integral atomic_fetch_sub( std::atomic<Integral>* obj, Integral arg );
 (1) (dal C++11)
template< class Integral >
Integral atomic_fetch_sub( volatile std::atomic<Integral>* obj, Integral arg );
 (2) (dal C++11)
template< class Integral >

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

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

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

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

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

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

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

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

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

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

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

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

Indice

[modifica] Parametri

obj -
puntatore all'oggetto atomica da modificare
Original:
pointer to the atomic object to modify
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arg -
il valore da sottrarre dal valore memorizzato nell'oggetto atomica
Original:
the value to subtract from the value stored in the atomic object
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order -
sycnhronization la memoria di ordinazione per questa operazione: tutti i valori sono consentiti .
Original:
the memory sycnhronization ordering for this operation: all values are permitted.
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[modifica] Valore di ritorno

Il valore contenuto in precedenza dall'oggetto atomico puntato da obj
Original:
The value held previously by the atomic object pointed to by obj
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[modifica] Eccezioni

noexcept specification:  
noexcept
  (dal C++11)

[modifica] Possibile implementazione

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

[modifica] Esempio

Multiple threads may use fetch_sub to concurrently process an indexed container 

#include <string>
#include <thread>
#include <vector>
#include <iostream>
#include <atomic>
#include <numeric>
 
const int N = 10000;
std::atomic<int> cnt;
std::vector<int> data(N);
 
void reader(int id) 
{
    for(;;) {
        int idx = atomic_fetch_sub_explicit(&cnt, 1, std::memory_order_relaxed);
        if (idx >= 0) {
            std::cout << "reader " << std::to_string(id) << " processed item "
                      << std::to_string(data[idx]) << '\n';
        } else {
            std::cout << "reader " << std::to_string(id) << " done\n";
            break;
        }
    }
}
 
int main()
{
    std::iota(data.begin(), data.end(), 1);
    cnt = data.size() - 1;
 
    std::vector<std::thread> v;
    for (int n = 0; n < 10; ++n) {
        v.emplace_back(reader, n);
    }
    for (auto& t : v) {
        t.join();
    }
}

Output: 

reader 2 processed item 10000
reader 6 processed item 9994
reader 4 processed item 9996
reader 6 processed item 9992
<....>
reader 0 done
reader 5 done
reader 3 done
reader 9 done

[modifica] Vedi anche

(C++11)
sottrae atomicamente l'argomento dal valore memorizzato nell'oggetto atomica e ottiene il valore contenuto in precedenza
Original:
atomically subtracts the argument from the value stored in the atomic object and obtains the value held previously
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(metodo pubblico) [modifica]
(C++11)
(C++11)
aggiunge un non-atomico valore a un oggetto atomico e ottiene il valore precedente della atomico
Original:
adds a non-atomic value to an atomic object and obtains the previous value of the atomic
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(funzione di modello) [modifica]
C documentation for atomic_fetch_sub, atomic_fetch_sub_explicit
Estratto da "https://it.cppreference.com/mwiki/index.php?title=cpp/atomic/atomic_fetch_sub&oldid=25026"