- std::atomic_flag
std::atomic_flag是一個原子的布爾類型,可支援兩種原子操作:
- test_and_set, 如果atomic_flag對象被設定,則傳回true; 如果atomic_flag對象未被設定,則設定之,傳回false
- clear. 清楚atomic_flag對象
std::atomic_flag可用于多線程之間的同步操作,類似于linux中的信号量。使用atomic_flag可實作mutex.
#include <iostream>
#include <atomic>
#include <vector>
#include <thread>
#include <sstream>
std::atomic_flag lock = ATOMIC_FLAG_INIT;
std::stringstream stream;
void append_numer(int x)
{
while (lock.test_and_set());
stream << "thread#" << x << "\n";
lock.clear();
}
int main()
{
std::vector<std::thread> ths;
for (int i=0; i<10; i++)
ths.push_back(std::thread(append_numer, i));
for (int i=0; i<10; i++)
ths[i].join();
std::cout << stream.str();
return 0;
}
2. std::atomic
std::atomic對int, char, bool等資料結構進行原子性封裝,在多線程環境中,對std::atomic對象的通路不會造成競争-冒險。利用std::atomic可實作資料結構的無鎖設計。
#include <iostream>
#include <atomic>
#include <vector>
#include <thread>
#include <sstream>
std::atomic<bool> ready(false);
std::atomic_flag winner = ATOMIC_FLAG_INIT;
void count1m(int i)
{
while (!ready);
for (int i=0; i<1000000; i++);
if (!winner.test_and_set())
std::cout << "winner: " << i << std::endl;
}
int main()
{
std::vector<std::thread> ths;
for (int i=0; i<10; i++)
ths.push_back(std::thread(count1m, i));
ready = true;
for (int i=0; i<10; i++)
ths[i].join();
return 0;
}
在上例中,執行read=true之後,所有線程結束空等。winner被初始化為ATOMIC_FLAG_INIT,最先執行winner.test_and_set并傳回false的線程為winner。
#include <iostream>
#include <atomic>
#include <vector>
#include <thread>
#include <sstream>
std::atomic<int> foo(0);
void set_foo(int x)
{
foo = x;
}
void print_foo()
{
while (foo == 0)
{
std::this_thread::yield();
}
std::cout << "x: " << foo << std::endl;
}
int main()
{
std::thread print_th(print_foo);
std::thread set_th(set_foo, 10);
print_th.join();
set_th.join();
return 0;
}
在上例總,set_foo用于設定atomic<int>對象的值,print_foo用于列印atomic<int>對象的值。std::atomic對象的值的讀取和寫入可使用load和store實作。
#include <iostream>
#include <cassert>
#include <atomic>
#include <vector>
#include <unistd.h>
#include <thread>
#include <sstream>
std::atomic<int> foo(0);
std::atomic_flag lock = ATOMIC_FLAG_INIT;
void add_foo()
{
while (1)
{
foo++;
// foo = foo + 1;
while (lock.test_and_set());
std::cout <<"add: " << foo << std::endl;
lock.clear();
usleep(1000);
}
}
void sub_foo()
{
while (1)
{
foo--;
// foo = foo - 1;
while (lock.test_and_set());
std::cout << "sub: " << foo << std::endl;
lock.clear();
usleep(1000);
}
}
int main()
{
std::thread th2 = std::thread(add_foo);
std::thread th1 = std::thread(sub_foo);
th1.join();
th2.join();
return 0;
}
atomic<int>支援++和--的原子操作。
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