为什么代码在线程之间突变共享变量显然不会受到竞争条件的影响？(Why does code mutating a shared variable across threads apparently NOT suffer from a race condition?)

 我正在使用Cygwin GCC并运行此代码： 
 
#include <iostream>
#include <thread>
#include <vector>
using namespace std;

unsigned u = 0;

void foo()
{
    u++;
}

int main()
{
    vector<thread> threads;
    for(int i = 0; i < 1000; i++) {
        threads.push_back (thread (foo));
    }
    for (auto& t : threads) t.join();

    cout << u << endl;
    return 0;
}
 
 用行编译： g++ -Wall -fexceptions -g -std=c++14 -c main.cpp -o main.o 
 
 它打印1000，这是正确的。 但是，由于线程覆盖了先前递增的值，我预计会导致较少的数字。 为什么这个代码不会相互访问？ 
 
 我的测试机有4个核心，我没有限制我所知道的程序。 
 
 当用更复杂的东西替换共享foo的内容时，问题仍然存在，例如 
 
if (u % 3 == 0) {
    u += 4;
} else {
    u -= 1;
}

I'm using Cygwin GCC and run this code:
 
#include <iostream>
#include <thread>
#include <vector>
using namespace std;

unsigned u = 0;

void foo()
{
    u++;
}

int main()
{
    vector<thread> threads;
    for(int i = 0; i < 1000; i++) {
        threads.push_back (thread (foo));
    }
    for (auto& t : threads) t.join();

    cout << u << endl;
    return 0;
}
 
Compiled with the line: g++ -Wall -fexceptions -g -std=c++14 -c main.cpp -o main.o.
 
It prints 1000, which is correct. However, I expected a lesser number due to threads overwriting a previously incremented value. Why does this code not suffer from mutual access?
 
My test machine has 4 cores, and I put no restrictions on the program that I know of.
 
The problem persists when replacing the content of the shared foo with something more complex, e.g.
 
if (u % 3 == 0) {
    u += 4;
} else {
    u -= 1;
}

原文：https://stackoverflow.com/questions/41816495