gcc linux利用CAS实现无锁编程

利用__sync_bool_compare_and_swap()，实现无锁编程

CAS即compare and swap,每次都会和old 值进行比较，如果没有发生更改再用new值给ptr赋值，然后返回true

#include <bits/stdc++.h>
#include <pthread.h>
#include <stdlib.h>
#include <unistd.h>

#ifdef __GNUC__
//#define ATOMIC
#define CAS
#define _ATOMIC_ADD_(x, y) __sync_fetch_and_add(x, y)
#define _ATOMIC_SUB_(x, y) __sync_fetch_and_sub(x, y)
#define _CAS_(ptr, old, new) __sync_bool_compare_and_swap(ptr, old, new)
#else
#define _ATOMIC_ADD_(x, y) ;
#define _ATOMIC_SUB_(x, y) ;
#define _CAS_(ptr, old, new) ;
#endif
int val = 0;

void* worker1(void* arg)
{
#ifdef ATOMIC
    for (int i = 0; i < 50000; i++)
        _ATOMIC_ADD_(&val, 1);
#endif
#ifdef CAS
    int old = val;
    int i = 0;
    for (; i < 10000; i++) {
        while (!_CAS_(&val, old, old + 1)) {
            old = val;
        }   
    }
    printf("%d", i);
#endif
}

void* worker2(void* arg)
{
#ifdef ATOMIC
    for (int i = 0; i < 50000; i++)
        _ATOMIC_SUB_(&val, 1);
#endif
#ifdef CAS
    int i = 0;
    int old = val;
    for (; i < 10000; i++) {
        while (!_CAS_(&val, old, old - 1)) {
            old = val;
        }
    }
    printf("%d", i);
#endif
}

int main()
{
    pthread_t tid[2];
    pthread_create(&tid[0], NULL, worker1, NULL);
    pthread_create(&tid[1], NULL, worker2, NULL);
    pthread_join(tid[0], NULL);
    pthread_join(tid[1], NULL);
    std::cout << val << std::endl;
    return 0;
}