场景

1.原子操作一般用在多线程执行的逻辑里, 比如统计业务个数 ++count, 或者下载文件个数,下载大小, 设置对象状态等. 如果不用原子操作, 那么可能在并发情况下, 原本的事务指令A会被事务指令2抢占而导致出现数据被覆盖.

说明

1.macOS下提供了 <libkern/OSAtomic.h> 来处理 C 方式的原子操作. 如果是object-c方式的原子属性可以使用关键字 atomic.

2.C++11(vs2012以上)也提供了方便使用可移植的 Atomic Library,

例子

#include <CoreFoundation/CoreFoundation.h> #import <Foundation/Foundation.h> #include <time.h> #include <vector> #include <atomic> #include <algorithm> #include <pthread.h> #include <libkern/OSAtomic.h> void TestAtomic(){ NSLog(@"TestAtomic"); volatile int64_t foo = 1; NSLog(@"foo is %lld",foo); // ++ OSAtomicIncrement64(&foo); NSLog(@"++ OSAtomicIncrement64: %lld",foo); // -- OSAtomicDecrement64(&foo); NSLog(@"-- OSAtomicDecrement64: %lld",foo); // = OSAtomicCompareAndSwap64(foo, 64, &foo); NSLog(@"= OSAtomicCompareAndSwap64: %lld",foo); // += OSAtomicAdd64(36, &foo); NSLog(@"+= OSAtomicAdd64: %lld",foo); // -= OSAtomicAdd64(-36, &foo); NSLog(@"-= OSAtomicAdd64: %lld",foo); } void TestCppAtomicLibrary(){ NSLog(@"TestCppAtomicLibrary"); std::atomic<int64_t> foo(1); NSLog(@"foo is %lld",foo.load()); // ++ ++foo; NSLog(@"++ OSAtomicIncrement64: %lld",foo.load()); // -- --foo; NSLog(@"-- OSAtomicDecrement64: %lld",foo.load()); // = foo = 64; NSLog(@"= OSAtomicCompareAndSwap64: %lld",foo.load()); // += foo+=36; NSLog(@"+= OSAtomicAdd64: %lld",foo.load()); // -= foo-=36; NSLog(@"-= OSAtomicAdd64: %lld",foo.load()); } #define THREAD_COUNT 100 #define FOR_NUMBER 10000 static volatile int32_t count = 0; static pthread_mutex_t mutex; static pthread_cond_t cond; static volatile int64_t foo = 1; static std::atomic<int64_t> foo1(1); void* RunCAtomic(void*){ for(int i = 0; i< FOR_NUMBER;++i){ OSAtomicIncrement64(&foo); // fail if ++foo OSAtomicAdd64(36, &foo); } for(int i = 0; i< FOR_NUMBER;++i){ OSAtomicDecrement64(&foo); OSAtomicAdd64(-36, &foo); } OSAtomicIncrement32(&count); if(OSAtomicCompareAndSwap32(THREAD_COUNT, 0, &count)){ assert(count == 0); pthread_mutex_lock(&mutex); pthread_cond_signal(&cond); pthread_mutex_unlock(&mutex); } return NULL; } void* RunCppAtomic(void*){ for(int i = 0; i< FOR_NUMBER;++i){ ++foo1; foo1+=36; } for(int i = 0; i< FOR_NUMBER;++i){ --foo1; foo1-=36; } OSAtomicIncrement32(&count); if(OSAtomicCompareAndSwap32(THREAD_COUNT, 0, &count)){ assert(count == 0); pthread_mutex_lock(&mutex); pthread_cond_signal(&cond); pthread_mutex_unlock(&mutex); } return NULL; } int main(int argc, const char * argv[]) { NSAutoreleasePool* pool = [NSAutoreleasePool new]; TestAtomic(); TestCppAtomicLibrary(); pthread_mutex_init(&mutex, NULL); pthread_cond_init(&cond, NULL); pthread_attr_t att; pthread_attr_init(&att); pthread_attr_setdetachstate(&att, PTHREAD_CREATE_JOINABLE); NSLog(@"Begin CAtomic"); for(int i = 0; i < THREAD_COUNT;++i){ pthread_t t; pthread_create(&t,&att,RunCAtomic,NULL); } pthread_mutex_lock(&mutex); pthread_cond_wait(&cond, &mutex); pthread_mutex_unlock(&mutex); assert(foo == 1); NSLog(@"End CAtomic"); NSLog(@"Begin CppAtomic"); for(int i = 0; i < THREAD_COUNT;++i){ pthread_t t; pthread_create(&t,&att,RunCppAtomic,NULL); } pthread_mutex_lock(&mutex); pthread_cond_wait(&cond, &mutex); pthread_mutex_unlock(&mutex); assert(foo1.load() == 1); NSLog(@"End CAtomic"); [pool drain]; return 0; }

输出:

2017-07-11 10:26:18.162 TestObjc[744:303] TestAtomic 2017-07-11 10:26:18.165 TestObjc[744:303] foo is 1 2017-07-11 10:26:18.165 TestObjc[744:303] ++ OSAtomicIncrement64: 2 2017-07-11 10:26:18.166 TestObjc[744:303] -- OSAtomicDecrement64: 1 2017-07-11 10:26:18.166 TestObjc[744:303] = OSAtomicCompareAndSwap64: 64 2017-07-11 10:26:18.167 TestObjc[744:303] += OSAtomicAdd64: 100 2017-07-11 10:26:18.167 TestObjc[744:303] -= OSAtomicAdd64: 64 2017-07-11 10:26:18.168 TestObjc[744:303] TestCppAtomicLibrary 2017-07-11 10:26:18.168 TestObjc[744:303] foo is 1 2017-07-11 10:26:18.169 TestObjc[744:303] ++ OSAtomicIncrement64: 2 2017-07-11 10:26:18.169 TestObjc[744:303] -- OSAtomicDecrement64: 1 2017-07-11 10:26:18.170 TestObjc[744:303] = OSAtomicCompareAndSwap64: 64 2017-07-11 10:26:18.170 TestObjc[744:303] += OSAtomicAdd64: 100 2017-07-11 10:26:18.171 TestObjc[744:303] -= OSAtomicAdd64: 64 2017-07-11 10:26:18.171 TestObjc[744:303] Begin CAtomic 2017-07-11 10:26:18.259 TestObjc[744:303] End CAtomic 2017-07-11 10:26:18.260 TestObjc[744:303] Begin CppAtomic 2017-07-11 10:26:18.360 TestObjc[744:303] End CAtomic

参考

atomic Windows 原子操作