在下有理解不到位,或是有更好的建议,欢迎批评指正!
相同点:关键段和互斥量都可以用来控制线程互斥访问资源。
不同点:关键段只能用于单进程间的多线程互斥,而互斥量可以用于多进程间的多线程互斥,而且互斥量可以处理“遗弃”(即某个个进程的某个线程占用了互斥量,但是它因为某些原因非正常关闭了,互斥量也没有释放,这是系统就是检测,处理这种情况,释放互斥量,以免其他线程一直等待下去)的问题。
按这样理解的话,在单个进程内使用关键段与互斥量应该可以达到类似的结果,真的是这样子吗?
个人用比较常见的生成者消费者模型稍作变型,成了4个生产者,4个消费者,4个临界区资源:
用信号量full,empty表示临界区的资源,用关键段处理对变量的互斥访问。
#include<stdio.h> #include<process.h> #include<windows.h> volatile long g_nLoginCount; const int THREAD_NUM = 10; volatile long g_num; HANDLE g_Mutex; HANDLE g_Semaphore_full,g_Semaphore_empty; //信号量 CRITICAL_SECTION g_thread; int num=10; unsigned int __stdcall producer(void *pPM){ int i; for(i=0;i<4;i++){ Sleep(100); WaitForSingleObject(g_Semaphore_empty,INFINITE); Sleep(100); EnterCriticalSection(&g_thread); //用关键段不会有问题 // WaitForSingleObject(g_Mutex,INFINITE); //用互斥量就会出问题 g_num++; printf("生产者ID:%d 累积SUM : %d\n",GetCurrentThreadId(),g_num); // ReleaseMutex(&g_Mutex); LeaveCriticalSection(&g_thread); ReleaseSemaphore(g_Semaphore_full,1,NULL);//信号量++ Sleep(100); } return 0; } unsigned int __stdcall customer(void *pPM){ int ok=1; while(1){ //Sleep(0); Sleep(100); Sleep(0); WaitForSingleObject(g_Semaphore_full,INFINITE); printf(" 消费者ID:%d 累积SUM : %d\n",GetCurrentThreadId(),g_num); ReleaseSemaphore(g_Semaphore_empty,1,NULL);//信号量++ } return 0; } int main(){ g_Semaphore_full = CreateSemaphore(NULL,0,4,NULL);//当前0个资源,最大允许4个同时访 g_Semaphore_empty = CreateSemaphore(NULL,0,4,NULL);//当前0个资源,最大允许4个同时访 g_Mutex = CreateMutex(NULL,FALSE,NULL); InitializeCriticalSection(&g_thread); HANDLE handle[10]; ReleaseSemaphore(g_Semaphore_empty,4,NULL);//信号量++ int i; for(i=0;i<4;i++){ handle[i] = (HANDLE)_beginthreadex(NULL,0,producer,NULL,0,NULL); } for(i=0;i<4;i++){ handle[i+4] = (HANDLE)_beginthreadex(NULL,0,customer,NULL,0,NULL); } WaitForMultipleObjects(THREAD_NUM,handle,TRUE,INFINITE); getchar(); //一定要在这里设置使主线程停止,否则执行到后面的话,子线程就被关闭了 for(i=0;i<6;i++) CloseHandle(handle[i]); CloseHandle(g_Semaphore_full); CloseHandle(g_Semaphore_empty); CloseHandle(g_Mutex); return 0; }
这样是正常的。
但是用互斥量处理对g_num变量的互斥访问的时候:
#include<stdio.h> #include<process.h> #include<windows.h> volatile long g_nLoginCount; const int THREAD_NUM = 10; volatile long g_num; HANDLE g_Mutex; HANDLE g_Semaphore_full,g_Semaphore_empty; //信号量 CRITICAL_SECTION g_thread; int num=10; unsigned int __stdcall producer(void *pPM){ int i; for(i=0;i<4;i++){ Sleep(100); WaitForSingleObject(g_Semaphore_empty,INFINITE); Sleep(100); // EnterCriticalSection(&g_thread); //用关键段不会有问题 WaitForSingleObject(g_Mutex,INFINITE); //用互斥量就会出问题 g_num++; printf("生产者ID:%d 累积SUM : %d\n",GetCurrentThreadId(),g_num); ReleaseMutex(&g_Mutex); // LeaveCriticalSection(&g_thread); ReleaseSemaphore(g_Semaphore_full,1,NULL);//信号量++ Sleep(100); } return 0; } unsigned int __stdcall customer(void *pPM){ int ok=1; while(1){ //Sleep(0); Sleep(100); Sleep(0); WaitForSingleObject(g_Semaphore_full,INFINITE); printf(" 消费者ID:%d 累积SUM : %d\n",GetCurrentThreadId(),g_num); ReleaseSemaphore(g_Semaphore_empty,1,NULL);//信号量++ } return 0; } int main(){ g_Semaphore_full = CreateSemaphore(NULL,0,4,NULL);//当前0个资源,最大允许4个同时访 g_Semaphore_empty = CreateSemaphore(NULL,0,4,NULL);//当前0个资源,最大允许4个同时访 g_Mutex = CreateMutex(NULL,FALSE,NULL); InitializeCriticalSection(&g_thread); HANDLE handle[10]; ReleaseSemaphore(g_Semaphore_empty,4,NULL);//信号量++ int i; for(i=0;i<4;i++){ handle[i] = (HANDLE)_beginthreadex(NULL,0,producer,NULL,0,NULL); } for(i=0;i<4;i++){ handle[i+4] = (HANDLE)_beginthreadex(NULL,0,customer,NULL,0,NULL); } WaitForMultipleObjects(THREAD_NUM,handle,TRUE,INFINITE); getchar(); //一定要在这里设置使主线程停止,否则执行到后面的话,子线程就被关闭了 for(i=0;i<6;i++) CloseHandle(handle[i]); CloseHandle(g_Semaphore_full); CloseHandle(g_Semaphore_empty); CloseHandle(g_Mutex); return 0; }
观察生产者的ID:成了有序的情况,这就是说,它使得一个线程完全执行完了之后在执行另外一个线程,这跟关键段的作用也差的太大了!!到底是什么情况??