多线程学习之一：线程对共享全局变量的访问

   多个线程之间，可以访问共享变量，但存在风险。因为各个线程的各语句执行顺序可以是任意的，所以访问共享全局变量时需要互斥加锁，才能保证结果可靠。

  在VC中，线程操作的几个接口函数：

  线程相关：

     创建线程：

         CreateThread（...）；

     等待线程结束：

        WaitForMultipleObjects(...);

     获取当前线程Id:

        GetCurrentThreadId(void);         

 

  Mutex相关：

     创建Mutex:

        CreateMutex(...)；

     等待请utex:

        WaitForSingleObject(handle,time)；

     释放Mutex: 

        ReleaseMutex(mutex);

 

  程序1：不加互斥，全局变量结果不确定；

  程序2：加了互斥，全局变量结果确定。

 

附：

程序1：

#include <stdio.h>
#include <stdlib.h>
#include <windows.h>
#include <WINBASE.h>

#define MAX_NUM  100
int count =0;

unsigned long thread_func(void* pArg);

int main()
{
   unsigned long tid[2] = {0};
   unsigned long selfTid = 0;
   HANDLE handle[2] = {NULL};
   
   selfTid = GetCurrentThreadId();
   printf("enter main tid: %u/r/n",selfTid);

   count = 0;
   handle[0] = CreateThread(NULL,2048,(LPTHREAD_START_ROUTINE)thread_func,NULL,NULL,&tid[0]);
   handle[1] = CreateThread(NULL,2048,(LPTHREAD_START_ROUTINE)thread_func,NULL,NULL,&tid[1]);
   WaitForMultipleObjects(2,handle,TRUE,0xFFFFFFFF);
  
   if(count == 2 * MAX_NUM)
   {
      printf("OK,count is: %u/r/n",count);  
   }
   else
   {
      printf("why?count is: %u/r/n",count);  
   }

   printf("tid[0]: %u,tid[1]:%u/r/n",tid[0],tid[1]);

   printf("return from main tid: %u/r/n",selfTid);
   CloseHandle(handle[0]);
   CloseHandle(handle[1]);
   return 0;
}

unsigned long thread_func(void* pArg)
{
  int i=0;
  unsigned long tid =0;
  int iTmp =0;

  tid =GetCurrentThreadId();
  printf("enter tid: %u/r/n",tid);

  for(i=0; i<MAX_NUM; i++)
  {
      iTmp = count +1;
   //Sleep(20);
   for(int j=0;j<1000000;j++);
   count = iTmp;
  }
 
  printf("return from tid: %u/r/n",tid);
  return 0;
}

  运行结果1：

enter main tid: 3588
enter tid: 1884
enter tid: 300
return from tid: 1884
return from tid: 300
why?count is: 121
tid[0]: 1884,tid[1]:300
return from main tid: 3588
Press any key to continue

 

  运行结果2：

enter main tid: 3856
enter tid: 3048
enter tid: 3396
return from tid: 3048
return from tid: 3396
why?count is: 122
tid[0]: 3048,tid[1]:3396
return from main tid: 3856
Press any key to continue  

 

 

程序2：

#include <stdio.h>
#include <stdlib.h>
#include <windows.h>
#include <WINBASE.h>

#define MAX_NUM  100
int count =0;

unsigned long thread_func(void* pArg);
HANDLE hMutex =0;

int main()
{
   unsigned long tid[2] = {0};
   unsigned long selfTid = 0;
   HANDLE handle[2] = {NULL};
   //hMutex = OpenMutex(NULL,FALSE,"mutext02");
   if(NULL == hMutex)
   {
     hMutex = CreateMutex(NULL,FALSE,"mutext02");
   }
   
   selfTid = GetCurrentThreadId();
   printf("enter main tid: %u,mutex;%u/r/n",selfTid,hMutex);

   count = 0;
   handle[0] = CreateThread(NULL,2048,(LPTHREAD_START_ROUTINE)thread_func,NULL,NULL,&tid[0]);
   handle[1] = CreateThread(NULL,2048,(LPTHREAD_START_ROUTINE)thread_func,NULL,NULL,&tid[1]);
   WaitForMultipleObjects(2,handle,TRUE,INFINITE);
  
   if(count == 2 * MAX_NUM)
   {
      printf("OK,count is: %u/r/n",count);  
   }
   else
   {
      printf("why?count is: %u/r/n",count);  
   }

   printf("tid[0]: %u,tid[1]:%u/r/n",tid[0],tid[1]);

   printf("return from main tid: %u/r/n",selfTid);
   CloseHandle(handle[0]);
   CloseHandle(handle[1]);
   CloseHandle(hMutex);
   return 0;
}

unsigned long thread_func(void* pArg)
{
  int i=0;
  unsigned long tid =0;
  int iTmp =0;

  tid =GetCurrentThreadId();
  printf("enter tid: %u,mutex;%u/r/n",tid,hMutex);

  for(i=0; i<MAX_NUM; i++)
  {
   WaitForSingleObject(hMutex,INFINITE);
      iTmp = count +1;
   Sleep(20);
   for(int j=0;j<1000000;j++);
   count = iTmp;
   ReleaseMutex(hMutex);
  }
 
  printf("return from tid: %u/r/n",tid);
  return 0;
}

  运行结果：

enter main tid: 2704,mutex;2032
enter tid: 1580,mutex;2032
enter tid: 3392,mutex;2032
return from tid: 1580
return from tid: 3392
OK,count is: 200
tid[0]: 1580,tid[1]:3392
return from main tid: 2704
Press any key to continue