C++多线程（三）

多线程同步之Critical Sections（功能与Mutex相同，保证某一时刻只有一个线程能够访问共享资源，但是不是内核对象，所以访问速度要比Mutex快，但是增没有等待超时的功能，所以有可能会导致死锁，使用时可以根据实际的情况选择其一）

一 Critical Sections

1） 因为Critical Sections不是内核对象，所以只能用来统一进程内线程间的同步，不能用来多个不同进程间的线程的同步。

2） 如果在Critical Sections中间突然程序crash或是exit而没有调用LeaveCriticalSection，则结果是改线程所对应的内核不能被释放，该线程成为死线程。

3） 要比其他的内核对象的速度要快。

二 使用CriticalSections的简单实例，Stack在push的时候可以分为3个步骤，看下面的代码，但是如果在第2步后此线程中断切换到其他的线程，其他的线程push后再返回执行时，此线程继续执行，这样有可能刚才其他线程push就会被覆盖了，在stack里找不到了。（下面的代码在debug下使用了CriticalSection，release下可能有问题）

#include <windows.h>
#include <process.h>
#include <stdio.h>
/**//////////////////////////////////////////////
//stack:
struct Node
{
struct Node *next;
int data;
};
struct Stack
{
struct Node *head;
#ifdef _DEBUG
CRITICAL_SECTION critical_sec;
#endif

Stack()
{
head = NULL;
#ifdef _DEBUG
InitializeCriticalSection(&critical_sec);
#endif
}
~Stack()
{
if(head != NULL) {

if(NULL == head->next) {
delete head;
head = NULL;
} else {
Node *p = head;
Node *q = head->next;

while(q != NULL){
delete p;
p = q;
q = q->next;
};
delete p;
p = NULL;
}
}
#ifdef _DEBUG
DeleteCriticalSection(&critical_sec);
#endif
}
void Push (int num)
{
//enter critical section, add a new node and then
#ifdef _DEBUG
EnterCriticalSection (&critical_sec);
#endif
Node * node = new Node();
node->next = head;
node->data = num;
head = node;
printf("Stack:%d/n",num);
//leave critical section
#ifdef _DEBUG
LeaveCriticalSection (&critical_sec);
#endif
}
int Pop ()
{
#ifdef _DEBUG
EnterCriticalSection (&critical_sec);
#endif
int result = 0;
if(head!= NULL)
{
result = head->data;
if(head->next != NULL)
{
Node *temp = head->next;
delete head;
head = temp;
}
else
head = NULL;
}
#ifdef _DEBUG
LeaveCriticalSection (&critical_sec);
#endif
return result;
}
};

/**///////////////////////////////////////////////////////
//test:
unsigned  __stdcall Thread1(void * pVoid)
{
Stack *stack = ((Stack*)pVoid);
for(int i = 200; i < 220; ++i)
stack->Push(i);
return 1;
}
unsigned __stdcall Thread2(void *pVoid)
{
Stack *stack = ((Stack*)pVoid);
for(int i = 0; i<20; ++i)
stack->Push(i);
return 1;
}
int main()
{
Stack stack;
stack.Push(1000);
stack.Push(1000);

HANDLE   hth1;
unsigned  uiThread1ID;
hth1 = (HANDLE)_beginthreadex( NULL, // security
0, // stack size
Thread1,
(void*)&stack, // arg list
CREATE_SUSPENDED, // so we can later call ResumeThread()
&uiThread1ID );

if ( hth1 == 0 )
printf("Failed to create thread 1/n");

DWORD   dwExitCode;
GetExitCodeThread( hth1, &dwExitCode );  // should be STILL_ACTIVE = 0x00000103 = 259
printf( "initial thread 1 exit code = %u/n", dwExitCode );

HANDLE   hth2;
unsigned  uiThread2ID;
hth2 = (HANDLE)_beginthreadex( NULL, // security
0, // stack size
Thread2,
(void*)&stack, // arg list
CREATE_SUSPENDED, // so we can later call ResumeThread()
&uiThread2ID );

if ( hth2 == 0 )
printf("Failed to create thread 2/n");

GetExitCodeThread( hth2, &dwExitCode );  // should be STILL_ACTIVE = 0x00000103 = 259
printf( "initial thread 2 exit code = %u/n", dwExitCode );

ResumeThread( hth1 );
ResumeThread( hth2 );

WaitForSingleObject( hth1, INFINITE );
WaitForSingleObject( hth2, INFINITE );

GetExitCodeThread( hth1, &dwExitCode );
printf( "thread 1 exited with code %u/n", dwExitCode );

GetExitCodeThread( hth2, &dwExitCode );
printf( "thread 2 exited with code %u/n", dwExitCode );

CloseHandle( hth1 );
CloseHandle( hth2 );

printf("Primary thread terminating./n");
return 0;
}