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除了臨界區(qū)對(duì)象進(jìn)能夠被用于一個(gè)進(jìn)程內(nèi)的線程使用外,臨界區(qū)對(duì)象同于其他互斥體對(duì)象一樣,也可以提供同步機(jī)制.事件,互斥體和信號(hào)量對(duì)象也能夠被用于單個(gè)進(jìn)程實(shí)現(xiàn)資源互斥訪問,與此相比,為了實(shí)現(xiàn)同步互斥訪問共享資源,臨界區(qū)對(duì)象提供一種更加輕巧,快速和有效的機(jī)制.象互斥體對(duì)象一樣,臨界區(qū)對(duì)象也可以被一個(gè)使用它來阻止同步訪問共享資源的線程所用于.例如:一個(gè)進(jìn)程隨時(shí)可以使用一個(gè)臨界區(qū)對(duì)象來阻止其他線程修改一個(gè)局?jǐn)?shù)據(jù)結(jié)構(gòu).
進(jìn)程負(fù)責(zé)為一個(gè)臨界區(qū)分配內(nèi)存.典型地,就是宣布一個(gè)CRITICAL_SECTION類型的變量.線程在使用它之前,臨界區(qū)對(duì)象必須使用InitializeCriticalSection函數(shù)進(jìn)行初始化.線程使用EnterCriticalSection或TryEnterCriticalSection函數(shù)獲取臨界區(qū)所有權(quán),并通過LeaveCriticalSection函數(shù)釋放臨界區(qū)所用權(quán).如果當(dāng)前臨界區(qū)對(duì)象被其他線程所擁有,EnterCriticalSection將無限等待為了臨界區(qū)所有權(quán).相比之下,一個(gè)互斥體對(duì)象為了互斥,等待函數(shù)會(huì)有一個(gè)指定的時(shí)間段.TryEnterCriticalSection函數(shù)無阻塞正在調(diào)用的線程而進(jìn)入臨界區(qū).
一旦線程擁有臨界區(qū),它必須額外的調(diào)用EnterCriticalSection或無阻塞他的執(zhí)行的TryEnterCriticalSection函數(shù).這樣以來,當(dāng)?shù)却粋€(gè)已存在的臨界區(qū)的擁有權(quán)時(shí),就不會(huì)出現(xiàn)死鎖自己.每當(dāng)它進(jìn)入臨界區(qū)時(shí),就必須調(diào)用LeaveCriticalSection來釋放對(duì)臨界區(qū)的擁有權(quán).
當(dāng)臨界區(qū)被初始化之后,進(jìn)程內(nèi)的任一個(gè)線程都可以調(diào)用DeleteCriticalSection函數(shù)來釋放被分配了的系統(tǒng)資源.DeleteCriticalSection函數(shù)被調(diào)用之后,這個(gè)臨界區(qū)對(duì)象就不能夠再被擁有為了同步機(jī)制了.
一旦一個(gè)臨界區(qū)對(duì)象被特有化之后,通過調(diào)用EnterCriticalSection函數(shù)而關(guān)聯(lián)度其它的線程將等待為了它的擁有權(quán).沒有等到擁有權(quán)的線程釋 放去繼續(xù)運(yùn)行.
SECTICAL_SECTION結(jié)構(gòu):
typedef struct _RTL_CRITICAL_SECTION {
PRTL_CRITICAL_SECTION_DEBUG DebugInfo;
//
// The following three fields control entering and exiting the critical
// section for the resource
//
LONG LockCount;
LONG RecursionCount;
HANDLE OwningThread; // from the thread's ClientId->UniqueThread
HANDLE LockSemaphore;
ULONG_PTR SpinCount; // force size on 64-bit systems when packed
} RTL_CRITICAL_SECTION, *PRTL_CRITICAL_SECTION;
typedef RTL_CRITICAL_SECTION CRITICAL_SECTION;
typedef PRTL_CRITICAL_SECTION PCRITICAL_SECTION;
typedef PRTL_CRITICAL_SECTION LPCRITICAL_SECTION; 操作臨界區(qū)的相關(guān)函數(shù)
(1) InitializeCriticalSection
初始化一個(gè)臨界區(qū)對(duì)象,而臨界區(qū)對(duì)象(數(shù)據(jù)成員)由系統(tǒng)自動(dòng)維護(hù).
VOID InitializeCriticalSection(
LPCRITICAL_SECTION lpCriticalSection //pointer to critical section object
);
(2) DeleteCriticalSection
釋放一個(gè)沒有被占有的臨界區(qū)對(duì)象的所有資源.
VOID DeleteCriticalSection(
LPCRITICAL_SECTION lpCriticalSection //pointer to critical section object
);
(3) EnterCriticalSection
用來等待臨界區(qū)對(duì)象的所有權(quán),當(dāng)調(diào)用線程賦予所有權(quán),本函數(shù)返回,如果沒能等待到,那么導(dǎo)致線程暫停.
VOID EnterCriticalSection(
LPCRITICAL_SECTION lpCriticalSection //pointer to critical section object
);
(4) LeaveCriticalSection
釋放指定臨界區(qū)對(duì)象所有權(quán)
void LeaveCriticalSection(
LPCRITICAL_SECTION lpCriticalSection //pointer to critical section object
);
例子:
CRITICAL_SECTION g_cs; //臨界區(qū)對(duì)象
void main()
{
HANDLE thread1 = CreateThread(NULL, 0, Fun1Proc, NULL, 0, NULL);
HANDLE thread2 = CreateThread(NULL, 0, Fun2Proc, NULL, 0, NULL);
CloseHandle(thread1);
CloseHandle(thread2);
//初始化一個(gè)臨界區(qū)對(duì)象
InitializeCriticalSection(&g_cs);
Sleep(4000);
//釋放一個(gè)沒有被占有的臨界區(qū)對(duì)象的所有資源
DeleteCriticalSection(&g_cs); } DWORD WINAPI Fun1Proc(LPVOID lpParam)
{
while(TRUE)
{
EnterCriticalSection(&g_cs);
if (ticket>0)
{
cout<<"thread1 sells: "<<ticket--<<endl;
Sleep(1);
}
else
break;
LeaveCriticalSection(&g_cs);
}
return 0;
} DWORD WINAPI Fun1Proc(LPVOID lpParam)
{
while(TRUE)
{
EnterCriticalSection(&g_cs);
if (ticket>0)
{
cout<<"thread2 sells: "<<ticket--<<endl;
Sleep(1);
}
else
break;
LeaveCriticalSection(&g_cs);
}
return 0;
} //以下是我自己編寫實(shí)現(xiàn)臨界區(qū)的類:
//頭文件 CriticalSection.h
#pragma once
class CCriticalSection
{
private:
CRITICAL_SECTION m_CS;
public:
CCriticalSection()
{
::InitializeCriticalSection(&m_CS);
}
~CCriticalSection()
{
::DeleteCriticalSection(&m_CS);
}
operator LPCRITICAL_SECTION()
{
return &m_CS;
}
void Lock()
{
::EnterCriticalSection(&m_CS);
}
void UnLock()
{
::LeaveCriticalSection(&m_CS);
}
};
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