寫在前面：由于內容較多，所以斷續沒有寫完的內容。

11、刪除日志文件(Deleting The Rollback Journal)
一旦更改寫入設備，日志文件將會被刪除，這是事務真正提交的時刻。如果在這之前系統發生崩潰，就會進行恢復處理，使得數據庫和沒發生改變一樣；如果在這之后系統發生崩潰，表明所有的更改都已經寫入磁盤。SQLite就是根據日志存在情況決定是否對數據庫進行恢復處理。

刪除文件本質上不是一個原子操作，但是從用戶進程的角度來看是一個原子操作，所以一個事務看起來是一個原子操作。
在許多系統中，刪除文件也是一個高代價的操作。作為優化，SQLite可以配置成把日志文件的長度截為0或者把日志文件頭清零。

?

12、釋放鎖(Releasing The Lock)
作為原子提交的最后一步，釋放排斥鎖使得其它進程可以開始訪問數據庫了。
下圖中，我們指明了當鎖被釋放的時候用戶空間所擁有的信息已經被清空了.對于老版本的SQLite你可這么認為。但最新的SQLite會保存些用戶空間的緩存不會被清空—萬一下一個事務開始的時候，這些數據剛好可以用上呢。重新利用這些內存要比再次從操作系統磁盤緩存或者硬盤中讀取要來得輕松與快捷得多，何樂而不為呢？在再次使用這些數據之前，我們必須先取得一個共享鎖，同時我們還不得不去檢查一下，保證還沒有其他進程在我們擁有共享鎖之前對數據庫文件進行了修改。數據庫文件的第一頁中有一個計數器，數據庫文件每做一次修改，這個計數器就會增長一下。我們可以通過檢查這個計數器就可得知是否有其他進程修改過數據庫文件。如果數據庫文件已經被修改過了，那么用戶內存空間的緩存就不得不清空，并重新讀入。大多數情況下，這種情況不大會發生，因此用戶空間的內存緩存將是有效的，這對于性能提高來說作用是顯著的。

?

以上兩步是在sqlite3BtreeCommit()---btree.c函數中實現的。

代碼如下：

?

Code
//提交事務,至此一個事務完成.主要做兩件事:
//刪除日志文件,釋放數據庫文件的寫鎖
int?sqlite3BtreeCommit(Btree?*p){
??BtShared?*pBt?=?p->pBt;

??btreeIntegrity(p);

??/*?If?the?handle?has?a?write-transaction?open,?commit?the?shared-btrees?
??**?transaction?and?set?the?shared?state?to?TRANS_READ.
??*/
??if(?p->inTrans==TRANS_WRITE?){
????int?rc;
????assert(?pBt->inTransaction==TRANS_WRITE?);
????assert(?pBt->nTransaction>0?);

???//調用pager，提交事務
????rc?=?sqlite3pager_commit(pBt->pPager);
????if(?rc!=SQLITE_OK?){
??????return?rc;
????}
????pBt->inTransaction?=?TRANS_READ;
????pBt->inStmt?=?0;
??}
??unlockAllTables(p);

??/*?If?the?handle?has?any?kind?of?transaction?open,?decrement?the?transaction
??**?count?of?the?shared?btree.?If?the?transaction?count?reaches?0,?set
??**?the?shared?state?to?TRANS_NONE.?The?unlockBtreeIfUnused()?call?below
??**?will?unlock?the?pager.
??*/
??if(?p->inTrans!=TRANS_NONE?){
????pBt->nTransaction--;
????if(?0==pBt->nTransaction?){
??????pBt->inTransaction?=?TRANS_NONE;
????}
??}
}

//提交事務，主要調用pager_unwritelock()函數
int?sqlite3pager_commit(Pager?*pPager){
??int?rc;
??PgHdr?*pPg;

??if(?pPager->errCode?){
????return?pPager->errCode;
??}
??if(?pPager->state<PAGER_RESERVED?){
????return?SQLITE_ERROR;
??}
??TRACE2("COMMIT?%d\n",?PAGERID(pPager));
??if(?MEMDB?){
????pPg?=?pager_get_all_dirty_pages(pPager);
????while(?pPg?){
??????clearHistory(PGHDR_TO_HIST(pPg,?pPager));
??????pPg->dirty?=?0;
??????pPg->inJournal?=?0;
??????pPg->inStmt?=?0;
??????pPg->needSync?=?0;
??????pPg->pPrevStmt?=?pPg->pNextStmt?=?0;
??????pPg?=?pPg->pDirty;
????}
????pPager->pDirty?=?0;
#ifndef?NDEBUG
????for(pPg=pPager->pAll;?pPg;?pPg=pPg->pNextAll){
??????PgHistory?*pHist?=?PGHDR_TO_HIST(pPg,?pPager);
??????assert(?!pPg->alwaysRollback?);
??????assert(?!pHist->pOrig?);
??????assert(?!pHist->pStmt?);
????}
#endif
????pPager->pStmt?=?0;
????pPager->state?=?PAGER_SHARED;
????return?SQLITE_OK;
??}
??if(?pPager->dirtyCache==0?){
????/*?Exit?early?(without?doing?the?time-consuming?sqlite3OsSync()?calls)
????**?if?there?have?been?no?changes?to?the?database?file.?*/
????assert(?pPager->needSync==0?);
????rc?=?pager_unwritelock(pPager);
????pPager->dbSize?=?-1;
????return?rc;
??}
??assert(?pPager->journalOpen?);
??rc?=?sqlite3pager_sync(pPager,?0,?0);
??
??//刪除文件,釋放寫鎖
??if(?rc==SQLITE_OK?){
????rc?=?pager_unwritelock(pPager);
????pPager->dbSize?=?-1;
??}
??return?rc;
}

//對數據庫加read?lock，刪除日志文件
static?int?pager_unwritelock(Pager?*pPager){
??PgHdr?*pPg;
??int?rc;
??assert(?!MEMDB?);
??if(?pPager->state<PAGER_RESERVED?){
????return?SQLITE_OK;
??}
??sqlite3pager_stmt_commit(pPager);
??if(?pPager->stmtOpen?){
????sqlite3OsClose(&pPager->stfd);
????pPager->stmtOpen?=?0;
??}
??if(?pPager->journalOpen?){

???//關閉日志文件
????sqlite3OsClose(&pPager->jfd);
????pPager->journalOpen?=?0;
????//刪除日志文件
????sqlite3OsDelete(pPager->zJournal);
????sqliteFree(?pPager->aInJournal?);
????pPager->aInJournal?=?0;
????for(pPg=pPager->pAll;?pPg;?pPg=pPg->pNextAll){
??????pPg->inJournal?=?0;
??????pPg->dirty?=?0;
??????pPg->needSync?=?0;
#ifdef?SQLITE_CHECK_PAGES
??????pPg->pageHash?=?pager_pagehash(pPg);
#endif
????}
????pPager->pDirty?=?0;
????pPager->dirtyCache?=?0;
????pPager->nRec?=?0;
??}else{
????assert(?pPager->aInJournal==0?);
????assert(?pPager->dirtyCache==0?||?pPager->useJournal==0?);
??}

??//釋放寫鎖，加讀鎖
??rc?=?sqlite3OsUnlock(pPager->fd,?SHARED_LOCK);
??pPager->state?=?PAGER_SHARED;
??pPager->origDbSize?=?0;
??pPager->setMaster?=?0;
??pPager->needSync?=?0;
??pPager->pFirstSynced?=?pPager->pFirst;
??return?rc;
}

下圖可進一步描述該過程：

?

最后來看看sqlite3BtreeSync()和sqlite3BtreeCommit()是如何被調用的。

一般來說，事務提交方式為自動提交的話，在虛擬機中的OP_Halt指令實現提交事務，相關代碼如下：

?

Code
//虛擬機停機指令
case?OP_Halt:?{????????????/*?no-push?*/
??p->pTos?=?pTos;
??p->rc?=?pOp->p1;
??p->pc?=?pc;
??p->errorAction?=?pOp->p2;
??if(?pOp->p3?){
????sqlite3SetString(&p->zErrMsg,?pOp->p3,?(char*)0);
??}
??//設置虛擬機狀態SQLITE_MAGIC_RUN?為?SQLITE_MAGIC_HALT,
??//并提交事務
??rc?=?sqlite3VdbeHalt(p);
??assert(?rc==SQLITE_BUSY?||?rc==SQLITE_OK?);
??if(?rc==SQLITE_BUSY?){
????p->rc?=?SQLITE_BUSY;
????return?SQLITE_BUSY;
??}
??return?p->rc???SQLITE_ERROR?:?SQLITE_DONE;
}

//當虛擬機要停機時,調用該函數,如果VDBE改變了數據庫且為自動
//提交模式，則提交這些改變
int?sqlite3VdbeHalt(Vdbe?*p){
??sqlite3?*db?=?p->db;
??int?i;
??int?(*xFunc)(Btree?*pBt)?=?0;??/*?Function?to?call?on?each?btree?backend?*/
??int?isSpecialError;????????????/*?Set?to?true?if?SQLITE_NOMEM?or?IOERR?*/

??/*?This?function?contains?the?logic?that?determines?if?a?statement?or
??**?transaction?will?be?committed?or?rolled?back?as?a?result?of?the
??**?execution?of?this?virtual?machine.?
??**
??**?Special?errors:
??**
??**?????If?an?SQLITE_NOMEM?error?has?occured?in?a?statement?that?writes?to
??**?????the?database,?then?either?a?statement?or?transaction?must?be?rolled
??**?????back?to?ensure?the?tree-structures?are?in?a?consistent?state.?A
??**?????statement?transaction?is?rolled?back?if?one?is?open,?otherwise?the
??**?????entire?transaction?must?be?rolled?back.
??**
??**?????If?an?SQLITE_IOERR?error?has?occured?in?a?statement?that?writes?to
??**?????the?database,?then?the?entire?transaction?must?be?rolled?back.?The
??**?????I/O?error?may?have?caused?garbage?to?be?written?to?the?journal?
??**?????file.?Were?the?transaction?to?continue?and?eventually?be?rolled?
??**?????back?that?garbage?might?end?up?in?the?database?file.
??**?????
??**?????In?both?of?the?above?cases,?the?Vdbe.errorAction?variable?is?
??**?????ignored.?If?the?sqlite3.autoCommit?flag?is?false?and?a?transaction
??**?????is?rolled?back,?it?will?be?set?to?true.
??**
??**?Other?errors:
??**
??**?No?error:
??**
??*/

??if(?sqlite3MallocFailed()?){
????p->rc?=?SQLITE_NOMEM;
??}
??if(?p->magic!=VDBE_MAGIC_RUN?){
????/*?Already?halted.??Nothing?to?do.?*/
????assert(?p->magic==VDBE_MAGIC_HALT?);
????return?SQLITE_OK;
??}
??//釋放虛擬機中所有的游標
??closeAllCursors(p);
??checkActiveVdbeCnt(db);

??/*?No?commit?or?rollback?needed?if?the?program?never?started?*/
??if(?p->pc>=0?){

????/*?Check?for?one?of?the?special?errors?-?SQLITE_NOMEM?or?SQLITE_IOERR?*/
????isSpecialError?=?((p->rc==SQLITE_NOMEM?||?p->rc==SQLITE_IOERR)?1:0);
????if(?isSpecialError?){
??????/*?This?loop?does?static?analysis?of?the?query?to?see?which?of?the
??????**?following?three?categories?it?falls?into:
??????**
??????**?????Read-only
??????**?????Query?with?statement?journal
??????**?????Query?without?statement?journal
??????**
??????**?We?could?do?something?more?elegant?than?this?static?analysis?(i.e.
??????**?store?the?type?of?query?as?part?of?the?compliation?phase),?but?
??????**?handling?malloc()?or?IO?failure?is?a?fairly?obscure?edge?case?so?
??????**?this?is?probably?easier.?Todo:?Might?be?an?opportunity?to?reduce?
??????**?code?size?a?very?small?amount?though
??????*/
??????int?isReadOnly?=?1;
??????int?isStatement?=?0;
??????assert(p->aOp?||?p->nOp==0);
??????for(i=0;?i<p->nOp;?i++){?
????????switch(?p->aOp[i].opcode?){
??????????case?OP_Transaction:
????????????isReadOnly?=?0;
????????????break;
??????????case?OP_Statement:
????????????isStatement?=?1;
????????????break;
????????}
??????}
??
??????/*?If?the?query?was?read-only,?we?need?do?no?rollback?at?all.?Otherwise,
??????**?proceed?with?the?special?handling.
??????*/
??????if(?!isReadOnly?){
????????if(?p->rc==SQLITE_NOMEM?&&?isStatement?){
??????????xFunc?=?sqlite3BtreeRollbackStmt;
????????}else{
??????????/*?We?are?forced?to?roll?back?the?active?transaction.?Before?doing
??????????**?so,?abort?any?other?statements?this?handle?currently?has?active.
??????????*/
??????????sqlite3AbortOtherActiveVdbes(db,?p);
??????????sqlite3RollbackAll(db);
??????????db->autoCommit?=?1;
????????}
??????}
????}
??
????/*?If?the?auto-commit?flag?is?set?and?this?is?the?only?active?vdbe,?then
????**?we?do?either?a?commit?or?rollback?of?the?current?transaction.?
????**
????**?Note:?This?block?also?runs?if?one?of?the?special?errors?handled?
????**?above?has?occured.?
????*/
????//如果自動提交事務，則提交事務
????if(?db->autoCommit?&&?db->activeVdbeCnt==1?){
??????if(?p->rc==SQLITE_OK?||?(p->errorAction==OE_Fail?&&?!isSpecialError)?){
????/*?The?auto-commit?flag?is?true,?and?the?vdbe?program?was?
????????**?successful?or?hit?an?'OR?FAIL'?constraint.?This?means?a?commit?
????????**?is?required.
????????*/
????????//提交事務
????????int?rc?=?vdbeCommit(db);
????????if(?rc==SQLITE_BUSY?){
??????????return?SQLITE_BUSY;
????????}else?if(?rc!=SQLITE_OK?){
??????????p->rc?=?rc;
??????????sqlite3RollbackAll(db);
????????}else{
??????????sqlite3CommitInternalChanges(db);
????????}
??????}else{
????????sqlite3RollbackAll(db);
??????}
????}else?if(?!xFunc?){
??????if(?p->rc==SQLITE_OK?||?p->errorAction==OE_Fail?){
????????xFunc?=?sqlite3BtreeCommitStmt;
??????}else?if(?p->errorAction==OE_Abort?){
????????xFunc?=?sqlite3BtreeRollbackStmt;
??????}else{
????????sqlite3AbortOtherActiveVdbes(db,?p);
????????sqlite3RollbackAll(db);
????????db->autoCommit?=?1;
??????}
????}
??
????/*?If?xFunc?is?not?NULL,?then?it?is?one?of?sqlite3BtreeRollbackStmt?or
????**?sqlite3BtreeCommitStmt.?Call?it?once?on?each?backend.?If?an?error?occurs
????**?and?the?return?code?is?still?SQLITE_OK,?set?the?return?code?to?the?new
????**?error?value.
????*/
????assert(!xFunc?||
??????xFunc==sqlite3BtreeCommitStmt?||
??????xFunc==sqlite3BtreeRollbackStmt
????);
????for(i=0;?xFunc?&&?i<db->nDb;?i++){?
??????int?rc;
??????Btree?*pBt?=?db->aDb[i].pBt;
??????if(?pBt?){
????????rc?=?xFunc(pBt);
????????if(?rc?&&?(p->rc==SQLITE_OK?||?p->rc==SQLITE_CONSTRAINT)?){
??????????p->rc?=?rc;
??????????sqlite3SetString(&p->zErrMsg,?0);
????????}
??????}
????}
??
????/*?If?this?was?an?INSERT,?UPDATE?or?DELETE?and?the?statement?was?committed,?
????**?set?the?change?counter.?
????*/
????if(?p->changeCntOn?&&?p->pc>=0?){
??????if(?!xFunc?||?xFunc==sqlite3BtreeCommitStmt?){
????????sqlite3VdbeSetChanges(db,?p->nChange);
??????}else{
????????sqlite3VdbeSetChanges(db,?0);
??????}
??????p->nChange?=?0;
????}
??
????/*?Rollback?or?commit?any?schema?changes?that?occurred.?*/
????if(?p->rc!=SQLITE_OK?&&?db->flags&SQLITE_InternChanges?){
??????sqlite3ResetInternalSchema(db,?0);
??????db->flags?=?(db->flags?|?SQLITE_InternChanges);
????}
??}

??/*?We?have?successfully?halted?and?closed?the?VM.??Record?this?fact.?*/
??if(?p->pc>=0?){
????db->activeVdbeCnt--;
??}
??p->magic?=?VDBE_MAGIC_HALT;
??checkActiveVdbeCnt(db);

??return?SQLITE_OK;
}

//提交事務,主要調用:
//sqlite3BtreeSync()---同步btree,?sqlite3BtreeCommit()---提交事務
static?int?vdbeCommit(sqlite3?*db){
??int?i;
??int?nTrans?=?0;??/*?Number?of?databases?with?an?active?write-transaction?*/
??int?rc?=?SQLITE_OK;
??int?needXcommit?=?0;

??for(i=0;?i<db->nDb;?i++){?
????Btree?*pBt?=?db->aDb[i].pBt;
????if(?pBt?&&?sqlite3BtreeIsInTrans(pBt)?){
??????needXcommit?=?1;
??????if(?i!=1?)?nTrans++;
????}
??}

??/*?If?there?are?any?write-transactions?at?all,?invoke?the?commit?hook?*/
??if(?needXcommit?&&?db->xCommitCallback?){
????sqlite3SafetyOff(db);
????rc?=?db->xCommitCallback(db->pCommitArg);
????sqlite3SafetyOn(db);
????if(?rc?){
??????return?SQLITE_CONSTRAINT;
????}
??}

??/*?The?simple?case?-?no?more?than?one?database?file?(not?counting?the
??**?TEMP?database)?has?a?transaction?active.???There?is?no?need?for?the
??**?master-journal.
??**
??**?If?the?return?value?of?sqlite3BtreeGetFilename()?is?a?zero?length
??**?string,?it?means?the?main?database?is?:memory:.??In?that?case?we?do
??**?not?support?atomic?multi-file?commits,?so?use?the?simple?case?then
??**?too.
??*/
??//簡單的情況,只有一個數據庫文件,不需要master-journal
??if(?0==strlen(sqlite3BtreeGetFilename(db->aDb[0].pBt))?||?nTrans<=1?){
????for(i=0;?rc==SQLITE_OK?&&?i<db->nDb;?i++){?
??????Btree?*pBt?=?db->aDb[i].pBt;
??????if(?pBt?){
??????????//同步btree
????????rc?=?sqlite3BtreeSync(pBt,?0);
??????}
????}

????/*?Do?the?commit?only?if?all?databases?successfully?synced?*/
????//commite事務
????if(?rc==SQLITE_OK?){
??????for(i=0;?i<db->nDb;?i++){
????????Btree?*pBt?=?db->aDb[i].pBt;
????????if(?pBt?){
??????????sqlite3BtreeCommit(pBt);
????????}
??????}
????}
??}

??/*?The?complex?case?-?There?is?a?multi-file?write-transaction?active.
??**?This?requires?a?master?journal?file?to?ensure?the?transaction?is
??**?committed?atomicly.
??*/
#ifndef?SQLITE_OMIT_DISKIO
??else{
????int?needSync?=?0;
????char?*zMaster?=?0;???/*?File-name?for?the?master?journal?*/
????char?const?*zMainFile?=?sqlite3BtreeGetFilename(db->aDb[0].pBt);
????OsFile?*master?=?0;

????/*?Select?a?master?journal?file?name?*/
????do?{
??????u32?random;
??????sqliteFree(zMaster);
??????sqlite3Randomness(sizeof(random),?&random);
??????zMaster?=?sqlite3MPrintf("%s-mj%08X",?zMainFile,?random&0x7fffffff);
??????if(?!zMaster?){
????????return?SQLITE_NOMEM;
??????}
????}while(?sqlite3OsFileExists(zMaster)?);

????/*?Open?the?master?journal.?*/
????rc?=?sqlite3OsOpenExclusive(zMaster,?&master,?0);
????if(?rc!=SQLITE_OK?){
??????sqliteFree(zMaster);
??????return?rc;
????}
?
????/*?Write?the?name?of?each?database?file?in?the?transaction?into?the?new
????**?master?journal?file.?If?an?error?occurs?at?this?point?close
????**?and?delete?the?master?journal?file.?All?the?individual?journal?files
????**?still?have?'null'?as?the?master?journal?pointer,?so?they?will?roll
????**?back?independently?if?a?failure?occurs.
????*/
????for(i=0;?i<db->nDb;?i++){?
??????Btree?*pBt?=?db->aDb[i].pBt;
??????if(?i==1?)?continue;???/*?Ignore?the?TEMP?database?*/
??????if(?pBt?&&?sqlite3BtreeIsInTrans(pBt)?){
????????char?const?*zFile?=?sqlite3BtreeGetJournalname(pBt);
????????if(?zFile[0]==0?)?continue;??/*?Ignore?:memory:?databases?*/
????????if(?!needSync?&&?!sqlite3BtreeSyncDisabled(pBt)?){
??????????needSync?=?1;
????????}
????????rc?=?sqlite3OsWrite(master,?zFile,?strlen(zFile)+1);
????????if(?rc!=SQLITE_OK?){
??????????sqlite3OsClose(&master);
??????????sqlite3OsDelete(zMaster);
??????????sqliteFree(zMaster);
??????????return?rc;
????????}
??????}
????}


????/*?Sync?the?master?journal?file.?Before?doing?this,?open?the?directory
????**?the?master?journal?file?is?store?in?so?that?it?gets?synced?too.
????*/
????zMainFile?=?sqlite3BtreeGetDirname(db->aDb[0].pBt);
????rc?=?sqlite3OsOpenDirectory(master,?zMainFile);
????if(?rc!=SQLITE_OK?||
??????????(needSync?&&?(rc=sqlite3OsSync(master,0))!=SQLITE_OK)?){
??????sqlite3OsClose(&master);
??????sqlite3OsDelete(zMaster);
??????sqliteFree(zMaster);
??????return?rc;
????}

????/*?Sync?all?the?db?files?involved?in?the?transaction.?The?same?call
????**?sets?the?master?journal?pointer?in?each?individual?journal.?If
????**?an?error?occurs?here,?do?not?delete?the?master?journal?file.
????**
????**?If?the?error?occurs?during?the?first?call?to?sqlite3BtreeSync(),
????**?then?there?is?a?chance?that?the?master?journal?file?will?be
????**?orphaned.?But?we?cannot?delete?it,?in?case?the?master?journal
????**?file?name?was?written?into?the?journal?file?before?the?failure
????**?occured.
????*/
????for(i=0;?i<db->nDb;?i++){?
??????Btree?*pBt?=?db->aDb[i].pBt;
??????if(?pBt?&&?sqlite3BtreeIsInTrans(pBt)?){
????????rc?=?sqlite3BtreeSync(pBt,?zMaster);
????????if(?rc!=SQLITE_OK?){
??????????sqlite3OsClose(&master);
??????????sqliteFree(zMaster);
??????????return?rc;
????????}
??????}
????}
????sqlite3OsClose(&master);

????/*?Delete?the?master?journal?file.?This?commits?the?transaction.?After
????**?doing?this?the?directory?is?synced?again?before?any?individual
????**?transaction?files?are?deleted.
????*/
????rc?=?sqlite3OsDelete(zMaster);
????assert(?rc==SQLITE_OK?);
????sqliteFree(zMaster);
????zMaster?=?0;
????rc?=?sqlite3OsSyncDirectory(zMainFile);
????if(?rc!=SQLITE_OK?){
??????/*?This?is?not?good.?The?master?journal?file?has?been?deleted,?but
??????**?the?directory?sync?failed.?There?is?no?completely?safe?course?of
??????**?action?from?here.?The?individual?journals?contain?the?name?of?the
??????**?master?journal?file,?but?there?is?no?way?of?knowing?if?that
??????**?master?journal?exists?now?or?if?it?will?exist?after?the?operating
??????**?system?crash?that?may?follow?the?fsync()?failure.
??????*/
??????return?rc;
????}

????/*?All?files?and?directories?have?already?been?synced,?so?the?following
????**?calls?to?sqlite3BtreeCommit()?are?only?closing?files?and?deleting
????**?journals.?If?something?goes?wrong?while?this?is?happening?we?don't
????**?really?care.?The?integrity?of?the?transaction?is?already?guaranteed,
????**?but?some?stray?'cold'?journals?may?be?lying?around.?Returning?an
????**?error?code?won't?help?matters.
????*/
????for(i=0;?i<db->nDb;?i++){?
??????Btree?*pBt?=?db->aDb[i].pBt;
??????if(?pBt?){
????????sqlite3BtreeCommit(pBt);
??????}
????}
??}
#endif

??return?rc;
} 轉自：http://www.cnblogs.com/hustcat/

轉載于:https://www.cnblogs.com/meiyou/archive/2009/09/03/1559864.html

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