sqlite 句柄－sqlite 基礎教程(3) · sqlite基礎教程

<table border="2" width="600" height="80" cellspacing="1" cellpadding="1" align="left"><tbody><tr><td><div style="text-align:center"><strong>聲明</strong></div><span style="color:#ff0000">歡迎轉載，但是請尊重作者勞動成果，轉載請保留此框內聲明，謝謝。<br/>文章出處：<a href="http://blog.csdn.net/iukey" target="_blank">http://blog.csdn.net/iukey</a></span></td></tr></tbody></table> 要操縱一個數據庫你就得有一個這個數據庫的句柄(又碰到這個難以理解的詞了，不過確實還沒得一個更好的詞來替代它)。其實你跟本不需要去在乎這個詞叫什么，你只要搞清楚他是一個什么玩意兒。就如同鞋子為什么叫鞋子，仔細想想確實也難以理解，不過清楚他的功能就OK了，不是嗎？句柄在很多地方我們見到過，最常見的就是文件的句柄，我們要操縱一個文件，我們就要取得一個文件的句柄。句柄是個什么東東呢？其實很簡單，句柄是一個東東的描述，他被定義為一個結構體，這個結構體可能會包含要描述的東東的具體信息，比如位置、大小、類型等等。我們有了這個描述信息我們就能去找到這個東東，然后操縱它。一句話：句柄是物體的描述結構體。我們來看看sqlite的句柄是怎么定義的(不要被嚇到了，代碼直接跳過就好)： ~~~ struct sqlite3 { sqlite3_vfs *pVfs; /* OS Interface */ int nDb; /* Number of backends currently in use */ Db *aDb; /* All backends */ int flags; /* Miscellaneous flags. See below */ unsigned int openFlags; /* Flags passed to sqlite3_vfs.xOpen() */ int errCode; /* Most recent error code (SQLITE_*) */ int errMask; /* & result codes with this before returning */ u8 autoCommit; /* The auto-commit flag. */ u8 temp_store; /* 1: file 2: memory 0: default */ u8 mallocFailed; /* True if we have seen a malloc failure */ u8 dfltLockMode; /* Default locking-mode for attached dbs */ signed char nextAutovac; /* Autovac setting after VACUUM if >=0 */ u8 suppressErr; /* Do not issue error messages if true */ u8 vtabOnConflict; /* Value to return for s3_vtab_on_conflict() */ int nextPagesize; /* Pagesize after VACUUM if >0 */ int nTable; /* Number of tables in the database */ CollSeq *pDfltColl; /* The default collating sequence (BINARY) */ i64 lastRowid; /* ROWID of most recent insert (see above) */ u32 magic; /* Magic number for detect library misuse */ int nChange; /* Value returned by sqlite3_changes() */ int nTotalChange; /* Value returned by sqlite3_total_changes() */ sqlite3_mutex *mutex; /* Connection mutex */ int aLimit[SQLITE_N_LIMIT]; /* Limits */ struct sqlite3InitInfo { /* Information used during initialization */ int iDb; /* When back is being initialized */ int newTnum; /* Rootpage of table being initialized */ u8 busy; /* TRUE if currently initializing */ u8 orphanTrigger; /* Last statement is orphaned TEMP trigger */ } init; int nExtension; /* Number of loaded extensions */ void **aExtension; /* Array of shared library handles */ struct Vdbe *pVdbe; /* List of active virtual machines */ int activeVdbeCnt; /* Number of VDBEs currently executing */ int writeVdbeCnt; /* Number of active VDBEs that are writing */ int vdbeExecCnt; /* Number of nested calls to VdbeExec() */ void (*xTrace)(void*,const char*); /* Trace function */ void *pTraceArg; /* Argument to the trace function */ void (*xProfile)(void*,const char*,u64); /* Profiling function */ void *pProfileArg; /* Argument to profile function */ void *pCommitArg; /* Argument to xCommitCallback() */ int (*xCommitCallback)(void*); /* Invoked at every commit. */ void *pRollbackArg; /* Argument to xRollbackCallback() */ void (*xRollbackCallback)(void*); /* Invoked at every commit. */ void *pUpdateArg; void (*xUpdateCallback)(void*,int, const char*,const char*,sqlite_int64); #ifndef SQLITE_OMIT_WAL int (*xWalCallback)(void *, sqlite3 *, const char *, int); void *pWalArg; #endif void(*xCollNeeded)(void*,sqlite3*,int eTextRep,const char*); void(*xCollNeeded16)(void*,sqlite3*,int eTextRep,const void*); void *pCollNeededArg; sqlite3_value *pErr; /* Most recent error message */ char *zErrMsg; /* Most recent error message (UTF-8 encoded) */ char *zErrMsg16; /* Most recent error message (UTF-16 encoded) */ union { volatile int isInterrupted; /* True if sqlite3_interrupt has been called */ double notUsed1; /* Spacer */ } u1; Lookaside lookaside; /* Lookaside malloc configuration */ #ifndef SQLITE_OMIT_AUTHORIZATION int (*xAuth)(void*,int,const char*,const char*,const char*,const char*); /* Access authorization function */ void *pAuthArg; /* 1st argument to the access auth function */ #endif #ifndef SQLITE_OMIT_PROGRESS_CALLBACK int (*xProgress)(void *); /* The progress callback */ void *pProgressArg; /* Argument to the progress callback */ int nProgressOps; /* Number of opcodes for progress callback */ #endif #ifndef SQLITE_OMIT_VIRTUALTABLE Hash aModule; /* populated by sqlite3_create_module() */ VtabCtx *pVtabCtx; /* Context for active vtab connect/create */ VTable **aVTrans; /* Virtual tables with open transactions */ int nVTrans; /* Allocated size of aVTrans */ VTable *pDisconnect; /* Disconnect these in next sqlite3_prepare() */ #endif FuncDefHash aFunc; /* Hash table of connection functions */ Hash aCollSeq; /* All collating sequences */ BusyHandler busyHandler; /* Busy callback */ int busyTimeout; /* Busy handler timeout, in msec */ Db aDbStatic[2]; /* Static space for the 2 default backends */ Savepoint *pSavepoint; /* List of active savepoints */ int nSavepoint; /* Number of non-transaction savepoints */ int nStatement; /* Number of nested statement-transactions */ u8 isTransactionSavepoint; /* True if the outermost savepoint is a TS */ i64 nDeferredCons; /* Net deferred constraints this transaction. */ int *pnBytesFreed; /* If not NULL, increment this in DbFree() */ #ifdef SQLITE_ENABLE_UNLOCK_NOTIFY /* The following variables are all protected by the STATIC_MASTER **mutex, not by sqlite3.mutex. They are used by code in notify.c. ** **When X.pUnlockConnection==Y, that means that X is waiting for Y to **unlock so that it can proceed. ** **When X.pBlockingConnection==Y, that means that something that X tried **tried to do recently failed with an SQLITE_LOCKED error due to locks **held by Y. */ sqlite3 *pBlockingConnection; /* Connection that caused SQLITE_LOCKED */ sqlite3 *pUnlockConnection; /* Connection to watch for unlock */ void *pUnlockArg; /* Argument to xUnlockNotify */ void (*xUnlockNotify)(void **, int); /* Unlock notify callback */ sqlite3 *pNextBlocked; /* Next in list of all blocked connections */ #endif }; ~~~ ~~~ typedef struct sqlite3 sqlite3;// ~~~ 是不是被嚇到了，沒關系，這段代碼本來就是我貼出來嚇唬你的，我也沒認真研究過這個代碼，也不想去研究，除非哪天有人出高價請我去研究，我現在只知道怎么用就好了。這個 sqlite3 結構體就是被用來描述我們磁盤里的數據庫文件的，有了這個描述符我們就可以對這個數據庫進行各種操作了，操作的具體內情我們不必要了解，我們只需要知道怎么去調用API就好了，當然有時候還是要了解一點點內情的，這個以后碰到再講。我用這么長的話加一大段嚇人的代碼只有一個目的：不要對句柄有恐懼感。好了，開始我們的正題，sqlite 里面你要操縱數據庫我們先得創建一個句柄，然后后面所有對數據庫得操作都會用到這個句柄。 ~~~ sqlite3* pdb; ~~~ 就這么簡單，這樣一個ssqlite的句柄我們就創建完成了，我們以后針對數據庫的操作都離不開它了。你可能還有疑問，我也知道你的疑問是什么，請看下回分解。