要操纵一个数据库你就得有一个这个数据库的句柄(又碰到这个难以理解的词了，不过确实还没得一个更好的词来替代它)。其实你跟本不需要去在乎这个词叫什么，你只要搞清楚他是一个什么玩意儿。就如同鞋子为什么叫鞋子，仔细想想确实也难以理解，不过 清楚他的功能就OK了，不是吗？
句柄在很多地方我们见到过，最常见的就是文件的句柄，我们要操纵一个文件，我们就要取得一个文件的句柄。句柄是个什么东东呢？其实很简单，句柄是一个东东的描述，他被定义为一个结构体，这个结构体可能会包含要描述的东东的具体信息，比如位置、大小、类型等等。我们有了这个描述信息我们就能去找到这个东东，然后操纵它。
一句话：句柄是物体的描述结构体。
我们来看看sqlite的句柄是怎么定义的(不要被吓到了，代码直接跳过就好)：
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- 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
- };
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- typedef struct sqlite3 sqlite3;//
是不是被吓到了，没关系，这段代码本来就是我贴出来吓唬你的，我也没认真研究过这个代码，也不想去研究，除非哪天有人出高价请我去研究，我现在只知道怎么用就好了。
这个 sqlite3 结构体就是被用来描述我们磁盘里的数据库文件的，有了这个描述符我们就可以对这个数据库进行各种操作了，操作的具体内情我们不必要了解，我们只需要知道怎么去调用API就好了，当然有时候还是要了解一点点内情的，这个以后碰到再讲。
我用这么长的话加一大段吓人的代码只有一个目的：不要对句柄有恐惧感。
好了，开始我们的正题，sqlite 里面你要操纵数据库我们先得创建一个句柄，然后后面所有对数据库得操作都会用到这个句柄。
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- sqlite3* pdb;
就 这么简单，这样一个ssqlite的句柄我们就创建完成了，我们以后针对数据库的操作都离不开它了。你可能还有疑问，我也知道你的 疑问是什么，请看下回分解。
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