sqlite 句柄－sqlite 基础教程(3)
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文章出处：http://blog.csdn.net/iukey
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)：
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; /* 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_WALint (*xWalCallback)(void *, sqlite3 *, const char *, int);void *pWalArg;#endifvoid(*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_AUTHORIZATIONint (*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_CALLBACKint (*xProgress)(void *); /* The progress callback */void *pProgressArg; /* Argument to the progress callback */int nProgressOps; /* Number of opcodes for progress callback */#endif#ifndef SQLITE_OMIT_VIRTUALTABLEHash 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() */#endifFuncDefHash aFunc; /* Hash table of connection functions */Hash aCollSeq; /* All collating sequences */BusyHandler busyHandler; /* Busy callback */int busyTimeout; /* Busy handler timeout, in msec */Db aDbStatic; /* 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。
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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