sqlite 句柄-sqlite 基础教程(3)

最后更新于:2022-04-01 16:09:19

<table border="2" width="600" height="80" cellspacing="1" cellpadding="1" align="left"><tbody><tr><td><div style="text-align:center"><strong>声明</strong></div><span style="color:#ff0000">欢迎转载,但是请尊重作者劳动成果,转载请保留此框内声明,谢谢。<br/>文章出处:<a href="http://blog.csdn.net/iukey" target="_blank">http://blog.csdn.net/iukey</a></span></td></tr></tbody></table> 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[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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