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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