sqlite 控制代碼－sqlite 基礎教程(3)

聲明 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。 文章出處：http://blog.csdn.net/iukey

要操縱一個資料庫你就得有一個這個資料庫的控制代碼(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麼，你只要搞清楚他是一個什麼玩意兒。就如同鞋子為什麼叫鞋子，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？

控制代碼在很多地方我們見到過，最常見的就是檔案的控制代碼，我們要操縱一個檔案，我們就要取得一個檔案的控制代碼。控制代碼是個什麼東東呢？其實很簡單，控制代碼是一個東東的描述，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資訊，比如位置、大小、類型等等。我們有了這個描述資訊我們就能去找到這個東東，然後操縱它。

一句話：控制代碼是物體的描述結構體。

我們來看看sqlite的控制代碼是怎麼定義的(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，沒關係，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，我現在只知道怎麼用就好了。

這個 sqlite3 結構體就是被用來描述我們磁碟裡的資料庫檔案的，有了這個描述符我們就可以對這個資料庫進行各種操作了，操作的具體內情我們不必要瞭解，我們只需要知道怎麼去調用API就好了，當然有時候還是要瞭解一點點內情的，這個以後碰到再講。

我用這麼長的話加一大段嚇人的代碼只有一個目的：不要對控制代碼有恐懼感。

好了，開始我們的正題，sqlite 裡面你要操縱資料庫我們先得建立一個控制代碼，然後後面所有對資料庫得操作都會用到這個控制代碼。

sqlite3* pdb;

就 這麼簡單，這樣一個ssqlite的控制代碼我們就建立完成了，我們以後針對資料庫的操作都離不開它了。你可能還有疑問，我也知道你的 疑問是什麼，請看下回分解。