int main(int argc, char **argv)
{
    int rc, i,  id, cid;
    char *name;
    char *sql;
    char *zErr;
    sqlite3 *db; sqlite3_stmt *stmt;
    sql="select id,name,cid from episodes";
    //打开数据库
    sqlite3_open("test.db", &db);
    //编译sql语句
    sqlite3_prepare(db, sql, strlen(sql), &stmt, NULL);
    //调用VM，执行VDBE程序
    rc = sqlite3_step(stmt);
    while(rc == SQLITE_ROW) {
        id = sqlite3_column_int(stmt, );
        name = ();
        cid = sqlite3_column_int(stmt, );
        if(name != NULL){
            fprintf(stderr, "Row:  id=%i, cid=%i, name='%s'\n", id,cid,name);
        } else {
            /* Field is NULL */
            fprintf(stderr, "Row:  id=%i, cid=%i, name=NULL\n", id,cid);
        }
        rc = sqlite3_step(stmt);
    }
    //释放资源
    sqlite3_finalize(stmt);
    //关闭数据库
    sqlite3_close(db);
    ;
}

//sqlite3.h
typedef struct sqlite3_stmt sqlite3_stmt;

//虚拟机数据结构 vdbeInt.h
struct Vdbe {
  sqlite3 *db;        /* The whole database */
  Vdbe *pPrev,*pNext; /* Linked list of VDBEs with the same Vdbe.db */
  FILE *trace;        /* Write an execution trace here, if not NULL */
  int nOp;            /* Number of instructions in the program(指令的条数) */
  int nOpAlloc;       /* Number of slots allocated for aOp[]*/
  Op *aOp;            /* Space to hold the virtual machine's program(指令)*/
  int nLabel;         /* Number of labels used */
  int nLabelAlloc;    /* Number of slots allocated in aLabel[] */
  int *aLabel;        /* Space to hold the labels */
  Mem *aStack;        /* The operand stack, except string values(栈空间) */
  Mem *pTos;          /* Top entry in the operand stack(栈顶指针) */
  Mem **apArg;        /* Arguments to currently executing user function */
  Mem *aColName;      /* Column names to return */
  int nCursor;        /* Number of slots in apCsr[] */
  Cursor **apCsr;     /* One element of this array for each open cursor(游标数组) */
  int nVar;           /* Number of entries in aVar[] */
  Mem *aVar;          /* Values for the OP_Variable opcode*/
  char **azVar;       /* Name of variables */
  int okVar;          /* True if azVar[] has been initialized */
  int magic;              /* Magic number for sanity checking */
  int nMem;               /* Number of memory locations currently allocated */
  Mem *aMem;              /* The memory locations(保存临时变量的Mem)*/
  int nCallback;          /* Number of callbacks invoked so far(回调的次数) */
  int cacheCtr;           /* Cursor row cache generation counter */
  Fifo sFifo;             /* A list of ROWIDs */
  int contextStackTop;    /* Index of top element in the context stack */
  int contextStackDepth;  /* The size of the "context" stack */
  Context *contextStack;  /* Stack used by opcodes ContextPush & ContextPop*/
  int pc;                 /* The program counter(初始程序计数器) */
  int rc;                 /* Value to return(返回结果) */
  unsigned uniqueCnt;     /* Used by OP_MakeRecord when P2!=0 */
  int errorAction;        /* Recovery action to do in case of an error */
  int inTempTrans;        /* True if temp database is transactioned */
  ];   /* Return address stack for OP_Gosub & OP_Return */
  int returnDepth;        /* Next unused element in returnStack[] */
  int nResColumn;         /* Number of columns in one row of the result set */
  char **azResColumn;     /* Values for one row of result */
  int popStack;           /* Pop the stack this much on entry to VdbeExec()(出栈的项数) */
  char *zErrMsg;          /* Error message written here */
  u8 resOnStack;          /* True if there are result values on the stack(有结果在栈上则为真)*/
  u8 explain;             /* True if EXPLAIN present on SQL command */
  u8 changeCntOn;         /* True to update the change-counter */
  u8 aborted;             /* True if ROLLBACK in another VM causes an abort */
  u8 expired;             /* True if the VM needs to be recompiled */
  u8 minWriteFileFormat;  /* Minimum file format for writable database files */
  int nChange;            /* Number of db changes made since last reset */
  i64 startTime;          /* Time when query started - used for profiling */
#ifdef SQLITE_SSE
  int fetchId;          /* Statement number used by sqlite3_fetch_statement */
  int lru;              /* Counter used for LRU cache replacement */
#endif
};

int nOp;            /* Number of instructions in the program(指令的条数) */
Op *aOp;            /* Space to hold the virtual machine's program(指令)*/

、Goto()    ||0c

、Integer() ||

、OpenRead()||

、SetNumColumns()||

、Rewind() ||0a

、Rowid()   ||

、Column()   ||

、Column()   ||

、Callback()||

、Next(|

、Close

、Halt

、Transaction()||

、VerifyCookie()||

、Goto()    |||

Mem *aStack;        /* The operand stack, except string values(栈空间) */
Mem *pTos;          /* Top entry in the operand stack(栈顶指针) */

int sqlite3VdbeExec(
  Vdbe *p                    /* The VDBE */
)

/*执行VDBE程序.当从数据库中取出一行数据时,该函数会调用回调函数(如果有的话),或者返回SQLITE_ROW.*/
int sqlite3VdbeExec(
  Vdbe *p                    /* The VDBE */
){
  //指令计数器
  int pc;                    /* The program counter */
  //当前指令
  Op *pOp;                   /* Current operation */
  int rc = SQLITE_OK;        /* Value to return */
  //数据库
  sqlite3 *db = p->db;       /* The database */
  u8 encoding = ENC(db);     /* The database encoding */
  //栈顶
  Mem *pTos;                 /* Top entry in the operand stack */
  if( p->magic!=VDBE_MAGIC_RUN ) return SQLITE_MISUSE;
  //当前栈顶指针
  pTos = p->pTos;
  if( p->rc==SQLITE_NOMEM ){
  　/* This happens if a malloc() inside a call to sqlite3_column_text() or
  　** sqlite3_column_text16() failed.  */
    goto no_mem;
  }
  p->rc = SQLITE_OK;
  //如果需要进行出栈操作，则进行出栈操作
  if( p->popStack ){
    popStack(&pTos, p->popStack);
    p->popStack = ;
  }
  //表明栈中没有结果
  p->resOnStack = ;
  db->busyHandler.nBusy = ;
  //执行指令
  for(pc=p->pc; rc==SQLITE_OK; pc++){
    //取出操作码
    pOp = &p->aOp[pc];
    switch( pOp->opcode ){
        //跳到操作数P2指向的指令
        case OP_Goto: {             /* no-push */
          CHECK_FOR_INTERRUPT;
          //设置pc
          pc = pOp->p2 - ;
          break;
        }
        //P1入栈
        case OP_Integer: {
          //当前栈顶指针上移
          pTos++;
          //设为整型
          pTos->flags = MEM_Int;
          //取操作数P1,并赋值
          pTos->i = pOp->p1;
          break;
        }
        //其它指令的实现
    }//end switch
  }//end for
}

/*该指令执行后,PC将指向下一条指令.
**栈中栈顶的P1个值为查询的结果.该指令会导致sqlite3_step()函数将以SQLITE_ROW为返回码
**而结束运行.此时用户程序就可以通过sqlite3_column_XXX读取位于栈中的数据了.
**当sqlite3_step()再一次运行时,栈顶的P1个值会在执行Next指令前自动出栈.
*/
case OP_Callback: {            /* no-push */
  Mem *pMem;
  Mem *pFirstColumn;
  assert( p->nResColumn==pOp->p1 );

  /* Data in the pager might be moved or changed out from under us
  ** in between the return from this sqlite3_step() call and the
  ** next call to sqlite3_step().  So deephermeralize everything on
  ** the stack.  Note that ephemeral data is never stored in memory
  ** cells so we do not have to worry about them.
  */
  pFirstColumn = &pTos[-pOp->p1];
  for(pMem = p->aStack; pMem<pFirstColumn; pMem++){
    Deephemeralize(pMem);
  }

  /* Invalidate all ephemeral cursor row caches */
  p->cacheCtr = (p->cacheCtr + )|;

  /* Make sure the results of the current row are \000 terminated
  ** and have an assigned type.  The results are deephemeralized as
  ** as side effect.
  */
  for(; pMem<=pTos; pMem++ ){
    sqlite3VdbeMemNulTerminate(pMem);
    //设置结果集中的数据类型
    storeTypeInfo(pMem, encoding);
  }

  /* Set up the statement structure so that it will pop the current
  ** results from the stack when the statement returns.
  */
  p->resOnStack = ; //栈上有结果
  p->nCallback++;  //回调次数加1
  //出栈的数据个数,在下次执行VDBE时,会先进行出栈操作
  p->popStack = pOp->p1;
  //程序计数器加1
  p->pc = pc + ;

  //设置vdbe的栈顶指针,此时,栈中保存有结果
  p->pTos = pTos;
  /*注意:这里不是break,而是return; 向sqlite3_step()返回SQLITE_ROW.
  **当用户程序重新调用sqlite3_step()时,重新执行VDBE.
  */
  return SQLITE_ROW;
}

/*移动当前游标到表或索引的第一条记录.
**如果表为空且p2>0,则跳到p2处;如果p2为0且表不空，则执行下一条指令.
*/
case OP_Rewind: {        /* no-push */
  int i = pOp->p1;
  Cursor *pC;
  BtCursor *pCrsr;
  int res;

  assert( i>= && i<p->nCursor );
  //取得当前游标
  pC = p->apCsr[i];
  assert( pC!= );
   ){
    //调用B-tree模块,移动游标到第一条记录
    rc = sqlite3BtreeFirst(pCrsr, &res);
    pC->atFirst = res==;
    pC->deferredMoveto = ;
    pC->cacheStatus = CACHE_STALE;
  }else{
    res = ;
  }
  pC->nullRow = res;
   ){
    pc = pOp->p2 - ;
  }
  break;
}

/*解析当前游标指定的记录的数据
**p1为当前游标索引号,p2为列号
*/
case OP_Column: {
  u32 payloadSize;   /* Number of bytes in the record */
  int p1 = pOp->p1;  /* P1 value of the opcode */
  //列号
  int p2 = pOp->p2;  /* column number to retrieve */
  //VDBE游标
  Cursor *pC = ;    /* The VDBE cursor */
  char *zRec;        /* Pointer to complete record-data */
  //btree游标
  BtCursor *pCrsr;   /* The BTree cursor */
  u32 *aType;        /* aType[i] holds the numeric type of the i-th column */
  u32 *aOffset;      /* aOffset[i] is offset to start of data for i-th column */
  //列数
  u32 nField;        /* number of fields in the record */
  int len;           /* The length of the serialized data for the column */
  int i;             /* Loop counter */
  char *zData;       /* Part of the record being decoded */
  Mem sMem;          /* For storing the record being decoded */

  sMem.flags = ;
  assert( p1<p->nCursor );
  //栈顶指针上移
  pTos++;
  pTos->flags = MEM_Null;

  /* This block sets the variable payloadSize to be the total number of
  ** bytes in the record.
  **
  ** zRec is set to be the complete text of the record if it is available.
  ** The complete record text is always available for pseudo-tables
  ** If the record is stored in a cursor, the complete record text
  ** might be available in the  pC->aRow cache.  Or it might not be.
  ** If the data is unavailable,  zRec is set to NULL.
  **
  ** We also compute the number of columns in the record.  For cursors,
  ** the number of columns is stored in the Cursor.nField element.  For
  ** records on the stack, the next entry down on the stack is an integer
  ** which is the number of records.
  */
  //设置游标
  pC = p->apCsr[p1];

  assert( pC!= );
   ){
    /* The record is stored in a B-Tree */
    //移到当前游标
    rc = sqlite3VdbeCursorMoveto(pC);
    if( rc ) goto abort_due_to_error;
    zRec = ;
    pCrsr = pC->pCursor;
    if( pC->nullRow ){
      payloadSize = ;
    }else if( pC->cacheStatus==p->cacheCtr ){
      payloadSize = pC->payloadSize;
      zRec = (char*)pC->aRow;
    }else if( pC->isIndex ){
      i64 payloadSize64;
      sqlite3BtreeKeySize(pCrsr, &payloadSize64);
      payloadSize = payloadSize64;
    }else{
      //解析数据,payloadSize保存cell的数据字节数
      sqlite3BtreeDataSize(pCrsr, &payloadSize);
    }
    nField = pC->nField;
  }else if( pC->pseudoTable ){
    /* The record is the sole entry of a pseudo-table */
    payloadSize = pC->nData;
    zRec = pC->pData;
    pC->cacheStatus = CACHE_STALE;
    assert( payloadSize== || zRec!= );
    nField = pC->nField;
    pCrsr = ;
  }else{
    zRec = ;
    payloadSize = ;
    pCrsr = ;
    nField = ;
  }

  /* If payloadSize is 0, then just push a NULL onto the stack. */
   ){
    assert( pTos->flags==MEM_Null );
    break;
  }

  assert( p2<nField );

  /* Read and parse the table header.  Store the results of the parse
  ** into the record header cache fields of the cursor.
  */
  if( pC && pC->cacheStatus==p->cacheCtr ){
    aType = pC->aType;
    aOffset = pC->aOffset;
  }else{
    u8 *zIdx;        /* Index into header */
    u8 *zEndHdr;     /* Pointer to first byte after the header(指向header之后的第一个字节)*/
    u32 offset;      /* Offset into the data */
    int szHdrSz;     /* Size of the header size field at start of record */
    int avail;       /* Number of bytes of available data */

    //数据类型数组
    aType = pC->aType;
     ){
      //每个数据类型分配8字节---sizeof(aType)==4
      pC->aType = aType = sqliteMallocRaw( *nField*sizeof(aType) );
    }
     ){
      goto no_mem;
    }
    //每列数据的偏移
    pC->aOffset = aOffset = &aType[nField];
    pC->payloadSize = payloadSize;
    pC->cacheStatus = p->cacheCtr;

    /* Figure out how many bytes are in the header */
    if( zRec ){
      zData = zRec;
    }else{
      if( pC->isIndex ){
        zData = (char*)sqlite3BtreeKeyFetch(pCrsr, &avail);
      }else{
        //获取数据
        zData = (char*)sqlite3BtreeDataFetch(pCrsr, &avail);
      }
      /* If KeyFetch()/DataFetch() managed to get the entire payload,
      ** save the payload in the pC->aRow cache.  That will save us from
      ** having to make additional calls to fetch the content portion of
      ** the record.
      */
      if( avail>=payloadSize ){
        zRec = zData;
        pC->aRow = (u8*)zData;
      }else{
        pC->aRow = ;
      }
    }
    assert( zRec!= || avail>=payloadSize || avail>= );
    //获得header size
    szHdrSz = GetVarint((u8*)zData, offset);

    /* The KeyFetch() or DataFetch() above are fast and will get the entire
    ** record header in most cases.  But they will fail to get the complete
    ** record header if the record header does not fit on a single page
    ** in the B-Tree.  When that happens, use sqlite3VdbeMemFromBtree() to
    ** acquire the complete header text.
    */
    if( !zRec && avail<offset ){
      rc = sqlite3VdbeMemFromBtree(pCrsr, , offset, pC->isIndex, &sMem);
      if( rc!=SQLITE_OK ){
        goto op_column_out;
      }
      zData = sMem.z;
    }
/* 一个记录的例子:
** 08 | 08 |04 00 13 01 | 63 61 74 01
** 08: nSize，payload总的大小——后面8个字节
** 08: 关键字大小，对于整型则为关键字本身
** 04: header size，包括本身共4个字节——04 00 13 01
** 00: 第一列的数据类型——空类型
** 13: 第二列的数据类型——字符串，长为(19-13)/2=3——“cat”
** 01: 第三列的数据类型——整型，占一个字节——1
** 对于这里的zData保存的数据为:04 00 13 01 63 61 74 01
*/
    //header之后的数据，对于上例为:63 61 74 01
    zEndHdr = (u8 *)&zData[offset];
    //header数据的索引号，对于上例为:00 13 01
    zIdx = (u8 *)&zData[szHdrSz];

    /* Scan the header and use it to fill in the aType[] and aOffset[]
    ** arrays.  aType[i] will contain the type integer for the i-th
    ** column and aOffset[i] will contain the offset from the beginning
    ** of the record to the start of the data for the i-th column
    */
    /*扫描header，然后设置aType[]和aOffset[]数组; aType[i]为第i列的数据类型,aOffset[i]为第i列数据相对于记录的开始的偏移.*/
    ; i<nField; i++){
      if( zIdx<zEndHdr ){
        //计算每一列数据的偏移
        aOffset[i] = offset;
        //计算每一列的数据类型
        zIdx += GetVarint(zIdx, aType[i]);
        //offset指向下一列
        offset += sqlite3VdbeSerialTypeLen(aType[i]);
      }else{
        /* If i is less that nField, then there are less fields in this
        ** record than SetNumColumns indicated there are columns in the
        ** table. Set the offset for any extra columns not present in
        ** the record to 0. This tells code below to push a NULL onto the
        ** stack instead of deserializing a value from the record.
        */
        aOffset[i] = ;
      }
    }
    Release(&sMem);
    sMem.flags = MEM_Null;

    /* If we have read more header data than was contained in the header,
    ** or if the end of the last field appears to be past the end of the
    ** record, then we must be dealing with a corrupt database.
    */
    if( zIdx>zEndHdr || offset>payloadSize ){
      rc = SQLITE_CORRUPT_BKPT;
      goto op_column_out;
    }
  }

  /* Get the column information. If aOffset[p2] is non-zero, then
  ** deserialize the value from the record. If aOffset[p2] is zero,
  ** then there are not enough fields in the record to satisfy the
  ** request.  In this case, set the value NULL or to P3 if P3 is
  ** a pointer to a Mem object.
  */
  //获取P2指定的列的数据
  if( aOffset[p2] ){
    assert( rc==SQLITE_OK );
    if( zRec ){
      //取得该列的数据
      zData = &zRec[aOffset[p2]];
    }else{
      len = sqlite3VdbeSerialTypeLen(aType[p2]);
      rc = sqlite3VdbeMemFromBtree(pCrsr, aOffset[p2], len, pC->isIndex,&sMem);
      if( rc!=SQLITE_OK ){
        goto op_column_out;
      }
      zData = sMem.z;
    }
    //解析zData，并将结果保存在pTos中
    sqlite3VdbeSerialGet((u8*)zData, aType[p2], pTos);
    pTos->enc = encoding;
  }else{
    if( pOp->p3type==P3_MEM ){
      sqlite3VdbeMemShallowCopy(pTos, (Mem *)(pOp->p3), MEM_Static);
    }else{
      pTos->flags = MEM_Null;
    }
  }

  /* If we dynamically allocated space to hold the data (in the
  ** sqlite3VdbeMemFromBtree() call above) then transfer control of that
  ** dynamically allocated space over to the pTos structure.
  ** This prevents a memory copy.
  */
   ){
    assert( pTos->flags & MEM_Ephem );
    assert( pTos->flags & (MEM_Str|MEM_Blob) );
    assert( pTos->z==sMem.z );
    assert( sMem.flags & MEM_Term );
    pTos->flags &= ~MEM_Ephem;
    pTos->flags |= MEM_Dyn|MEM_Term;
  }

  /* pTos->z might be pointing to sMem.zShort[].  Fix that so that we
  ** can abandon sMem */
  rc = sqlite3VdbeMemMakeWriteable(pTos);

op_column_out:
  break;
}

/*移动游标，使其指向表的下一个记录
*/
case OP_Prev:          /* no-push */
case OP_Next: {        /* no-push */
  Cursor *pC;
  BtCursor *pCrsr;

  CHECK_FOR_INTERRUPT;
  assert( pOp->p1>= && pOp->p1<p->nCursor );
  pC = p->apCsr[pOp->p1];
  assert( pC!= );
   ){
    int res;
    if( pC->nullRow ){
      res = ;
    }else{
      assert( pC->deferredMoveto== );
      //调用B-tree模块,移动游标指向下一条记录
      rc = pOp->opcode==OP_Next ? sqlite3BtreeNext(pCrsr, &res) :
                                  sqlite3BtreePrevious(pCrsr, &res);
      pC->nullRow = res;
      pC->cacheStatus = CACHE_STALE;
    }
     ){
      pc = pOp->p2 - ;
      sqlite3_search_count++;
    }
  }else{
    pC->nullRow = ;
  }
  pC->rowidIsValid = ;
  break;
}