** Checkpoint database zDb. If zDb is NULL, the main database is checkpointed.
*/
int sqlite3_wal_checkpoint(sqlite3 *db, const char *zDb){
#ifdef SQLITE_OMIT_WAL
  return SQLITE_OK;
#else
  int rc;                         /* Return code */
  int iDb = 0;                    /* sqlite3.aDb[] index of db to checkpoint */

  sqlite3_mutex_enter(db->mutex);
  if( zDb ){
    iDb = sqlite3FindDbName(db, zDb);
  }
  if( iDb<0 ){
    rc = SQLITE_ERROR;
................................................................................
}

#ifndef SQLITE_OMIT_WAL
/*
** Run a checkpoint on database iDb. This is a no-op if database iDb is
** not currently open in WAL mode.
**
** If a transaction is open at either the database handle (db) or b-tree
** level, this function returns SQLITE_LOCKED and a checkpoint is not
** attempted. If an error occurs while running the checkpoint, an SQLite
** error code is returned (i.e. SQLITE_IOERR). Otherwise, SQLITE_OK.
**
** The mutex on database handle db should be held by the caller. The mutex
** associated with the specific b-tree being checkpointed is taken by
** this function while the checkpoint is running.




*/
int sqlite3Checkpoint(sqlite3 *db, int iDb){
  Btree *pBt;                     /* Btree handle to checkpoint */

  int rc;                         /* Return code */

  assert( sqlite3_mutex_held(db->mutex) );



  pBt = db->aDb[iDb].pBt;

  if( sqlite3BtreeIsInReadTrans(pBt) ){
    rc = SQLITE_LOCKED;
  }else{
    sqlite3BtreeEnter(pBt);
    rc = sqlite3PagerCheckpoint(sqlite3BtreePager(pBt));
    sqlite3BtreeLeave(pBt);
  }




  return rc;
}
#endif /* SQLITE_OMIT_WAL */

/*
** This function returns true if main-memory should be used instead of

** Checkpoint database zDb. If zDb is NULL, the main database is checkpointed.
*/
int sqlite3_wal_checkpoint(sqlite3 *db, const char *zDb){
#ifdef SQLITE_OMIT_WAL
  return SQLITE_OK;
#else
  int rc;                         /* Return code */
  int iDb = SQLITE_MAX_ATTACHED;  /* sqlite3.aDb[] index of db to checkpoint */

  sqlite3_mutex_enter(db->mutex);
  if( zDb ){
    iDb = sqlite3FindDbName(db, zDb);
  }
  if( iDb<0 ){
    rc = SQLITE_ERROR;
................................................................................
}

#ifndef SQLITE_OMIT_WAL
/*
** Run a checkpoint on database iDb. This is a no-op if database iDb is
** not currently open in WAL mode.
**
** If a transaction is open on the database being checkpointed, this 
** function returns SQLITE_LOCKED and a checkpoint is not attempted. If 
** an error occurs while running the checkpoint, an SQLite error code is 
** returned (i.e. SQLITE_IOERR). Otherwise, SQLITE_OK.
**
** The mutex on database handle db should be held by the caller. The mutex
** associated with the specific b-tree being checkpointed is taken by
** this function while the checkpoint is running.
**
** If iDb is passed SQLITE_MAX_ATTACHED, then all attached databases are
** checkpointed. If an error is encountered it is returned immediately -
** no attempt is made to checkpoint any remaining databases.
*/
int sqlite3Checkpoint(sqlite3 *db, int iDb){

  int rc = SQLITE_OK;             /* Return code */
  int i;                          /* Used to iterate through attached dbs */

  assert( sqlite3_mutex_held(db->mutex) );

  for(i=0; i<db->nDb && rc==SQLITE_OK; i++){
    if( i==iDb || iDb==SQLITE_MAX_ATTACHED ){
      Btree *pBt = db->aDb[i].pBt;
      if( pBt ){
        if( sqlite3BtreeIsInReadTrans(pBt) ){
          rc = SQLITE_LOCKED;
        }else{
          sqlite3BtreeEnter(pBt);
          rc = sqlite3PagerCheckpoint(sqlite3BtreePager(pBt));
          sqlite3BtreeLeave(pBt);
        }
      }
    }
  }

  return rc;
}
#endif /* SQLITE_OMIT_WAL */

/*
** This function returns true if main-memory should be used instead of