定义了一些内部结构体，文件块列表结构体，插入回滚日志的指针以及memory-journal 总结构体，如下所示
/* Forward references to internal structures */
typedef struct MemJournal MemJournal;
typedef struct FilePoint FilePoint;
typedef struct FileChunk FileChunk;
//文件块列表结构体
struct FileChunk {
FileChunk *pNext; /* Next chunk in the journal */
u8 zChunk[JOURNAL_CHUNKSIZE]; /* Content of this chunk */
};
//插入回滚日志的指针
struct FilePoint {
sqlite3_int64 iOffset; /* Offset from the beginning of the file */
FileChunk *pChunk; /* Specific chunk into which cursor points */
};
// memory-journal 总结构体
struct MemJournal {
sqlite3_io_methods *pMethod; /* Parent class. MUST BE FIRST */
FileChunk *pFirst; /* Head of in-memory chunk-list */
FilePoint endpoint; /* Pointer to the end of the file */
FilePoint readpoint; /* Pointer to the end of the last xRead() */
};