声明：本SQLite源码剖析系列为刘兴(http://deepfuture.iteye.com/)原创，未经笔者授权，任何人和机构不能转载
** CAPI3REF: OS Interface Open File Handle
**sqlite3_file对象代表一个在sqlite3_vfs | OS interface layer打开的文件，独立的OS接口实现将通过这个对象的子类增加自己的字段
** An [sqlite3_file] object represents an open file in the
** [sqlite3_vfs | OS interface layer]. Individual OS interface
** implementations will
** want to subclass this object by appending additional fields
** for their own use. The pMethods entry is a pointer to an
** [sqlite3_io_methods] object that defines methods for performing
** I/O operations on the open file.
*pMethods入口是指向sqlite3_io_methods对象的指针，该对象定义了在打开文件执行I／O操作的方法
*/
typedef struct sqlite3_file sqlite3_file;
struct sqlite3_file {
const struct sqlite3_io_methods *pMethods; /* Methods for an open file */
};
/*
** CAPI3REF: OS Interface File Virtual Methods Object
**OS接口文件的虚拟方法对象
*被sqlite3_vfs对象的xOpen方法打开的每个文件组成了sqlite3_file对象或该对象的子类，这些文件以指向这些对象的实例的指针形式存在
** Every file opened by the [sqlite3_vfs] xOpen method populates an
** [sqlite3_file] object (or, more commonly, a subclass of the
** [sqlite3_file] object) with a pointer to an instance of this object.
**该对象定义了多种对代表sqlite3_file对象的打开文件的操作
** This object defines the methods used to perform various operations
** against the open file represented by the [sqlite3_file] object.
**如果xopen方法设置sqlite3_file.pMethods元素为非NULL指针，则sqlite3_io_methods.xClose会被调用，即使在xOpen失败时也是如此。如果要防止xClose在xopen失败后调用，只能将sqlite3_file.pMethods设置为空
** If the xOpen method sets the sqlite3_file.pMethods element
** to a non-NULL pointer, then the sqlite3_io_methods.xClose method
** may be invoked even if the xOpen reported that it failed. The
** only way to prevent a call to xClose following a failed xOpen
** is for the xOpen to set the sqlite3_file.pMethods element to NULL.
**SQLITE_SYNC_NORMAL和SQLITE_SYNC_FULL是xSync的标志参数，首选是使用标准的fsync()，其次是使用Mac OS X 形式的fullsync。与SQLITE_SYNC_DATAONLY标志进行或运算（即|SQLITE_SYNC_DATAONLY）表明文件数据和它的非索引节点需要被同步
** The flags argument to xSync may be one of [SQLITE_SYNC_NORMAL] or
** [SQLITE_SYNC_FULL]. The first choice is the normal fsync().
** The second choice is a Mac OS X style fullsync. The [SQLITE_SYNC_DATAONLY]
** flag may be ORed in to indicate that only the data of the file
** and not its inode needs to be synced.
** The integer values to xLock() and xUnlock() are one of
**
- xLock()和 xUnlock()的整数值是下列之一：
- [SQLITE_LOCK_NONE],
**
- [SQLITE_LOCK_SHARED],
**
- [SQLITE_LOCK_RESERVED],
**
- [SQLITE_LOCK_PENDING], or
**
- [SQLITE_LOCK_EXCLUSIVE].
**
**
** xLock() increases the lock. xUnlock() decreases the lock.
xLock增加锁，xUnlock()解锁，xCheckReservedLock()检测是否存在数据库连接，本进程或其它进程是否在文件中施加RESERVED、PENDING或EXCLUSIVE锁，如果锁存在返回true,否则返回false
** The xCheckReservedLock() method checks whether any database connection,
** either in this process or in some other process, is holding a RESERVED,
** PENDING, or EXCLUSIVE lock on the file. It returns true
** if such a lock exists and false otherwise.