[大数据]Sqlite源码解读（二）

2021SC@SDUSC

本篇开始解读代码：

开头一堆注释主要说明此文件是基于Windows NT操作系统内核（区分于Unix）

开始分析代码：

#if !SQLITE_OS_WINNT && !defined(SQLITE_OMIT_WAL)

#? error "WAL mode requires support from the Windows NT kernel, compile\

?with SQLITE_OMIT_WAL."

如果不是windows NT 操作系统且没有引入WAL模式，则会报错。

关于WAL模式:Write ahead logging.WAL是一种日志模式，在3.7版本后被引入，用来保证日志一定先于对应的脏页落盘。Sqlite日志文件主要有DELETE和WAL两种。WAL较于DELETE的优点是：1.WAL支持读写并发，而DELETE读写互斥；2.WAL只需一次fsync调用，而且WAL的顺序写日志优于DELETE 的离散写日志。

#if !SQLITE_OS_WINNT && SQLITE_MAX_MMAP_SIZE>0

#? error "Memory mapped files require support from the Windows NT kernel,\

?compile with SQLITE_MAX_MMAP_SIZE=0."

#endif

mmap是一个内存映射文件的方法，将该文件的磁盘地址映射到进程的虚拟地址。这样进程就可以用指针读写操作该内存，省去了对read()和write（）系统函数的调用。

#ifndef NTDDI_WIN8

#? define NTDDI_WIN8???????????? ???????????0x06020000

#endif

#ifndef NTDDI_WINBLUE

#? define NTDDI_WINBLUE???????????????????? 0x06030000

#endif

#ifndef NTDDI_WINTHRESHOLD

#? define NTDDI_WINTHRESHOLD??????????????? 0x06040000

#endif

定义需要用到的SDK版本的常数

SDK包含了开发本Windows版本所需的函数、常数、API、相关工具及示例。

#ifndef SQLITE_WIN32_MAX_PATH_BYTES

#? define SQLITE_WIN32_MAX_PATH_BYTES?? (SQLITE_WIN32_MAX_PATH_CHARS*4)

#endif

#ifndef SQLITE_WINNT_MAX_PATH_BYTES

#? define SQLITE_WINNT_MAX_PATH_BYTES?? \

??????????????????????????? (sizeof(WCHAR) * SQLITE_WINNT_MAX_PATH_CHARS)

#endif

对Win32路径名长度规定，每个字符用4个字节表示，采用UTF8编码。

对WinNT路径名长度规定，用Wchar长度乘unicode最大char长度。

#if SQLITE_OS_WINCE

typedef struct winceLock {

? int nReaders;?????? /* Number of reader locks obtained */

? BOOL bPending;????? /* Indicates a pending lock has been obtained */

? BOOL bReserved;???? /* Indicates a reserved lock has been obtained */

? BOOL bExclusive;??? /* Indicates an exclusive lock has been obtained */

} winceLock;

#endif

WINCE是一个操作界面源于Windows的嵌入式新型操作系统。

创建wincelock结构体，并指出三个布尔变量。

typedef struct winFile winFile;

struct winFile {

? const sqlite3_io_methods *pMethod; /*** Must be first ***/

? sqlite3_vfs *pVfs;????? /* The VFS used to open this file */

? HANDLE h;?????????????? /* Handle for accessing the file */

? u8 locktype;??????????? /* Type of lock currently held on this file */

? short sharedLockByte;?? /* Randomly chosen byte used as a shared lock */

? u8 ctrlFlags;?????????? /* Flags.? See WINFILE_* below */

? DWORD lastErrno;??????? /* The Windows errno from the last I/O error */

#ifndef SQLITE_OMIT_WAL

? winShm *pShm;?????????? /* Instance of shared memory on this file */

#endif

? const char *zPath;????? /* Full pathname of this file */

? int szChunk;??????????? /* Chunk size configured by FCNTL_CHUNK_SIZE */

#if SQLITE_OS_WINCE

? LPWSTR zDeleteOnClose;? /* Name of file to delete when closing */

? HANDLE hMutex;????????? /* Mutex used to control access to shared lock */

? HANDLE hShared;???????? /* Shared memory segment used for locking */

? winceLock local;??????? /* Locks obtained by this instance of winFile */

? winceLock *shared;????? /* Global shared lock memory for the file? */

#endif

#if SQLITE_MAX_MMAP_SIZE>0

? int nFetchOut;??????????????? /* Number of outstanding xFetch references */

? HANDLE hMap;????????????????? /* Handle for accessing memory mapping */

? void *pMapRegion;???????????? /* Area memory mapped */

? sqlite3_int64 mmapSize;?????? /* Size of mapped region */

? sqlite3_int64 mmapSizeMax;??? /* Configured FCNTL_MMAP_SIZE value */

#endif

};

typedef struct winVfsAppData winVfsAppData;

struct winVfsAppData {

? const sqlite3_io_methods *pMethod; /* The file I/O methods to use. */

? void *pAppData;??????????????????? /* The extra pAppData, if any. */

? BOOL bNoLock;????????????????????? /* Non-zero if locking is disabled. */

};

创建了winFile结构体

注册时首先声明VFS接口函数，在tvfs_vfs结构体里

static sqlite3_vfs tvfs_vfs

在pAppData记录原来默认的VFS

pVfs->pAppData = (void *)p;//把信息存到pVfs->pAppData里

void *pAppData：该指针指向一个存储有各种文件io操作方法的结构体地址

在Win下默认使用的是win32 vfs，VFS首先要注册，使用sqlite3_vfs_register函数。

接下来是tcl命令参数。假设注册VFS时新建的命令是tvfs，使用格式为：tvfs shm FILE VALUE。

db eval {CREATE TABLE t1(a TEXT, b INTEGER)} #新建表

db eval {

?? INSERT INTO t1 VALUES('one',1);

?? INSERT INTO t1 VALUES('two',2);?

?? INSERT INTO t1 VALUES(NULL,3);

}? #插入行

??? puts [db eval {SELECT * FROM t1}] #显示表的所有行并输出结果