sqlite3结构体与实际打开的文件描述符之间的关系

我们了解sqlite3数据库是一个文件格式的数据库，那显然它必然至少会持有一个打开的文件描述符，本文简单介绍 sqlite3结构体本身与这个文件描述符之间的关系，以及调用sqlite3_open最终打开这个文件描述符的流程。

先上结论，在sqlite3的数据结构中，与文件描述符的定义关系大致如下流程所示：

struct sqlite3——>(struct Db)aDb——>(struct Btree *)pBt——>
(struct BtShared *)pBt——>(struct Pager *)pPager ——>
(struct sqlite3_file)fd = struct unixFile  ——>(int)h;

for reference
struct sqlite3 line 16840
aDb代表这个数据库句柄上打开的数据库文件，aDb[0]为主数据库，可以包含多个，比如内存数据库，以及attach上来的数据库。
struct Btree  .definition in line 66184
sqlite3_file的定义根据VFS的实现不同而不同。在默认的unix中，是如下定义
/*
** The unixFile structure is subclass of sqlite3_file specific to the unix
** VFS implementations.

*/
typedef struct unixFile unixFile;

下面分析一下打开数据库文件的流程
在最基本的使用方式中，我们通过如下的方式打开一个sqlite3数据库的句柄。

     sqlite3* db;
     int iRet = sqlite3_open("test.db",&db);

整体的调用链如下，中间还包括了其他一些填充的流程。

sqlite3_open——》openDatabase——》sqlite3BtreeOpen——》sqlite3PagerOpen——》
sqlite3OsOpen——》 pVfs->xOpen ——》unixOpen ——》robust_open

在我这里使用的 3.39.4 版本中，其实际调用了openDatabase函数。

/*
** Open a new database handle.

*/
SQLITE_API int sqlite3_open(
  const char *zFilename,
  sqlite3 **ppDb
){
  return openDatabase(zFilename, ppDb,
                      SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE, 0);
}

openDatabase函数中执行了数据库handle的分配和初始化，这里注意下openDatabase携带了4个参数，除了需要打开的数据库路径和需要返回出来的sqlite3的Handle，另外两个参数分别是打开的flag和应使用的操作系统接口对应的操作系统名称。这两个参数，在一个不同于sqlite3_open的接口 sqlite3_open_v2接口中是允许配置的，该接口的语义可以参考如下网址。https://www.sqlite.net.cn/c3ref/open.html
按照默认的不配置的老接口，会使用默认的sqlite3_vfs对象。该对象实际是sqlite3结构体的第一个参数，也是我们后面将会用到的很重要的一个成员。

static int openDatabase(
  const char *zFilename, /* Database filename UTF-8 encoded */
  sqlite3 **ppDb,        /* OUT: Returned database handle */
  unsigned int flags,    /* Operational flags */
  const char *zVfs       /* Name of the VFS to use */
){
  sqlite3 *db;                    /* Store allocated handle here */
  ...

  rc = sqlite3_initialize();
  if( rc ) return rc;
  ...

  db = sqlite3MallocZero( sizeof(sqlite3) );
  ...

  /*关键的路径参数被传递给了 sqlite3ParseUri，同时将给db的pVfs参数，也就是 sqlite3_vfs 对象进行了赋值，下文提供了sqlite3ParseUri的定义*/
  rc = sqlite3ParseUri(zVfs, zFilename, &flags, &db->pVfs, &zOpen, &zErrMsg);

  ...

    /* Open the backend database driver */
  rc = sqlite3BtreeOpen(db->pVfs, zOpen, db, &db->aDb[0].pBt, 0,
                        flags | SQLITE_OPEN_MAIN_DB);
 ...

}

sqlite3ParseUri完成了比较关键的解析，他接受了完整的路径入参，默认该路径为uri的形式，然后提取出路径赋值到 zOpen中，并根据zVfs的参数，给VFS赋值。因为这里没有赋值，所以默认返回vfsList中的第一个vfs。这个vfsList在sqlite3_os_init中被初始化和定义。sqlite3_os_init为内部初始化调用，在sqlite3_initialize中被初始化。是openDatabase函数的第一步。

SQLITE_PRIVATE int sqlite3ParseUri(
  const char *zDefaultVfs,        /* VFS to use if no "vfs=xxx" query option */
  const char *zUri,               /* Nul-terminated URI to parse */
  unsigned int *pFlags,           /* IN/OUT: SQLITE_OPEN_XXX flags */
  sqlite3_vfs **ppVfs,            /* OUT: VFS to use */
  char **pzFile,                  /* OUT: Filename component of URI */
  char **pzErrMsg                 /* OUT: Error message (if rc!=SQLITE_OK) */
){
}

然后根据上述流程在 openDatabase中，接力棒给到了sqlite3BtreeOpen，该函数执行了实际数据库流程的打开工作，这里比较复杂的流程是需要把打开的数据库实例对应的 BtShared 关联到BtTree，再关联到sqlite3的aDb数组。
真正执行打开的关键操作是在sqlite3PagerOpen。其会返回BtShared持有的pPager，BtShared代表一个数据库文件的实例。

line 69301
/*
** Open a database file.

**...

*/
SQLITE_PRIVATE int sqlite3BtreeOpen(
  sqlite3_vfs *pVfs,      /* VFS to use for this b-tree */
  const char *zFilename,  /* Name of the file containing the BTree database */
  sqlite3 *db,            /* Associated database handle */
  Btree **ppBtree,        /* Pointer to new Btree object written here */
  int flags,              /* Options */
  int vfsFlags            /* Flags passed through to sqlite3_vfs.xOpen() */
){

...

    rc = sqlite3PagerOpen(pVfs, &pBt->pPager, zFilename,
                          sizeof(MemPage), flags, vfsFlags, pageReinit);
...

sqlite3PagerOpen函数打开了一个Pager对象，并将其赋值给BtShared，根据Pager的定义，每一个打开的文件，都被一个独立的Pager实例结构体管理 。其打开调用了 sqlite3OsOpen，而sqlite3OsOpen 又调用了 pVfs->xOpen ，即我们之前提到的 sqlite3_vfs 中的xOpen的handle。

/*
** Each open file is managed by a separate instance of the "Pager" structure.

*/
typedef struct Pager Pager;
...

line 58590
SQLITE_PRIVATE int sqlite3PagerOpen(
  sqlite3_vfs *pVfs,       /* The virtual file system to use */
  Pager **ppPager,         /* OUT: Return the Pager structure here */
  const char *zFilename,   /* Name of the database file to open */
  int nExtra,              /* Extra bytes append to each in-memory page */
  int flags,               /* flags controlling this file */
  int vfsFlags,            /* flags passed through to sqlite3_vfs.xOpen() */
  void (*xReinit)(DbPage*) /* Function to reinitialize pages */
){
...

    rc = sqlite3OsOpen(pVfs, pPager->zFilename, pPager->fd, vfsFlags, &fout);
...

}

...

SQLITE_PRIVATE int sqlite3OsOpen(
  sqlite3_vfs *pVfs,
  const char *zPath,
  sqlite3_file *pFile,
  int flags,
  int *pFlagsOut
){
  int rc;
  DO_OS_MALLOC_TEST(0);
  /* 0x87f7f is a mask of SQLITE_OPEN_ flags that are valid to be passed
  ** down into the VFS layer.  Some SQLITE_OPEN_ flags (for example,
  ** SQLITE_OPEN_FULLMUTEX or SQLITE_OPEN_SHAREDCACHE) are blocked before
  ** reaching the VFS. */
  rc = pVfs->xOpen(pVfs, zPath, pFile, flags & 0x1087f7f, pFlagsOut);
  assert( rc==SQLITE_OK || pFile->pMethods==0 );
  return rc;
}

sqlite3_vfs的handle的定义在sqlite3_os_init中。默认open的定义为 unixOpen 函数，unixOpen函数的定义如下

static int unixOpen(
  sqlite3_vfs *pVfs,           /* The VFS for which this is the xOpen method */
  const char *zPath,           /* Pathname of file to be opened */
  sqlite3_file *pFile,         /* The file descriptor to be filled in */
  int flags,                   /* Input flags to control the opening */
  int *pOutFlags               /* Output flags returned to SQLite core */
){
    ...

    fd = robust_open(zName, openFlags, openMode);
    ...

    ...

    rc = fillInUnixFile(pVfs, fd, pFile, zPath, ctrlFlags);
    ...

}

robust_open实际执行了打开的动作，而fillInUnixFile 则是将fd填充到了对应 的pFile句柄里。

static int robust_open(const char *z, int f, mode_t m){
  int fd;
  mode_t m2 = m ? m : SQLITE_DEFAULT_FILE_PERMISSIONS;
  while(1){
#if defined(O_CLOEXEC)
    fd = osOpen(z,f|O_CLOEXEC,m2);
#else
    fd = osOpen(z,f,m2);
#endif
    if( fd<0 ){ if( errno="=EINTR" ) continue; break; } fd>=SQLITE_MINIMUM_FILE_DESCRIPTOR ) break;
    osClose(fd);
    sqlite3_log(SQLITE_WARNING,
                "attempt to open \"%s\" as file descriptor %d", z, fd);
    fd = -1;
    if( osOpen("/dev/null", O_RDONLY, m)<0 ) break; } if( fd>=0 ){
    if( m!=0 ){
      struct stat statbuf;
      if( osFstat(fd, &statbuf)==0
       && statbuf.st_size==0
       && (statbuf.st_mode&0777)!=m
      ){
        osFchmod(fd, m);
      }
    }
#if defined(FD_CLOEXEC) && (!defined(O_CLOEXEC) || O_CLOEXEC==0)
    osFcntl(fd, F_SETFD, osFcntl(fd, F_GETFD, 0) | FD_CLOEXEC);
#endif
  }
  return fd;
}
</0></0>

作为参考，附上sqlite3的结构体定义如下

struct sqlite3 {
  sqlite3_vfs *pVfs;            /* OS Interface */
  struct Vdbe *pVdbe;           /* List of active virtual machines */
  CollSeq *pDfltColl;           /* BINARY collseq for the database encoding */
  sqlite3_mutex *mutex;         /* Connection mutex */
  Db *aDb;                      /* All backends */
  int nDb;                      /* Number of backends currently in use */
  u32 mDbFlags;                 /* flags recording internal state */
  u64 flags;                    /* flags settable by pragmas. See below */
  i64 lastRowid;                /* ROWID of most recent insert (see above) */
  i64 szMmap;                   /* Default mmap_size setting */
  u32 nSchemaLock;              /* Do not reset the schema when non-zero */
  unsigned int openFlags;       /* Flags passed to sqlite3_vfs.xOpen() */
  int errCode;                  /* Most recent error code (SQLITE_*) */
  int errByteOffset;            /* Byte offset of error in SQL statement */
  int errMask;                  /* & result codes with this before returning */
  int iSysErrno;                /* Errno value from last system error */
  u32 dbOptFlags;               /* Flags to enable/disable optimizations */
  u8 enc;                       /* Text encoding */
  u8 autoCommit;                /* The auto-commit flag. */
  u8 temp_store;                /* 1: file 2: memory 0: default */
  u8 mallocFailed;              /* True if we have seen a malloc failure */
  u8 bBenignMalloc;             /* Do not require OOMs if true */
  u8 dfltLockMode;              /* Default locking-mode for attached dbs */
  signed char nextAutovac;      /* Autovac setting after VACUUM if >=0 */
  u8 suppressErr;               /* Do not issue error messages if true */
  u8 vtabOnConflict;            /* Value to return for s3_vtab_on_conflict() */
  u8 isTransactionSavepoint;    /* True if the outermost savepoint is a TS */
  u8 mTrace;                    /* zero or more SQLITE_TRACE flags */
  u8 noSharedCache;             /* True if no shared-cache backends */
  u8 nSqlExec;                  /* Number of pending OP_SqlExec opcodes */
  u8 eOpenState;                /* Current condition of the connection */
  int nextPagesize;             /* Pagesize after VACUUM if >0 */
  i64 nChange;                  /* Value returned by sqlite3_changes() */
  i64 nTotalChange;             /* Value returned by sqlite3_total_changes() */
  int aLimit[SQLITE_N_LIMIT];   /* Limits */
  int nMaxSorterMmap;           /* Maximum size of regions mapped by sorter */
  struct sqlite3InitInfo {      /* Information used during initialization */
    Pgno newTnum;               /* Rootpage of table being initialized */
    u8 iDb;                     /* Which db file is being initialized */
    u8 busy;                    /* TRUE if currently initializing */
    unsigned orphanTrigger : 1; /* Last statement is orphaned TEMP trigger */
    unsigned imposterTable : 1; /* Building an imposter table */
    unsigned reopenMemdb : 1;   /* ATTACH is really a reopen using MemDB */
    const char **azInit;        /* "type", "name", and "tbl_name" columns */
  } init;
  int nVdbeActive;              /* Number of VDBEs currently running */
  int nVdbeRead;                /* Number of active VDBEs that read or write */
  int nVdbeWrite;               /* Number of active VDBEs that read and write */
  int nVdbeExec;                /* Number of nested calls to VdbeExec() */
  int nVDestroy;                /* Number of active OP_VDestroy operations */
  int nExtension;               /* Number of loaded extensions */
  void **aExtension;            /* Array of shared library handles */
  union {
    void (*xLegacy)(void*,const char*);   /* mTrace==SQLITE_TRACE_LEGACY */
    int (*xV2)(u32,void*,void*,void*);    /* All other mTrace values */
  } trace;
  void *pTraceArg;                        /* Argument to the trace function */
#ifndef SQLITE_OMIT_DEPRECATED
  void (*xProfile)(void*,const char*,u64);  /* Profiling function */
  void *pProfileArg;                        /* Argument to profile function */
#endif
  void *pCommitArg;                 /* Argument to xCommitCallback() */
  int (*xCommitCallback)(void*);    /* Invoked at every commit. */
  void *pRollbackArg;               /* Argument to xRollbackCallback() */
  void (*xRollbackCallback)(void*); /* Invoked at every commit. */
  void *pUpdateArg;
  void (*xUpdateCallback)(void*,int, const char*,const char*,sqlite_int64);
  void *pAutovacPagesArg;           /* Client argument to autovac_pages */
  void (*xAutovacDestr)(void*);     /* Destructor for pAutovacPAgesArg */
  unsigned int (*xAutovacPages)(void*,const char*,u32,u32,u32);
  Parse *pParse;                /* Current parse */
#ifdef SQLITE_ENABLE_PREUPDATE_HOOK
  void *pPreUpdateArg;          /* First argument to xPreUpdateCallback */
  void (*xPreUpdateCallback)(   /* Registered using sqlite3_preupdate_hook() */
    void*,sqlite3*,int,char const*,char const*,sqlite3_int64,sqlite3_int64
  );
  PreUpdate *pPreUpdate;        /* Context for active pre-update callback */
#endif /* SQLITE_ENABLE_PREUPDATE_HOOK */
#ifndef SQLITE_OMIT_WAL
  int (*xWalCallback)(void *, sqlite3 *, const char *, int);
  void *pWalArg;
#endif
  void(*xCollNeeded)(void*,sqlite3*,int eTextRep,const char*);
  void(*xCollNeeded16)(void*,sqlite3*,int eTextRep,const void*);
  void *pCollNeededArg;
  sqlite3_value *pErr;          /* Most recent error message */
  union {
    volatile int isInterrupted; /* True if sqlite3_interrupt has been called */
    double notUsed1;            /* Spacer */
  } u1;
  Lookaside lookaside;          /* Lookaside malloc configuration */
#ifndef SQLITE_OMIT_AUTHORIZATION
  sqlite3_xauth xAuth;          /* Access authorization function */
  void *pAuthArg;               /* 1st argument to the access auth function */
#endif
#ifndef SQLITE_OMIT_PROGRESS_CALLBACK
  int (*xProgress)(void *);     /* The progress callback */
  void *pProgressArg;           /* Argument to the progress callback */
  unsigned nProgressOps;        /* Number of opcodes for progress callback */
#endif
#ifndef SQLITE_OMIT_VIRTUALTABLE
  int nVTrans;                  /* Allocated size of aVTrans */
  Hash aModule;                 /* populated by sqlite3_create_module() */
  VtabCtx *pVtabCtx;            /* Context for active vtab connect/create */
  VTable **aVTrans;             /* Virtual tables with open transactions */
  VTable *pDisconnect;          /* Disconnect these in next sqlite3_prepare() */
#endif
  Hash aFunc;                   /* Hash table of connection functions */
  Hash aCollSeq;                /* All collating sequences */
  BusyHandler busyHandler;      /* Busy callback */
  Db aDbStatic[2];              /* Static space for the 2 default backends */
  Savepoint *pSavepoint;        /* List of active savepoints */
  int nAnalysisLimit;           /* Number of index rows to ANALYZE */
  int busyTimeout;              /* Busy handler timeout, in msec */
  int nSavepoint;               /* Number of non-transaction savepoints */
  int nStatement;               /* Number of nested statement-transactions  */
  i64 nDeferredCons;            /* Net deferred constraints this transaction. */
  i64 nDeferredImmCons;         /* Net deferred immediate constraints */
  int *pnBytesFreed;            /* If not NULL, increment this in DbFree() */
#ifdef SQLITE_ENABLE_UNLOCK_NOTIFY
  /* The following variables are all protected by the STATIC_MAIN
  ** mutex, not by sqlite3.mutex. They are used by code in notify.c.

  **
  ** When X.pUnlockConnection==Y, that means that X is waiting for Y to
  ** unlock so that it can proceed.

  **
  ** When X.pBlockingConnection==Y, that means that something that X tried
  ** tried to do recently failed with an SQLITE_LOCKED error due to locks
  ** held by Y.

  */
  sqlite3 *pBlockingConnection; /* Connection that caused SQLITE_LOCKED */
  sqlite3 *pUnlockConnection;           /* Connection to watch for unlock */
  void *pUnlockArg;                     /* Argument to xUnlockNotify */
  void (*xUnlockNotify)(void **, int);  /* Unlock notify callback */
  sqlite3 *pNextBlocked;        /* Next in list of all blocked connections */
#endif
#ifdef SQLITE_USER_AUTHENTICATION
  sqlite3_userauth auth;        /* User authentication information */
#endif
};

sqlite3_vfs的定义

struct sqlite3_vfs {
  int iVersion;            /* Structure version number (currently 3) */
  int szOsFile;            /* Size of subclassed sqlite3_file */
  int mxPathname;          /* Maximum file pathname length */
  sqlite3_vfs *pNext;      /* Next registered VFS */
  const char *zName;       /* Name of this virtual file system */
  void *pAppData;          /* Pointer to application-specific data */
  int (*xOpen)(sqlite3_vfs*, const char *zName, sqlite3_file*,
               int flags, int *pOutFlags);
  int (*xDelete)(sqlite3_vfs*, const char *zName, int syncDir);
  int (*xAccess)(sqlite3_vfs*, const char *zName, int flags, int *pResOut);
  int (*xFullPathname)(sqlite3_vfs*, const char *zName, int nOut, char *zOut);
  void *(*xDlOpen)(sqlite3_vfs*, const char *zFilename);
  void (*xDlError)(sqlite3_vfs*, int nByte, char *zErrMsg);
  void (*(*xDlSym)(sqlite3_vfs*,void*, const char *zSymbol))(void);
  void (*xDlClose)(sqlite3_vfs*, void*);
  int (*xRandomness)(sqlite3_vfs*, int nByte, char *zOut);
  int (*xSleep)(sqlite3_vfs*, int microseconds);
  int (*xCurrentTime)(sqlite3_vfs*, double*);
  int (*xGetLastError)(sqlite3_vfs*, int, char *);
  /*
  ** The methods above are in version 1 of the sqlite_vfs object
  ** definition.  Those that follow are added in version 2 or later
  */
  int (*xCurrentTimeInt64)(sqlite3_vfs*, sqlite3_int64*);
  /*
  ** The methods above are in versions 1 and 2 of the sqlite_vfs object.

  ** Those below are for version 3 and greater.

  */
  int (*xSetSystemCall)(sqlite3_vfs*, const char *zName, sqlite3_syscall_ptr);
  sqlite3_syscall_ptr (*xGetSystemCall)(sqlite3_vfs*, const char *zName);
  const char *(*xNextSystemCall)(sqlite3_vfs*, const char *zName);
  /*
  ** The methods above are in versions 1 through 3 of the sqlite_vfs object.

  ** New fields may be appended in future versions.  The iVersion
  ** value will increment whenever this happens.

  */
};

Btree相关

/* A Btree handle
**
** A database connection contains a pointer to an instance of
** this object for every database file that it has open.  This structure
** is opaque to the database connection.  The database connection cannot
** see the internals of this structure and only deals with pointers to
** this structure.

**
** For some database files, the same underlying database cache might be
** shared between multiple connections.  In that case, each connection
** has it own instance of this object.  But each instance of this object
** points to the same BtShared object.  The database cache and the
** schema associated with the database file are all contained within
** the BtShared object.

**
** All fields in this structure are accessed under sqlite3.mutex.

** The pBt pointer itself may not be changed while there exists cursors
** in the referenced BtShared that point back to this Btree since those
** cursors have to go through this Btree to find their BtShared and
** they often do so without holding sqlite3.mutex.

*/
struct Btree {
  sqlite3 *db;       /* The database connection holding this btree */
  BtShared *pBt;     /* Sharable content of this btree */
  u8 inTrans;        /* TRANS_NONE, TRANS_READ or TRANS_WRITE */
  u8 sharable;       /* True if we can share pBt with another db */
  u8 locked;         /* True if db currently has pBt locked */
  u8 hasIncrblobCur; /* True if there are one or more Incrblob cursors */
  int wantToLock;    /* Number of nested calls to sqlite3BtreeEnter() */
  int nBackup;       /* Number of backup operations reading this btree */
  u32 iBDataVersion; /* Combines with pBt->pPager->iDataVersion */
  Btree *pNext;      /* List of other sharable Btrees from the same db */
  Btree *pPrev;      /* Back pointer of the same list */
#ifdef SQLITE_DEBUG
  u64 nSeek;         /* Calls to sqlite3BtreeMovetoUnpacked() */
#endif
#ifndef SQLITE_OMIT_SHARED_CACHE
  BtLock lock;       /* Object used to lock page 1 */
#endif
};

/*
** An instance of this object represents a single database file.

**
** A single database file can be in use at the same time by two
** or more database connections.  When two or more connections are
** sharing the same database file, each connection has it own
** private Btree object for the file and each of those Btrees points
** to this one BtShared object.  BtShared.nRef is the number of
** connections currently sharing this database file.

**
** Fields in this structure are accessed under the BtShared.mutex
** mutex, except for nRef and pNext which are accessed under the
** global SQLITE_MUTEX_STATIC_MAIN mutex.  The pPager field
** may not be modified once it is initially set as long as nRef>0.

** The pSchema field may be set once under BtShared.mutex and
** thereafter is unchanged as long as nRef>0.

**
** isPending:
**
**   If a BtShared client fails to obtain a write-lock on a database
**   table (because there exists one or more read-locks on the table),
**   the shared-cache enters 'pending-lock' state and isPending is
**   set to true.

**
**   The shared-cache leaves the 'pending lock' state when either of
**   the following occur:
**
**     1) The current writer (BtShared.pWriter) concludes its transaction, OR
**     2) The number of locks held by other connections drops to zero.

**
**   while in the 'pending-lock' state, no connection may start a new
**   transaction.

**
**   This feature is included to help prevent writer-starvation.

*/
struct BtShared {
  Pager *pPager;        /* The page cache */
  sqlite3 *db;          /* Database connection currently using this Btree */
  BtCursor *pCursor;    /* A list of all open cursors */
  MemPage *pPage1;      /* First page of the database */
  u8 openFlags;         /* Flags to sqlite3BtreeOpen() */
#ifndef SQLITE_OMIT_AUTOVACUUM
  u8 autoVacuum;        /* True if auto-vacuum is enabled */
  u8 incrVacuum;        /* True if incr-vacuum is enabled */
  u8 bDoTruncate;       /* True to truncate db on commit */
#endif
  u8 inTransaction;     /* Transaction state */
  u8 max1bytePayload;   /* Maximum first byte of cell for a 1-byte payload */
  u8 nReserveWanted;    /* Desired number of extra bytes per page */
  u16 btsFlags;         /* Boolean parameters.  See BTS_* macros below */
  u16 maxLocal;         /* Maximum local payload in non-LEAFDATA tables */
  u16 minLocal;         /* Minimum local payload in non-LEAFDATA tables */
  u16 maxLeaf;          /* Maximum local payload in a LEAFDATA table */
  u16 minLeaf;          /* Minimum local payload in a LEAFDATA table */
  u32 pageSize;         /* Total number of bytes on a page */
  u32 usableSize;       /* Number of usable bytes on each page */
  int nTransaction;     /* Number of open transactions (read + write) */
  u32 nPage;            /* Number of pages in the database */
  void *pSchema;        /* Pointer to space allocated by sqlite3BtreeSchema() */
  void (*xFreeSchema)(void*);  /* Destructor for BtShared.pSchema */
  sqlite3_mutex *mutex; /* Non-recursive mutex required to access this object */
  Bitvec *pHasContent;  /* Set of pages moved to free-list this transaction */
#ifndef SQLITE_OMIT_SHARED_CACHE
  int nRef;             /* Number of references to this structure */
  BtShared *pNext;      /* Next on a list of sharable BtShared structs */
  BtLock *pLock;        /* List of locks held on this shared-btree struct */
  Btree *pWriter;       /* Btree with currently open write transaction */
#endif
  u8 *pTmpSpace;        /* Temp space sufficient to hold a single cell */
  int nPreformatSize;   /* Size of last cell written by TransferRow() */
};

Original: https://blog.csdn.net/u010050543/article/details/128277309
Author: darkpush
Title: sqlite3结构体与实际打开的文件描述符之间的关系