SQLite

    }else{
      pp = &p->pNext;
    }
  }
}

/*
** Open a shared-memory area.  This implementation uses mmapped files.








**
** When opening a new shared-memory file, if no other instances of that
** file are currently open, in this process or in other processes, then
** the file must be truncated to zero length or have its header cleared.
*/
static int unixShmOpen(
  sqlite3_vfs *pVfs,    /* The VFS */
  const char *zName,    /* Name of file to mmap */
  sqlite3_shm **pShm    /* Write the unixShm object created here */
){
  struct unixShm *p = 0;             /* The connection to be opened */
  struct unixShmFile *pFile = 0;     /* The underlying mmapped file */
  int rc;                            /* Result code */
  struct unixFileId fid;             /* Unix file identifier */

  struct stat sStat;                 /* Result from stat() an fstat() */


  /* Allocate space for the new sqlite3_shm object */


  p = sqlite3_malloc( sizeof(*p) );
  if( p==0 ) return SQLITE_NOMEM;
  memset(p, 0, sizeof(*p));










  /* Look to see if there is an existing unixShmFile that can be used.
  ** If no matching unixShmFile currently exists, create a new one.
  */
  unixEnterMutex();
  rc = stat(zName, &sStat);
  if( rc==0 ){
    memset(&fid, 0, sizeof(fid));
    fid.dev = sStat.st_dev;
    fid.ino = sStat.st_ino;
    for(pFile = unixShmFileList; pFile; pFile=pFile->pNext){
      if( memcmp(&pFile->fid, &fid, sizeof(fid))==0 ) break;
    }
  }
  if( pFile==0 ){
    int nName = strlen(zName);
    pFile = sqlite3_malloc( sizeof(*pFile) + nName + 1 );
    if( pFile==0 ){
      rc = SQLITE_NOMEM;
      goto shm_open_err;
    }
    memset(pFile, 0, sizeof(*pFile));
    pFile->zFilename = (char*)&pFile[1];
    memcpy(pFile->zFilename, zName, nName+1);
    pFile->h = -1;
    pFile->pNext = unixShmFileList;
    unixShmFileList = pFile;

    pFile->mutex = sqlite3_mutex_alloc(SQLITE_MUTEX_FAST);
    if( pFile->mutex==0 ){
      rc = SQLITE_NOMEM;
      goto shm_open_err;
    }
    pFile->mutexBuf = sqlite3_mutex_alloc(SQLITE_MUTEX_FAST);
    if( pFile->mutexBuf==0 ){
      rc = SQLITE_NOMEM;
      goto shm_open_err;
    }

    pFile->h = open(zName, O_RDWR|O_CREAT, 0664);
    if( pFile->h<0 ){
      rc = SQLITE_CANTOPEN_BKPT;
      goto shm_open_err;
    }

    rc = fstat(pFile->h, &sStat);
    if( rc ){

  return SQLITE_OK;

  /* Jump here on any error */
shm_open_err:
  unixShmPurge();
  sqlite3_free(p);
  sqlite3_free(pFile);

  *pShm = 0;
  unixLeaveMutex();
  return rc;
}

/*
** Close a connectioon to shared-memory.

*/
static int unixShmClose(sqlite3_shm *pSharedMem){
  unixShm *p;            /* The connection to be closed */
  unixShmFile *pFile;    /* The underlying shared-memory file */
  unixShm **pp;          /* For looping over sibling connections */

  if( pSharedMem==0 ) return SQLITE_OK;
  p = (struct unixShm*)pSharedMem;
  pFile = p->pFile;

  /* If pFile->nRef has reached 0, then close the underlying
  ** shared-memory file, too */
  unixEnterMutex();
  assert( pFile->nRef>0 );
  pFile->nRef--;
  if( pFile->nRef==0 ){

    unixShmPurge();
  }
  unixLeaveMutex();

  return SQLITE_OK;
}

  sqlite3_mutex_leave(pFile->mutex);
  OSTRACE(("SHM-LOCK shmid-%d, pid-%d got %s\n",
           p->id, getpid(), azLkName[p->lockState]));
  if( pGotLock ) *pGotLock = p->lockState;
  return rc;
}

/*
** Delete a shared-memory segment from the system.
*/
static int unixShmDelete(sqlite3_vfs *pVfs, const char *zName){
  return pVfs->xDelete(pVfs, zName, 0);
}

#else
# define unixShmOpen    0
# define unixShmSize    0
# define unixShmGet     0
# define unixShmRelease 0
# define unixShmLock    0
# define unixShmClose   0
# define unixShmDelete  0
#endif /* #ifndef SQLITE_OMIT_WAL */

/*
************************ End of sqlite3_vfs methods ***************************
******************************************************************************/

/******************************************************************************

    unixSleep,            /* xSleep */                      \
    unixCurrentTime,      /* xCurrentTime */                \
    unixGetLastError,     /* xGetLastError */               \
    unixShmOpen,          /* xShmOpen */                    \
    unixShmSize,          /* xShmSize */                    \
    unixShmGet,           /* xShmGet */                     \
    unixShmRelease,       /* xShmRelease */                 \
    0,                    /* xShmPush */                    \
    0,                    /* xShmPull */                    \
    unixShmLock,          /* xShmLock */                    \
    unixShmClose,         /* xShmClose */                   \
    unixShmDelete,        /* xShmDelete */                  \
    0,                    /* xRename */                     \
    0,                    /* xCurrentTimeInt64 */           \
  }

  /*
  ** All default VFSes for unix are contained in the following array.
  **

  ** The methods above are in version 1 of the sqlite_vfs object
  ** definition.  Those that follow are added in version 2 or later
  */
  int (*xShmOpen)(sqlite3_vfs*, const char *zName, sqlite3_shm**);
  int (*xShmSize)(sqlite3_shm*, int reqSize, int *pNewSize);
  int (*xShmGet)(sqlite3_shm*, int reqMapSize, int *pMapSize, void**);
  int (*xShmRelease)(sqlite3_shm*);
  int (*xShmPush)(sqlite3_shm*);
  int (*xShmPull)(sqlite3_shm*);
  int (*xShmLock)(sqlite3_shm*, int desiredLock, int *gotLock);
  int (*xShmClose)(sqlite3_shm*);
  int (*xShmDelete)(sqlite3_vfs*, const char *zName);
  int (*xRename)(sqlite3_vfs*, const char *zOld, const char *zNew, int dirSync);
  int (*xCurrentTimeInt64)(sqlite3_vfs*, sqlite3_int64*);
  /*
  ** The methods above are in versions 1 and 2 of the sqlite_vfs object.
  ** New fields may be appended in figure versions.  The iVersion
  ** value will increment whenever this happens. 
  */

static int devsymCurrentTime(sqlite3_vfs*, double*);

static int devsymShmOpen(sqlite3_vfs *, const char *, sqlite3_shm **);
static int devsymShmSize(sqlite3_shm *, int , int *);
static int devsymShmGet(sqlite3_shm *, int , int *, void **);
static int devsymShmRelease(sqlite3_shm *);
static int devsymShmLock(sqlite3_shm *, int , int *);
static int devsymShmClose(sqlite3_shm *);
static int devsymShmDelete(sqlite3_vfs *, const char *);

static sqlite3_vfs devsym_vfs = {
  2,                     /* iVersion */
  sizeof(devsym_file),      /* szOsFile */
  DEVSYM_MAX_PATHNAME,      /* mxPathname */
  0,                     /* pNext */
  DEVSYM_VFS_NAME,          /* zName */

  devsymSleep,              /* xSleep */
  devsymCurrentTime,        /* xCurrentTime */
  0,                        /* xGetLastError */
  devsymShmOpen,
  devsymShmSize,
  devsymShmGet,
  devsymShmRelease,
  0,
  0,
  devsymShmLock,
  devsymShmClose,
  devsymShmDelete,
  0,
  0,
};

static sqlite3_io_methods devsym_io_methods = {
  1,                            /* iVersion */
  devsymClose,                      /* xClose */

}
static int devsymShmRelease(sqlite3_shm *p){
  return g.pVfs->xShmRelease(p);
}
static int devsymShmLock(sqlite3_shm *p, int desiredLock, int *gotLock){
  return g.pVfs->xShmLock(p, desiredLock, gotLock);
}
static int devsymShmClose(sqlite3_shm *p){
  return g.pVfs->xShmClose(p);
}
static int devsymShmDelete(sqlite3_vfs *pVfs, const char *zName){
  return g.pVfs->xShmDelete(g.pVfs, zName);
}

/*
** This procedure registers the devsym vfs with SQLite. If the argument is
** true, the devsym vfs becomes the new default vfs. It is the only publicly
** available function in this file.
*/
void devsym_register(int iDeviceChar, int iSectorSize){
  if( g.pVfs==0 ){
    g.pVfs = sqlite3_vfs_find(0);
    devsym_vfs.szOsFile += g.pVfs->szOsFile;
    devsym_vfs.xShmOpen = (g.pVfs->xShmOpen ? devsymShmOpen : 0);
    devsym_vfs.xShmSize = (g.pVfs->xShmSize ? devsymShmSize : 0);
    devsym_vfs.xShmGet = (g.pVfs->xShmGet ? devsymShmGet : 0);
    devsym_vfs.xShmRelease = (g.pVfs->xShmRelease ? devsymShmRelease : 0);
    devsym_vfs.xShmLock = (g.pVfs->xShmLock ? devsymShmLock : 0);
    devsym_vfs.xShmClose = (g.pVfs->xShmClose ? devsymShmClose : 0);
    devsym_vfs.xShmDelete = (g.pVfs->xShmDelete ? devsymShmDelete : 0);
    sqlite3_vfs_register(&devsym_vfs, 0);
  }
  if( iDeviceChar>=0 ){
    g.iDeviceChar = iDeviceChar;
  }else{
    g.iDeviceChar = 0;
  }

    fsDlSym,                                    /* xDlSym */
    fsDlClose,                                  /* xDlClose */
    fsRandomness,                               /* xRandomness */
    fsSleep,                                    /* xSleep */
    fsCurrentTime,                              /* xCurrentTime */
    0,                                          /* xShmOpen */
    0,                                          /* xShmSize */
    0,                                          /* xShmPush */
    0,                                          /* xShmPull */
    0,                                          /* xShmLock */
    0,                                          /* xShmClose */
    0,                                          /* xShmDelete */
    0,                                          /* xRename */
    0                                           /* xCurrentTimeInt64 */
  }, 
  0,                                            /* pFileList */

  u32 iCallback;             /* Value to pass to log callback (or 0) */
  sqlite3_shm *pWIndex;      /* The open wal-index file */
  int szWIndex;              /* Size of the wal-index that is mapped in mem */
  u32 *pWiData;              /* Pointer to wal-index content in memory */
  u8 lockState;              /* SQLITE_SHM_xxxx constant showing lock state */
  u8 readerType;             /* SQLITE_SHM_READ or SQLITE_SHM_READ_FULL */
  WalIndexHdr hdr;           /* Wal-index for current snapshot */

};


/*
** This structure is used to implement an iterator that iterates through
** all frames in the log in database page order. Where two or more frames
** correspond to the same database page, the iterator visits only the 

  assert( zDb );
  if( pVfs->xShmOpen==0 ) return SQLITE_CANTOPEN_BKPT;

  /* Allocate an instance of struct Wal to return. */
  *ppWal = 0;
  nWal = strlen(zDb);
  pRet = (Wal*)sqlite3MallocZero(sizeof(Wal) + pVfs->szOsFile + nWal+11);
  if( !pRet ) goto wal_open_out;
  pRet->pVfs = pVfs;
  pRet->pFd = (sqlite3_file *)&pRet[1];
  zWal = pVfs->szOsFile + (char*)pRet->pFd;
  sqlite3_snprintf(nWal+11, zWal, "%s-wal-index", zDb);
  rc = pVfs->xShmOpen(pVfs, zWal, &pRet->pWIndex);
  if( rc ) goto wal_open_out;

  /* Open file handle on the write-ahead log file. */
  zWal[nWal+4] = 0;
  flags = (SQLITE_OPEN_READWRITE|SQLITE_OPEN_CREATE|SQLITE_OPEN_MAIN_JOURNAL);
  rc = sqlite3OsOpen(pVfs, zWal, pRet->pFd, flags, &flags);

wal_open_out:
  if( rc!=SQLITE_OK ){
    if( pRet ){
      pVfs->xShmClose(pRet->pWIndex);
      sqlite3OsClose(pRet->pFd);
      sqlite3_free(pRet);
    }
  }
  *ppWal = pRet;
  return rc;
}

      rc = walCheckpoint(pWal, pFd, sync_flags, zBuf);
      if( rc==SQLITE_OK ){
        isDelete = 1;
      }
      walIndexUnmap(pWal);
    }

    pWal->pVfs->xShmClose(pWal->pWIndex);
    sqlite3OsClose(pWal->pFd);
    if( isDelete ){
      int nWal;
      char *zWal = &((char *)pWal->pFd)[pWal->pVfs->szOsFile];
      sqlite3OsDelete(pWal->pVfs, zWal, 0);
      nWal = sqlite3Strlen30(zWal);
      memcpy(&zWal[nWal], "-index", 7);
      pWal->pVfs->xShmDelete(pWal->pVfs, zWal);
    }
    sqlite3_free(pWal);
  }
  return rc;
}

/*