SQLite

** point to the same locking structure.  The locking structure keeps
** a reference count (so we will know when to delete it) and a "cnt"
** field that tells us its internal lock status.  cnt==0 means the
** file is unlocked.  cnt==-1 means the file has an exclusive lock.
** cnt>0 means there are cnt shared locks on the file.
**
** Any attempt to lock or unlock a file first checks the locking
** structure.  The fcntl() system call is only invoked to set a
** POSIX lock if the internal lock structure transitions between
** a locked and an unlocked state.
*/

/*
** An instance of the following structure serves as the key used
** to locate a particular lockInfo structure given its inode.
*/
struct inodeKey {
  dev_t dev;   /* Device number */
  ino_t ino;   /* Inode number */
};

/*
** An instance of the following structure is allocated for each inode.
** A single inode can have multiple file descriptors, so each OsFile
** structure contains a pointer to an instance of this object and this
** object keeps a count of the number of OsFiles pointing to it.
*/
struct lockInfo {
  struct inodeKey key;  /* The lookup key */
  int cnt;              /* 0: unlocked.  -1: write lock.  1...: read lock. */
  int nRef;             /* Number of pointers to this structure */
};

/*
** This hash table maps inodes (in the form of inodeKey structures) into
** pointers to lockInfo structures.
*/
static Hash lockHash = { SQLITE_HASH_BINARY, 0, 0, 0, 0, 0 };

/*
** Given a file descriptor, locate a lockInfo structure that describes

  int *pReadonly
){
#if OS_UNIX
  id->fd = open(zFilename, O_RDWR|O_CREAT, 0644);
  if( id->fd<0 ){
    id->fd = open(zFilename, O_RDONLY);
    if( id->fd<0 ){
      return SQLITE_CANTOPEN;
    }
    *pReadonly = 1;
  }else{
    *pReadonly = 0;
  }
  sqliteOsEnterMutex();
  id->pLock = findLockInfo(id->fd);

  }
  return SQLITE_OK;
#endif
#if OS_WIN
  HANDLE h;
  int fileflags;
  if( delFlag ){
    fileflags = FILE_ATTRIBUTE_TEMPORARY | FILE_FLAG_RANDOM_ACCESS
                     | FILE_FLAG_DELETE_ON_CLOSE;
  }else{
    fileflags = FILE_FLAG_RANDOM_ACCESS;
  }
  h = CreateFile(zFilename,
     GENERIC_READ | GENERIC_WRITE,
     0,

      int n = sqliteRandomByte() % sizeof(zChars);
      zBuf[j++] = zChars[n];
    }
    zBuf[j] = 0;
    if( !sqliteOsFileExists(zBuf) ) break;
  }
#endif
  return SQLITE_OK;
}

/*
** Close a file
*/
int sqliteOsClose(OsFile *id){
#if OS_UNIX

#define MX_LOCKBYTE 0xFFF0

#if OS_WIN

// get the platform id to decide how to calculate the lock offset

int mkPlatformId(void){

 static long init=0;
 static long lock=0;

 static int pid=VER_PLATFORM_WIN32_WINDOWS;
 OSVERSIONINFOA info;

 while (!init) {
  if (InterlockedIncrement(&lock)==1)
   {
    info.dwOSVersionInfoSize=sizeof(info);
    if (GetVersionEx(&info)) pid=info.dwPlatformId;
    init=1;
   }
  else
   Sleep(1);
  }
 return pid;
}

// locks and unlocks beyond eof. uses platformid to move the lock as far as possible.
int mklock(HANDLE h, WORD base, WORD size)
{
 if (mkPlatformId()==VER_PLATFORM_WIN32_WINDOWS)
  return LockFile(h,0xFFFF0000+base,0,size,0);

  return UnlockFile(h,0xFFFF0000+base,0,size,0);
 else
  return UnlockFile(h,base,0xFFFFFFFF,size,0);
}

//obtain the sync lock on a handle

void synclock(HANDLE h){
 while (!mklock(h,0,1)) Sleep(1);
}

void syncunlock(HANDLE h){
 mkunlock(h,0,1);
}

#endif

/*
** Change the status of the lock on the file "id" to be a readlock.

#if OS_WIN
  int rc;
  if( id->locked>0 ){
    rc = SQLITE_OK;
  }else{
    int lk = (sqliteRandomInteger() & 0x7ffffff)%MX_LOCKBYTE + 1;
    int res;

    synclock(id->h);
    if (id->locked<0) mkunlock(id->h,1,MX_LOCKBYTE); // release write lock if we have it
    res=mklock(id->h,lk,1);
    syncunlock(id->h);

    if( res ){
      id->locked = lk;
      rc = SQLITE_OK;
    }else{
      rc = SQLITE_BUSY;
    }
  }

#endif
#if OS_WIN
  int rc;
  if( id->locked<0 ){
    rc = SQLITE_OK;
  }else{
    int res;

    synclock(id->h);
    if (id->locked>0) mkunlock(id->h,id->locked,1); // release read lock
    res=mklock(id->h,1,MX_LOCKBYTE);
    syncunlock(id->h);

    if(res){
      id->locked = -1;
      rc = SQLITE_OK;
    }else{
      rc = SQLITE_BUSY;
    }
  }