Windows是怎样读网络文件的

Question

"读进来的数据都变化很慢，从几秒到几分钟才变化"是什么意思呢？老兄是不是采用的多层结构，客户端Cursor的方式？如果那样，改一下cursor的方式试试。我觉得使用select * from ....应该可以得到最新数据的

Answer 1

卫星证券行情数据库几乎每秒都在更新，所以每秒读进来的数据都应该有新的，但事实到几秒甚至几分钟才变化。文件其实是用FileOpen打开读的，用Tquery只不过证实一下数据是否没变化，结果如问题所述。是否有清文件缓存的函数，强制系统真正读文件而不是读缓存？Database Desktop却做得到，不知它内部怎样读文件的？

Answer 2

顺便提一下，我用BC写了同样功能的程序，在Windows的DOS窗口下运行，问题依旧。但在纯DOS方式下运行则结果正确。是否Windows对文件缓存的管理太自信了，以至不知文件已更新？

Answer 3

   FileOpen ,你close 再open应该,就更新了:-)

Answer 4

close 再open也不行,Windows的缓存可能很大，整个文件都在缓存了(文件才97K)。读网络实时变化的文件你们可能很少试过吧:-(

Answer 5

to  lzc(阿成仔)　:
  你在那家公司做？

Answer 6

摘抄微软《Network Programming for Windows》一段话： 
“Microsoft Windows offers applications the capability to communicate over a network using built-in file system services. This is sometimes referred to as the network operating system (NOS) capability. （Windows使应用程序能通过作系统内建的文件系统服务在网络上通信。有时候，我们将之称为“网络作系统”（NOS）能力。）” 
“This functionality allows applications to use common file system API functions, such as ReadFile and WriteFile, to access remote files across a network.（应用程序可自由使用一些常规的文件系统API函数，比如ReadFile和WriteFile 等通过网络访问远程文件）”

Answer 7

这是不是和 TDataBase 使用的事务级别有关？

Answer 8

   我以前作过，你则样的程序，不过没有你这样的问题，不过我是直接，读写
.dbf的，我给你源程序，你自己分析一下吧！unit dbfinter;interface
uses sysutils;
type
  EDBFERROR=class(Exception);
  dbf_header=record
        dbf_version:char;
        update_year:shortint;
        update_month:shortint;
        update_day:shortint;
        records:longint;
        header_length:word;
        record_length:word;
        user_argument:longint;
        reserve:array [0..15] of char;
  end;
  dbf_record=record
        field_name:array[0..10] of char;
        field_type:char;
        field_offset:longint;
        field_length:shortint;
        field_decade:shortint;
        reserved_space:array [0..13] of char;
  end;
  Tdbf=class(tobject)
     private
         head:dbf_header;
         field:array[1..100] of dbf_record;
         recordbuf:array[0..1024] of char;
     public
         filename:string;
         handle:integer;
         records:longint;
         fields:integer;
         broker:boolean;
         position:integer;
         function open(s:string):integer;
         function cell(r,c:integer):string;
         function cells(r,c:integer):string;
         function celln(r:integer;name:string):string;
         function cellns(r:integer;name:string):string;
         procedure close;
     end;implementation
uses  windows,Dialogs,forms,spx_main,ustru;function Tdbf.open(s:string):integer;
var r,i:integer;
    dbferr:edbferror;
begin
    position:=-1;
    filename:=s;
    handle:=fileopen(s,fmopenread or fmsharedenynone);
    result:=handle;
    if (handle > 0) then
    begin
        broker:=false;
        fileseek(handle,0,0);
        if fileread(handle,head,32) < 32 then
        begin
            dbferr:=edbferror.Create('DBFERROR');
            broker:=true;
            main_form.logmess(LOGT_MESSAGE,0,'文件'+filename+'不存在');
            raise dbferr;
            exit;
        end;
        r:=(head.header_length-32) div 32;
        for i:=1 to r do
        begin
            if fileread(handle,field[i],32) < 32 then
            begin
                dbferr:=edbferror.Create('DBFERROR');
                broker:=true;
                main_form.logmess(LOGT_MESSAGE,0,'文件'+filename+'不存在');
                raise dbferr;
                exit;
            end;
        end;
        records:=head.records;
        fields:=r;
    end
    else
    begin
        broker:=true;
        main_form.logmess(LOGT_MESSAGE,0,'文件'+filename+'不存在');
    end;
end;function Tdbf.cell(r,c:integer):string;
var
    offset:longint;
    len:integer;
    s:string[255];
    i:integer;
    dbferr:edbferror;
begin
    if broker = true then
    begin
        open(filename);
    end;
    offset:=head.header_length + (r-1)*head.record_length;
    if position <> r then
    begin
        fileseek(handle,offset,0);
        if fileread(handle,recordbuf,head.record_length) < head.record_length then
        begin
            dbferr:=edbferror.Create('DBFERROR');
            broker:=true;
            main_form.logmess(LOGT_MESSAGE,0,'文件'+filename+'不存在');
            raise dbferr;
            exit;
        end;
        position:=r;
    end;
    if (recordbuf[0] = char(0)) then
    begin
        recordbuf[0]:=char(32);
    end;
    if (c = 0) then
    begin
        offset:=0;
        len:=1;
    end
    else
    begin
        offset:=1;
        for i:=1 to c-1 do
            offset:=offset+field[i].field_length;
        len:=field[c].field_length;
    end;
    s:=copy(recordbuf,offset+1,len);
    if ((c <> 0) and (field[c].field_type = 'N')) then
    begin
        try
            if (field[c].field_decade <> 0) then
            begin
                if pos('.-',s) = 0 then
                begin
                    s:=format('%.'+inttostr(field[c].field_decade)+'f',[strtofloat(trim(s))]);
                end
                else
                begin
                    s:='0.00';
                end;
            end
            else
            begin
                s:=inttostr(strtoint(trim(s)));
            end;
            result:=s;
        except
            result:='0';
        end;
    end
    else
       result:=s;
end;function Tdbf.cells(r,c:integer):string;
var
    offset:longint;
    len:integer;
    s:string[255];
    i:integer;
    dbferr:edbferror;
begin
    if broker = true then
    begin
        open(filename);
    end;
    offset:=head.header_length + (r-1)*head.record_length;
    if position <> r then
    begin
        fileseek(handle,offset,0);
        if fileread(handle,recordbuf,head.record_length) < head.record_length then
        begin
            dbferr:=edbferror.Create('DBFERROR');
            broker:=true;
            main_form.logmess(LOGT_MESSAGE,0,'文件'+filename+'不存在');
            raise dbferr;
            exit;
        end;
        position:=r;
    end;
    if (recordbuf[0] = char(0)) then
    begin
        recordbuf[0]:=char(32);
    end;
    if (c = 0) then
    begin
        offset:=0;
        len:=1;
    end
    else
    begin
        offset:=1;
        for i:=1 to c-1 do
            offset:=offset+field[i].field_length;
        len:=field[c].field_length;
    end;
    s:=copy(recordbuf,offset+1,len);
    result:=s;
end;function Tdbf.celln(r:integer;name:string):string;
var i,c:integer;
begin
    if broker = true then
    begin
        open(filename);
    end;
    for i:=1 to fields do
    begin
        if (uppercase(field[i].field_name) = uppercase(name)) then
        begin
            c:=i;
            result:=cell(r,c);
            exit;
        end;
    end;
    result:='';
end;function Tdbf.cellns(r:integer;name:string):string;
var i,c:integer;
begin
    if broker = true then
    begin
        open(filename);
    end;
    for i:=1 to fields do
    begin
        if (uppercase(field[i].field_name) = uppercase(name)) then
        begin
            c:=i;
            result:=cells(r,c);
            exit;
        end;
    end;
    result:='';
end;procedure Tdbf.close;
begin
    if handle > 0 then
    begin
        try
           fileclose(handle);
        except
        end;
    end;
end;end.