ch16_03.htm
来自「by Randal L. Schwartz and Tom Phoenix I」· HTM 代码 · 共 222 行
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<html><head><title>Fixed-length Random-access Databases (Learning Perl, 3rd Edition)</title><link rel="stylesheet" type="text/css" href="../style/style1.css" /><meta name="DC.Creator" content="Randal L. Schwartz and Tom Phoenix" /><meta name="DC.Format" content="text/xml" scheme="MIME" /><meta name="DC.Language" content="en-US" /><meta name="DC.Publisher" content="O'Reilly & Associates, Inc." /><meta name="DC.Source" scheme="ISBN" content="0596001320L" /><meta name="DC.Subject.Keyword" content="stuff" /><meta name="DC.Title" content="Learning Perl, 3rd Edition" /><meta name="DC.Type" content="Text.Monograph" /></head><body bgcolor="#ffffff"><img alt="Book Home" border="0" src="gifs/smbanner.gif" usemap="#banner-map" /><map name="banner-map"><area shape="rect" coords="1,-2,616,66" href="index.htm" alt="Learning Perl, 3rd Edition" /><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch16_02.htm"><img alt="Previous" border="0" src="../gifs/txtpreva.gif" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"></a></td><td align="right" valign="top" width="228"><a href="ch16_04.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr></table></div><h2 class="sect1">16.3. Fixed-length Random-access Databases</h2><p><a name="INDEX-1057" /> <a name="INDEX-1058" />Another form of persistent datais the fixed-length, record-oriented disk file.<a href="#FOOTNOTE-350">[350]</a> In thisscheme, the data consists of a number of records of identical length.The numbering of the records is either not important or determined bysome indexing scheme.</p><blockquote class="footnote"> <a name="FOOTNOTE-350" /><p>[350]By"fixed-length," we don't mean that the file itselfis of a fixed length; it's each individual record that is of afixed length. In this section, we'll use an example file inwhich every record is 55 bytes long.</p> </blockquote><p>For example, we might want to store some information about eachbowler at Bedrock Lanes. Let's say we decide to have a seriesof records, one per bowler, in which the data holds theplayer's name, age, last five bowling scores, and the time anddate of his last game.</p><p>We need to decide upon a suitable format for this data. Let'ssay that after studying the available formats in the documentationfor <tt class="literal">pack</tt>, we decide to use 40 characters for theplayer's name, a one-byte integer for his age,<a href="#FOOTNOTE-351">[351]</a> fivetwo-byte integers for his last five scores,<a href="#FOOTNOTE-352">[352]</a> and a four-byte integer for thetimestamp of his most-recent game,<a href="#FOOTNOTE-353">[353]</a> giving a format string of <tt class="literal">"a40 CI5 L"</tt>. Each record is thus 55 bytes long. If we werereading all of the data in the database, we'd read chunks of 55bytes until we got to the end. If we wanted to go to the fifthrecord, we'd skip ahead 4 x 55 bytes (220 bytes) and readthe fifth record directly.</p><blockquote class="footnote"><a name="FOOTNOTE-351" /><p>[351]Since one byte may have 256 different values, this will holdages from 0 to 255 with ease. If Methuselah comes to bowl in Bedrock,we'll have to redesign the database. </p> </blockquote><blockquote class="footnote"> <a name="FOOTNOTE-352" /><p>[352]Wecan't use one-byte integers for the scores, because a bowlingscore can be as high as 300. Two-byte integers can hold values from 0to 65535 (if unsigned) or -32768 to 32767 (if signed). We can usesome of these extra values as special codes; for example, if a playerhas only three games on record, the other scores could be set to 9999to indicate this.</p> </blockquote><blockquote class="footnote"> <a name="FOOTNOTE-353" /><p>[353]The standard Unixtimestamp format (and the time value used by many other systems) is a32-bit integer, which fits into four bytes, of course. You'llprobably find it handy to use a module to manipulate time and dateformats.</p> </blockquote><p>Perl supports programs that use such a disk file. In order to do so,however, you need to learn a few more things, including how to:</p><ol><li><p>Open a disk file for both reading and writing</p></li><li><p>Move around in this file to an arbitrary position</p></li><li><p>Fetch data by a length rather than up to the next newline</p></li><li><p>Write data down in fixed-length blocks</p></li></ol><p>The <tt class="literal">open</tt><a name="INDEX-1059" /><a name="INDEX-1060" /> function has anadditional mode we haven't shown yet. If you use<tt class="literal">"+<"</tt> at the front of the filenameparameter's string, that is similar to using<tt class="literal">"<"</tt> to open the existing file for reading,except that it also asks for write permission on the file. Thus youcan have read/write access to the file:</p><blockquote><pre class="code">open(FRED, "<fred"); # open file fred for reading (error if file absent)open(FRED, "+<fred"); # open file fred read/write (error if file absent)</pre></blockquote><p>Similarly, <tt class="literal">"+>"</tt> says to create a new file (as<tt class="literal">">"</tt> would), but to have read access to it aswell, thus also giving read/write access:</p><blockquote><pre class="code">open(WILMA, ">wilma"); # make new file wilma (wiping out existing file)open(WILMA, "+>wilma"); # make new file wilma, but also with read access</pre></blockquote><p>Do you see the important difference between the two new modes? Bothgive read/write access to a file. But <tt class="literal">"+<"</tt> letsyou work with an existing file; it doesn't create it. Thesecond mode, <tt class="literal">"+>"</tt> isn't often useful,because it gives read/write access to a new, empty file that it hasjust created. That's mostly used for temporary (scratch) files.</p><p>Once we've got the file open, we need to move around in it. Youdo this with the <tt class="literal">seek</tt><a name="INDEX-1061" /> function:</p><blockquote><pre class="code">seek(FRED, 55 * $n, 0); # seek to start of record $n</pre></blockquote><p>The first parameter to <tt class="literal">seek</tt> is a filehandle, thesecond parameter gives the offset in bytes from the start of thefile, and the third parameter is zero.<a href="#FOOTNOTE-354">[354]</a> To get to a certain record in our file of bowling data,you'll need to skip over some other records. Since each recordis 55 bytes long, we'll multiply <tt class="literal">$n</tt> times<tt class="literal">55</tt> to find out which byte position we want. (Notethat the record numbers are thus zero-based; record zero is at thebeginning of the file.)</p><blockquote class="footnote"> <a name="FOOTNOTE-354" /><p>[354]Actually, thethird parameter is the "whence" parameter. You can use adifferent value than zero if you want to seek to a position relativeto the current position, or relative to the end of the file; see the<em class="emphasis">perlfunc</em>manpage for moreinformation. Most people will simply want to use zero here.</p></blockquote><p>Once the file pointer has been positioned with<tt class="literal">seek</tt>, the next input or output operation willstart at that position.</p><p>When we're ready to read from the file, we can't use theordinary line-input operator because that's made to read lines,not 55-byte records. There may not be a newline character in thisentire file, or it may appear in packed data in the middle of arecord. Instead, we'll use the<tt class="literal">read</tt><a name="INDEX-1062" /> function:</p><blockquote><pre class="code">my $buf; # The input buffer variablemy $number_read = read(FRED, $buf, 55);</pre></blockquote><p>As you can see, the first parameter to <tt class="literal">read</tt> isthe filehandle. The second parameter is a buffer variable; the dataread will be placed into this variable. (Yes, this is an odd way toget the result.) The third parameter is the number of bytes to read;here we've asked for 55 bytes, since that's the size ofour record. Normally, you can expect the length of<tt class="literal">$buf</tt> to be the specified number of bytes, and youcan expect that the return value (in <tt class="literal">$number_read</tt>)to be the same. But if your current position in the file is only fivebytes from the end when you request 55 bytes, you'll get onlyfive. Under normal circumstances, you'll get as many bytes asyou ask for.</p><p>Once you've got those 55 bytes, what can you do with them? Youcan unpack them (using the format we previously designed) to get thebowler's name and other information, of course:</p><blockquote><pre class="code">my($name, $age, $score_1, $score_2, $score_3, $score_4, $score_5, $when) = unpack "a40 C I5 L", $buf;</pre></blockquote><p>Since we can read the information from the file with<tt class="literal">read</tt>, can you guess how we can write it backinto the file? Sorry, it's not <tt class="literal">write</tt>; thatwas a trick question.<a href="#FOOTNOTE-355">[355]</a> You already know the correct function, which is<tt class="literal">print</tt><a name="INDEX-1063" />. But you have to be sure that the datastring is exactly the right size; if it's too large,you'll overwrite the next record's data, but ifit's too small, leftover data in the current record may bemixed with the new data. To ensure that the length is correct,we'll use <tt class="literal">pack</tt>. Let's say that Wilmahas just bowled a game and her new score is in<tt class="literal">$new_score</tt>. That will be the first of the fivemost-recent scores we keep for her (<tt class="literal">$score_5</tt>, asthe oldest one, will be discarded), and in place of<tt class="literal">$when</tt> (the timestamp of her previous game),we'll store the current time from the <tt class="literal">time</tt>function:</p><blockquote class="footnote"> <a name="FOOTNOTE-355" /><p>[355]Perl actually does have a<tt class="literal">write</tt> function, but that is used with formats,which are beyond the scope of this book. See the<em class="emphasis">perlform</em>manpage.</p></blockquote><blockquote><pre class="code">print FRED pack("a40 C I5 L", $name, $age, $new_score, $score_1, $score_2, $score_3, $score_4, time);</pre></blockquote><p>On some systems, you'll have to use <tt class="literal">seek</tt>whenever you switch from reading to writing, even if the currentposition in the file is already correct. It's not a bad idea,then, to always use <tt class="literal">seek</tt> right before reading orprinting.</p><p>Rather than use the two constant values <tt class="literal">"a40 C I5L"</tt> and <tt class="literal">55</tt> throughout the program, aswe've done here, it would generally be better to define themjust once near the top of the code. That way, if we ever need tochange the database format, we don't have to go searchingthrough our code for places where the number <tt class="literal">55</tt>appears. Here's one way you might define both of those values,using the <tt class="literal">length</tt><a name="INDEX-1064" /> function to determine the length of astring so you won't have to count bytes:<a name="INDEX-1065" /> <a name="INDEX-1066" /></p><blockquote><pre class="code">my $pack_format = "a40 C I5 L";my $pack_length = length pack($pack_format, "dummy data", 0, 1, 2, 3, 4, 5, 6);</pre></blockquote><hr width="684" align="left" /><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch16_02.htm"><img alt="Previous" border="0" src="../gifs/txtpreva.gif" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"><img alt="Home" border="0" src="../gifs/txthome.gif" /></a></td><td align="right" valign="top" width="228"><a href="ch16_04.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr><tr><td align="left" valign="top" width="228">16.2. Manipulating Data with pack and unpack</td><td align="center" valign="top" width="228"><a href="index/index.htm"><img alt="Book Index" border="0" src="../gifs/index.gif" /></a></td><td align="right" valign="top" width="228">16.4. Variable-length (Text) Databases</td></tr></table></div><hr width="684" align="left" /><img alt="Library Navigation Links" border="0" src="../gifs/navbar.gif" usemap="#library-map" /><p><p><font size="-1"><a href="copyrght.htm">Copyright © 2002</a> O'Reilly & Associates. 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