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Decoding Form Input?><html><head><title>Decoding Form Input (CGI Programming with Perl)</title><link href="../style/style1.css" type="text/css" rel="stylesheet" /><meta name="DC.Creator" content="Scott Guelich, Gunther Birznieks and Shishir Gundavaram" /><meta scheme="MIME" content="text/xml" name="DC.Format" /><meta content="en-US" name="DC.Language" /><meta content="O'Reilly & Associates, Inc." name="DC.Publisher" /><meta scheme="ISBN" name="DC.Source" content="1565924193L" /><meta name="DC.Subject.Keyword" content="stuff" /><meta name="DC.Title" content="CGI Programming with Perl" /><meta content="Text.Monograph" name="DC.Type" /></head><body bgcolor="#ffffff"><img src="gifs/smbanner.gif" alt="Book Home" usemap="#banner-map" border="0" /><map name="banner-map"><area alt="CGI Programming with Perl" href="index.htm" coords="0,0,466,65" shape="rect" /><area alt="Search this book" href="jobjects/fsearch.htm" coords="467,0,514,18" shape="rect" /></map><div class="navbar"><table border="0" width="515"><tr><td width="172" valign="top" align="left"><a href="ch04_02.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0" /></a></td><td width="171" valign="top" align="center"><a href="index.htm">CGI Programming with Perl</a></td><td width="172" valign="top" align="right"><a href="ch05_01.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr></table></div><hr align="left" width="515" /><h2 class="sect1">4.3. Decoding Form Input</h2><p><a name="INDEX-896" /> <a name="INDEX-897" /> <a name="INDEX-898" /> <a name="INDEX-899" />In order to<a name="INDEX-900" />access the information contained withinthe form, we must decode the data that is sent to us. The<a name="INDEX-901" />algorithm for decoding form data is:</p><ol><li><p>Read the query string from <tt class="literal">$ENV{QUERY_STRING}</tt>.</p></li><li><p>If the <tt class="literal">$ENV{REQUEST_METHOD}</tt> is POST, determine thesize of the request using <tt class="literal">$ENV{CONTENT_LENGTH}</tt> andread that amount of data from the standard input. Append this data tothe data read from the query string, if present (this should bejoined with "<a name="INDEX-902" />&").</p></li><li><p>Split the result on the "<a name="INDEX-903" /> <a name="INDEX-904" />&" character, which separatesname-value pairs (the format is<tt class="literal">name=value&name=value...</tt>).</p></li><li><p>Split each name-value pair on the "s" character to getthe name and value.</p></li><li><p>Decode the <a name="INDEX-905" /><a name="INDEX-906" /><a name="INDEX-907" />URL-encodedcharacters in the name and value.</p></li><li><p>Associate each name with its value(s); remember that each option namemay have multiple values.</p></li></ol><p>A form sends its parameters as the body of a <a name="INDEX-908" />POST request, or as the query string of aGET request. However, it is possible to create a form that uses thePOST method and direct it to a URL containing a query string. Thus,it is possible to get a <a name="INDEX-909" /><a name="INDEX-910" />query string with a POST request.</p><p>Here is a first attempt at our<a name="INDEX-911" />subroutine:</p><blockquote><pre class="code">sub parse_form_data { my %form_data; my $name_value; my @name_value_pairs = split /&/, $ENV{QUERY_STRING}; if ( $ENV{REQUEST_METHOD} eq 'POST' ) { my $query = ""; read( STDIN, $query, $ENV{CONTENT_LENGTH} ) == $ENV{CONTENT_LENGTH} or return undef; push @name_value_pairs, split /&/, $query; } foreach $name_value ( @name_value_pairs ) { my( $name, $value ) = split /=/, $name_value; $name =~ tr/+/ /; $name =~ s/%([\da-f][\da-f])/chr( hex($1) )/egi; $value = "" unless defined $value; $value =~ tr/+/ /; $value =~ s/%([\da-f][\da-f])/chr( hex($1) )/egi; $form_data{$name} = $value; } return %form_data;}</pre></blockquote><p>You can use<tt class="function">parse_form_data</tt><a name="INDEX-912" /> <a name="INDEX-913" /> like this:</p><blockquote><pre class="code">my %query = parse_form_data( ) or error( "Invalid request" );my $activity = $query{activity};</pre></blockquote><p>We split the query string into name-value pairs and then store eachpair in <tt class="literal">@name_value_pairs</tt>. Since the client putsampersands between key-value pairs, the<em class="emphasis">split</em><a name="INDEX-914" /> command specifies an ampersand as thedelimiter. If the request method is POST, then we also read thecontent of the request from STDIN. If the number of bytes that weread does not match the number that we expect, we return<tt class="literal">undef</tt>. This could happen if the user presses theirbrowser's Stop button while sending a request.</p><p>We then loop over each of the name-value pairs and spit them into<tt class="literal">$name</tt> and <tt class="literal">$value</tt>. It ispossible that a parameter can be passed without an equal sign or avalue. This happens for<a name="INDEX-915" /><a name="INDEX-916" /> <a name="INDEX-917" /><ISINDEX> forms, which arevirtually never used anymore, or for <a name="INDEX-918" /> <a name="INDEX-919" />manuallyconstructed URLs. By setting the <tt class="literal">$value</tt> to anempty string when it isn't defined, we avoid warnings fromPerl.</p><p>We replace each <tt class="literal">+</tt> with a<a name="INDEX-920" />space character. We then decode<a name="INDEX-921" /><a name="INDEX-922" />URL-encoded characters by replacingstrings that start with <tt class="literal">%</tt> and that are followed bytwo hexadecimal characters using the expression that we discussed in<a href="ch02_01.htm">Chapter 2, "The Hypertext Transport Protocol "</a>. We then add the name and value pair toour <a name="INDEX-923" /><a name="INDEX-924" /><a name="INDEX-925" />hash, which we return when we are done.</p><p>You may have noticed that there is a problem with our subroutine; itoccurs in the hash assignment near the end of the subroutine:</p><blockquote><pre class="code">$form_data{$name} = $value;</pre></blockquote><p>If the form has elements that share the same name, or if there is a<a name="INDEX-926" />scrolling box that supports<a name="INDEX-927" />multiple values, then it is possible forus to receive multiple values for the same name. For example, if youchoose "One" and "Two" in a select list withthe variable name "numbers," the query string would looklike:</p><blockquote><pre class="code">numbers=One&numbers=Two</pre></blockquote><p>Our example earlier would save only the last value in the hash. Thereare a couple different ways we could solve this, but neither isideal. First, we could convert the value of the hash into an arrayreference for multiple values by replacing the hash assignment withthe following lines:</p><blockquote><pre class="code">if ( exists $form_data{$name} ) { if ( ref $form_data{$name} ) { push @{ $form_data{$name} }, $value; } else { $form_data{$name} = [ $form_data{$name}, $value ]; }else { $form_data{$name} = $value;}</pre></blockquote><p>This code is somewhat complex, but because it is hidden in oursubroutine, this isn't really an issue. The real problem withthis approach is that CGI scripts using this subroutine need to knowwhich elements can have multiple values and must test each one or runthe risk of mistakenly believing the user entered something like"ARRAY(0x19abcde)", which is <a name="INDEX-928" />Perl's scalar representationof an array reference. Code to access the values of the"numbers" element would look like this:</p><blockquote><pre class="code">my %query = parse_form_data( ) or error( "Invalid request" );my @numbers = ref( $query{numbers} ) ? @{ $query{numbers} } : $query{numbers};</pre></blockquote><p>This syntax is awkward. Another approach is to store the multiplevalues as a single text string that is delimited by a certaincharacter, such as a tab or "<tt class="literal">\0</tt>". Thisis easier to code in the subroutine:</p><blockquote><pre class="code">if ( exists $form_data{$name} ) { $form_data{$name} .= "\t$value";else { $form_data{$name} = $value;}</pre></blockquote><p>It is also easier to read in the CGI script:</p><blockquote><pre class="code">my %query = parse_form_data( ) or error( "Invalid request" );my @numbers = split "\t", $query{numbers};</pre></blockquote><p>However, there is still a potential for corrupted data if the CGIscript is not expecting multiple values.</p><p>Fortunately, there is a better solution. Instead of writing an inputsubroutine ourselves, we can use CGI.pm, which provides an effectivesolution to this problem along with many <a name="INDEX-929" /> <a name="INDEX-930" /> <a name="INDEX-931" /> <a name="INDEX-932" />other <a name="INDEX-933" />useful <a name="INDEX-934" /> <a name="INDEX-935" />features. The nextchapter discusses CGI.pm.</p><hr align="left" width="515" /><div class="navbar"><table border="0" width="515"><tr><td width="172" valign="top" align="left"><a href="ch04_02.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0" /></a></td><td width="171" valign="top" align="center"><a href="index.htm"><img src="../gifs/txthome.gif" alt="Home" border="0" /></a></td><td width="172" valign="top" align="right"><a href="ch05_01.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr><tr><td width="172" valign="top" align="left">4.2. Form Tags</td><td width="171" valign="top" align="center"><a href="index/index.htm"><img src="../gifs/index.gif" alt="Book Index" border="0" /></a></td><td width="172" valign="top" align="right">5. CGI.pm</td></tr></table></div><hr align="left" width="515" /><img src="../gifs/navbar.gif" alt="Library Navigation Links" usemap="#library-map" border="0" /><p><font size="-1"><a href="copyrght.htm">Copyright © 2001</a> O'Reilly & Associates. All rights reserved.</font></p><map name="library-map"><area href="../index.htm" coords="1,1,83,102" shape="rect" /><area href="../lnut/index.htm" coords="81,0,152,95" shape="rect" /><area href="../run/index.htm" coords="172,2,252,105" shape="rect" /><area href="../apache/index.htm" coords="238,2,334,95" shape="rect" /><area href="../sql/index.htm" coords="336,0,412,104" shape="rect" /><area href="../dbi/index.htm" coords="415,0,507,101" shape="rect" /><area href="../cgi/index.htm" coords="511,0,601,99" shape="rect" /></map></body></html>⌨️ 快捷键说明
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