CGI(3)

💅 CGI(3) — Handle Common Gateway Interface requests and responses 💅

🌸 NAME 🌸

CGI — Handle Common Gateway Interface requests and responses. This is the CGI(3) manual page, bestie 💖.

✨ SYNOPSIS ✨

okay so here's the whole vibe in code, no changes, copy-paste safe:

           use strict;
           use warnings;

           use CGI;

               # create a CGI object (query) for use
           my $q = CGI->new;

           # Process an HTTP request
           my @values  = $q->multi_param('form_field');
           my $value   = $q->param('param_name');

           my $fh      = $q->upload('file_field');

           my $riddle  = $q->cookie('riddle_name');
           my %answers = $q->cookie('answers');

           # Prepare various HTTP responses
           print $q->header();
           print $q->header('application/json');

           my $cookie1 = $q->cookie(
               -name  => 'riddle_name',
               -value => "The Sphynx's Question"
           );

           my $cookie2 = $q->cookie(
               -name  => 'answers',
               -value => \%answers
           );

           print $q->header(
               -type    => 'image/gif',
               -expires => '+3d',
               -cookie  => [ $cookie1,$cookie2 ]
           );

           print $q->redirect('http://somewhere.else/in/movie/land');

🦋 DESCRIPTION 🦋

CGI.pm is a stable, complete and mature solution for processing and preparing HTTP requests and responses. Major features include processing form submissions, file uploads, reading and writing cookies, query string generation and manipulation, and processing and preparing HTTP headers. iconic behavior, honestly ✨

CGI.pm performs very well in a vanilla CGI.pm environment and also comes with built-in support for mod_perl and mod_perl2 as well as FastCGI.

It has the benefit of having developed and refined over 20 years with input from dozens of contributors and being deployed on thousands of websites. CGI.pm was included in the perl distribution from perl v5.4 to v5.20, and it has now been removed from the perl core... 👀

💀 CGI.pm HAS BEEN REMOVED FROM THE PERL CORE 💀

<http://perl5.git.perl.org/perl.git/commitdiff/e9fa5a80>

If you upgrade to a new version of perl or if you rely on a system or vendor perl and get an updated version of perl through a system update, then you will have to install CGI.pm yourself with cpan/cpanm/a vendor package/manually. To make this a little easier the CGI::Fast module has been split into its own distribution, meaning you do not need access to a compiler to install CGI.pm.

The rationale for this decision is that CGI.pm is no longer considered good practice for developing web applications, including quick prototyping and small web scripts. There are far better, cleaner, quicker, easier, safer, more scalable, more extensible, more modern alternatives available at this point in time. These will be documented with CGI::Alternatives.

For more discussion on the removal of CGI.pm from core please see:

<http://www.nntp.perl.org/group/perl.perl5.porters/2013/05/msg202130.html>

Note that the v4 releases of CGI.pm will retain back compatibility as much as possible, however you may need to make some minor changes to your code if you are using deprecated methods or some of the more obscure features of the module. If you plan to upgrade to v4.00 and beyond you should read the Changes file for more information and test your code against CGI.pm before deploying it.

🚫 HTML Generation functions should no longer be used 🚫

All HTML generation functions within CGI.pm are no longer being maintained. Any issues, bugs, or patches will be rejected unless they relate to fundamentally broken page rendering. periodt.

The rationale for this is that the HTML generation functions of CGI.pm are an obfuscation at best and a maintenance nightmare at worst. You should be using a template engine for better separation of concerns. See CGI::Alternatives for an example of using CGI.pm with the Template::Toolkit module.

These functions, and perldoc for them, are considered deprecated, they are no longer being maintained and no fixes or features for them will be accepted. They will, however, continue to exist in CGI.pm without any deprecation warnings ("soft" deprecation) so you can continue to use them if you really want to. All documentation for these functions has been moved to CGI::HTML::Functions.

🌈 Programming style 🌈

There are two styles of programming with CGI.pm, an object-oriented (OO) style and a function-oriented style. You are recommended to use the OO style as CGI.pm will create an internal default object when the functions are called procedurally and you will not have to worry about method names clashing with perl builtins. slay, fewer collisions 💅

In the object-oriented style you create one or more CGI objects and then use object methods to create the various elements of the page. Each CGI object starts out with the list of named parameters that were passed to your CGI script by the server. You can modify the objects, save them to a file or database and recreate them. Because each object corresponds to the "state" of the CGI script, and because each object's parameter list is independent of the others, this allows you to save the state of the script and restore it later.

For example, using the object oriented style:

           #!/usr/bin/env perl

           use strict;
           use warnings;

           use CGI;                             # load CGI routines

           my $q = CGI->new;                    # create new CGI object
           print $q->header;                    # create the HTTP header

In the function-oriented style, there is one default CGI object that you rarely deal with directly. Instead you just call functions to retrieve CGI parameters, manage cookies, and so on. The following example is identical to above, in terms of output, but uses the function-oriented interface. The main differences are that we now need to import a set of functions into our name space (usually the "standard" functions), and we don't need to create the CGI object.

           #!/usr/bin/env perl

           use strict;
           use warnings;

           use CGI qw/:standard/;           # load standard CGI routines
           print header();                  # create the HTTP header

The examples in this document mainly use the object-oriented style. See HOW TO IMPORT FUNCTIONS for important information on function-oriented programming in CGI.pm.

🎀 Calling CGI.pm routines 🎀

Most CGI.pm routines accept several arguments, sometimes as many as 20 optional ones! To simplify this interface, all routines use a named argument calling style that looks like this:

           print $q->header(
               -type    => 'image/gif',
               -expires => '+3d',
           );

Each argument name is preceded by a dash. Neither case nor order matters in the argument list: -type, -Type, and -TYPE are all acceptable. In fact, only the first argument needs to begin with a dash. If a dash is present in the first argument CGI.pm assumes dashes for the subsequent ones. lowkey flexible, highkey useful ✨

Several routines are commonly called with just one argument. In the case of these routines you can provide the single argument without an argument name. header() happens to be one of these routines. In this case, the single argument is the document type.

           print $q->header('text/html');

Other such routines are documented below.

Sometimes named arguments expect a scalar, sometimes a reference to an array, and sometimes a reference to a hash. Often, you can pass any type of argument and the routine will do whatever is most appropriate. For example, the param() routine is used to set a CGI parameter to a single or a multi-valued value. The two cases are shown below:

           $q->param(
               -name  => 'veggie',
               -value => 'tomato',
           );

           $q->param(
               -name  => 'veggie',
               -value => [ qw/tomato tomahto potato potahto/ ],
           );

Many routines will do something useful with a named argument that it doesn't recognize. For example, you can produce non-standard HTTP header fields by providing them as named arguments:

           print $q->header(
               -type            => 'text/html',
               -cost            => 'Three smackers',
               -annoyance_level => 'high',
               -complaints_to   => 'bit bucket',
           );

This will produce the following nonstandard HTTP header:

           HTTP/1.0 200 OK
           Cost: Three smackers
           Annoyance-level: high
           Complaints-to: bit bucket
           Content-type: text/html

Notice the way that underscores are translated automatically into hyphens. the way that _ becomes - is so iconic 💅

🦋 Creating a new query object (object-oriented style) 🦋

           my $q = CGI->new;

This will parse the input (from POST, GET and DELETE methods) and store it into a perl5 object called $q. Note that because the input parsing happens at object instantiation you have to set any CGI package variables that control parsing before you call CGI->new.

Any filehandles from file uploads will have their position reset to the beginning of the file.

🌸 Creating a new query object from an input file 🌸

           my $q = CGI->new( $input_filehandle );

If you provide a file handle to the new() method, it will read parameters from the file (or STDIN, or whatever). The file can be in any of the forms describing below under debugging (i.e. a series of newline delimited TAG=VALUE pairs will work). Conveniently, this type of file is created by the save() method (see below). Multiple records can be saved and restored.

Perl purists will be pleased to know that this syntax accepts references to file handles, or even references to filehandle globs, which is the "official" way to pass a filehandle. You can also initialize the CGI object with a FileHandle or IO::File object.

If you are using the function-oriented interface and want to initialize CGI state from a file handle, the way to do this is with restore_parameters(). This will (re)initialize the default CGI object from the indicated file handle.

           open( my $in_fh,'<',"test.in") || die "Couldn't open test.in for read: $!";
           restore_parameters( $in_fh );
           close( $in_fh );

You can also initialize the query object from a hash reference:

           my $q = CGI->new( {
               'dinosaur' => 'barney',
               'song'     => 'I love you',
               'friends'  => [ qw/ Jessica George Nancy / ]
           } );

or from a properly formatted, URL-escaped query string:

           my $q = CGI->new('dinosaur=barney&color=purple');

or from a previously existing CGI object (currently this clones the parameter list, but none of the other object-specific fields, such as autoescaping):

           my $old_query = CGI->new;
           my $new_query = CGI->new($old_query);

To create an empty query, initialize it from an empty string or hash:

           my $empty_query = CGI->new("");

              -or-

           my $empty_query = CGI->new({});

🎀 Fetching a list of keywords from the query 🎀

           my @keywords = $q->keywords

Okay so if the script got invoked as the result of an ISINDEX search, you can grab those parsed keywords as an array with the keywords() method. Cute, right? 💅

Fetching the names of all the parameters passed to your script 🎀

my @names = $q->multi_param

my @names = $q->param

So if your script got invoked with a parameter list (think name1=value1&name2=value2&name3=value3), then param() / multi_param() will hand you back the parameter names as a list. 📋

But if the script got invoked as an ISINDEX script and contains a string with no ampersands in it (like value1+value2+value3), then you get exactly one parameter, its name is keywords, and it's holding the +-delimited keywords. iykyk 🌟

The array of parameter names comes back in the same order the browser submitted them. Usually that's the same order the parameters are defined in the form — but bestie, note that this isn't part of the spec, so it's not guaranteed. Don't build your whole life on it. 😭

Fetching the value or values of a single named parameter 🦋

my @values = $q->multi_param('foo');

    -or-

my $value = $q->param('foo');

    -or-

my @values = $q->param('foo'); # list context, discouraged and will raise
                               # a warning (use ->multi_param instead)

Pass param() / multi_param() a single argument and it fetches the value of that named parameter. If the parameter is multivalued (classic case: multiple selections in a scrolling list), you can ask for an array. Otherwise the method returns the first value. That's it, that's the vibe. ✨

Warning, and I mean it — calling param() in list context can lead to vulnerabilities if you don't sanitise user input, because it's possible to inject other param keys and values into your code. That is literally why multi_param() exists: to make it obvious that a list is coming back. But note that param() can still be called in list context and will return a list for back compatibility. 🙈

Here's the kind of code that gets you got — the call to param gets evaluated in list context, so it can possibly inject extra keys and values into the hash:

my %user_info = (
    id   => 1,
    name => $q->param('name'),
);

The fix? Force scalar context on the call to ->param by prefixing it with scalar:

name => scalar $q->param('name'),

If you call param() in list context with an argument, a warning gets raised by CGI.pm. You can silence that warning by setting $CGI::LIST_CONTEXT_WARN to 0, or — honestly better — just use the multi_param() method instead. 🫶

If a value isn't given in the query string, like in the queries name1=&name2=, it comes back as an empty string.

And if the parameter doesn't exist at all? Then param() returns undef in scalar context, and the empty list in list context. Poof, gone. 💨

Setting the value(s) of a named parameter 🌸

$q->param('foo','an','array','of','values');

This sets the value for the named parameter 'foo' to an array of values. It's one way to change the value of a field AFTER the script has already been invoked once. Glow-up for your form. 💖

param() also recognizes a named parameter style of calling, described in more detail later:

$q->param(
    -name   => 'foo',
    -values => ['an','array','of','values'],
);

$q->param(
    -name  => 'foo',
    -value => 'the value',
);

Appending additional values to a named parameter ⭐

$q->append(
    -name   =>'foo',
    -values =>['yet','more','values'],
);

This adds a value, or a whole list of values, to the named parameter. If the parameter already exists, the new values get tacked onto the end. If it doesn't exist yet, the parameter gets created. Main character energy: it just shows up. 👑

Note that this method only recognizes the named argument calling syntax. No shortcuts, no vibes-based guessing. 💅

Importing all parameters into a namespace 🧚

$q->import_names('R');

This creates a series of variables in the 'R' namespace. For example, $R::foo, @R:foo. For keyword lists, a variable @R::keywords will appear. If you don't give a namespace, this method assumes 'Q'.

WARNING: don't import anything into 'main' — that is a major security risk! 😭

NOTE 1: Variable names get transformed as necessary into legal perl variable names. All non-legal characters are transformed into underscores. If you need to keep the original names, you should use the param() method instead to access CGI variables by name.

Honestly? You should probably not use this method at all, given the caveats above and the security risks. Consider this your gentle anti-recommendation. 🙈

Deleting a parameter completely 💀

$q->delete('foo','bar','baz');

This completely clears a list of parameters. It's sometimes useful for resetting parameters that you don't want passed down between script invocations. Clean slate, iconic. ✨

If you're using the function call interface, use "Delete()" instead, so you don't collide with perl's built-in delete operator. Name collision? Not on our watch. 🫶

Deleting all parameters 🌈

$q->delete_all();

This clears the CGI object completely. It might be useful to make sure all the defaults are taken when you create a fill-out form. Fresh start, no baggage. 🦋

Use Delete_all() instead if you're using the function call interface.

Handling non-urlencoded arguments 🍓

If the POSTed data is not of type application/x-www-form-urlencoded or multipart/form-data, then the POSTed data will not be processed — instead it comes back as-is in a parameter named POSTDATA. To retrieve it, use code like this:

my $data = $q->param('POSTDATA');

Likewise, if PUTed and PATCHed data can be retrieved with code like this:

my $data = $q->param('PUTDATA');

my $data = $q->param('PATCHDATA');

(If you don't know what the preceding means, worry not. It only affects people trying to use CGI for XML processing and other specialized tasks. Not your problem bestie 💅)

PUTDATA/POSTDATA/PATCHDATA are also available via upload_hook, and as file uploads via the -putdata_upload option. 🎀

Direct access to the parameter list 🔥

$q->param_fetch('address')->[1] = '1313 Mockingbird Lane';
unshift @{$q->param_fetch(-name=>'address')},'George Munster';

If you need access to the parameter list in a way that isn't covered by the methods in the previous sections, you can get a direct reference to it by calling the param_fetch() method with the name of the parameter. It returns an array reference to the named parameter, which you can then manipulate in any way you like. Go feral. 😈

You can also use a named argument style using the -name argument.

Fetching the parameter list as a hash 👑

my $params = $q->Vars;
print $params->{'address'};
my @foo = split("\0",$params->{'foo'});
my %params = $q->Vars;

use CGI ':cgi-lib';
my $params = Vars();

Many people want to fetch the entire parameter list as a hash where the keys are the names of the CGI parameters and the values are the parameters' values. That's exactly what the Vars() method does. 💖

Call it in a scalar context and it returns the parameter list as a tied hash reference — changing a key changes the value of the parameter in the underlying CGI parameter list. Call it in a list context and it returns the parameter list as an ordinary hash. That one lets you read the contents of the parameter list, but not change it. Read-only bestie. 🙈

When you use this, the thing you must watch out for is multivalued CGI parameters. A hash can't distinguish between scalar and list context, so multivalued parameters come back as a packed string separated by the "\0" (null) character. You have to split that packed string to get at the individual values. This is the convention introduced long ago by Steve Brenner in his cgi-lib.pl module for perl version 4, and it may be replaced in future versions with array references. History! 🏛️

If you want to use Vars() as a function, import the :cgi-lib set of function calls (also see the section on CGI-LIB compatibility).

Saving the state of the script to a file 💾

$q->save(\*FILEHANDLE)

This writes the current state of the form to the filehandle you provide. You can read it back in by providing a filehandle to the new() method. Note that the filehandle can be a file, a pipe, or whatever. Versatile. ✨

The format of the saved file is:

NAME1=VALUE1
NAME1=VALUE1'
NAME2=VALUE2
NAME3=VALUE3
=

Both name and value are URL escaped. Multi-valued CGI parameters are represented as repeated names. A session record is delimited by a single = symbol. You can write out multiple records and read them back in with several calls to new. You can even do this across several sessions by opening the file in append mode, which lets you create primitive guest books, or keep a history of users' queries. Nostalgic internet vibes fr fr 🫶

Here's a short example of creating multiple session records:

use strict;
use warnings;
use CGI;

open (my $out_fh,'>>','test.out') || die "Can't open test.out: $!";
my $records = 5;
for ( 0 .. $records ) {
    my $q = CGI->new;
    $q->param( -name => 'counter',-value => $_ );
    $q->save( $out_fh );
}
close( $out_fh );

# reopen for reading
open (my $in_fh,'<','test.out') || die "Can't open test.out: $!";
while (!eof($in_fh)) {
    my $q = CGI->new($in_fh);
    print $q->param('counter'),"\n";
}

The file format used for save/restore is identical to the one used by the Whitehead Genome Center's data exchange format "Boulderio", and can be manipulated and even databased using Boulderio utilities. See Boulder for further details. 🧬

If you want to use this method from the function-oriented (non-OO) interface, the exported name for this method is save_parameters().

Retrieving cgi errors 😭

Errors can happen while processing user input — especially when processing uploaded files. When these errors occur, CGI stops processing and returns an empty parameter list. You can test for the existence and nature of errors with the cgi_error() function. The error messages are formatted as HTTP status codes, so you can either fold the error text into a page, or use it as the value of the HTTP status:

if ( my $error = $q->cgi_error ) {
    print $q->header( -status => $error );
    print "Error: $error";
    exit 0;
}

When using the function-oriented interface (see the next section), errors may only occur the first time you call param(). Be ready for this! 🚨

Using the function-oriented interface 🌟

To use the function-oriented interface, you must specify which CGI.pm routines, or sets of routines, to import into your script's namespace. There's a small overhead that comes with this importation, but it isn't much. Barely a blip. 💫

use strict;
use warnings;

use CGI qw/ list of methods /;

The listed methods get imported straight into your current package, so you can just call them — no need to build a CGI object first. Like, at all. Here's how you'd import param() and header() and then use them directly:

use strict;
use warnings;

use CGI qw/ param header /;
print header('text/plain');
my $zipcode = param('zipcode');

More often though, you'll import whole sets of functions by referring to the group by name. Every function set starts with a : character, like :cgi (which is the CGI protocol handling squad).

Here's the function-set menu, bestie:

• :cgi — imports all the CGI-handling methods, like param(), path_info() and all those besties.
• :all — imports literally every available method. For the full list, go peek at the CGI.pm code where the %EXPORT_TAGS variable lives. (N.B. the :cgi-lib imports are not included in the :all import — you have to import :cgi-lib yourself to get those, iykyk)

Heads up: in the interests of execution speed, CGI.pm does not use the standard Exporter syntax for specifying load symbols. This may change in the future. ✨

🎀 Pragmas 🎀

So besides the function sets, there are also a bunch of pragmas you can import. Pragmas always start with a hyphen and they change the way CGI.pm functions in various ways. And the vibes are immaculate, because pragmas, function sets, and individual functions can all be imported in the same use() line. For example, this use statement imports the cgi set of functions and turns on debugging mode (pragma -debug):

use strict;
use warninigs;
use CGI qw/ :cgi -debug /;

The current pragma lineup is:

• -no_undef_params — keeps CGI.pm from including undef params in the parameter list. No ghosts, no gremlins. 💀
• -utf8 — makes CGI.pm treat all parameters as text strings instead of binary strings (see perlunitut for the difference), assuming UTF-8 for the encoding.

CGI.pm does the decoding from the UTF-8 encoded input data, and it restricts this decoding to input text — as distinct from binary upload data, which is left completely untouched. Because of that, a ':utf8' layer must not be used on STDIN. 🫶

If you don't use this option, you can manually pick which fields are expected to return utf-8 strings and convert them with code like this:

use strict;
use warnings;

use CGI;
use Encode qw/ decode /;

my $cgi   = CGI->new;
my $param = $cgi->param('foo');
$param    = decode( 'UTF-8',$param );

• -putdata_upload, -postdata_upload, -patchdata_upload — makes $cgi->param('PUTDATA');, $cgi->param('PATCHDATA');, and $cgi->param('POSTDATA'); act like file uploads named PUTDATA, PATCHDATA, and POSTDATA. See "Handling non-urlencoded arguments" and "Processing a file upload field". PUTDATA/POSTDATA/PATCHDATA are also available via upload_hook.
• -nph — makes CGI.pm produce a header appropriate for an NPH (no parsed header) script. You may need to do other things as well to tell the server that the script is NPH. See the discussion of NPH scripts below. 💅
• -newstyle_urls — separates the name=value pairs in CGI parameter query strings with semicolons instead of ampersands. Like this:

?name=fred;age=24;favorite_color=3

Semicolon-delimited query strings are always accepted, and will be emitted by self_url() and query_string(). newstyle_urls became the default in version 2.64. Slay.

• -oldstyle_urls — separates the name=value pairs in CGI parameter query strings with ampersands instead of semicolons. This is no longer the default — it's giving retro.
• -no_debug — turns off the command-line processing features. If you want to run a CGI.pm script from the command line and you don't want it reading CGI parameters from the command line or STDIN, use this pragma:

use CGI qw/ -no_debug :standard /;

• -debug — turns on full debugging. On top of reading CGI arguments from the command-line processing, CGI.pm will pause and try to read arguments from STDIN, producing the message "(offline mode: enter name=value pairs on standard input)" features.

See the section on debugging for more details.

---

💖 GENERATING DYNAMIC DOCUMENTS 💖

Most of CGI.pm's functions are all about creating documents on the fly. Generally you produce the HTTP header first, then the document itself. CGI.pm hands you functions for generating HTTP headers of various types.

Each of these functions produces a fragment of HTTP which you can print out directly so the browser processes it, append it to a string, or save it to a file for later. Iconic, honestly. 🌈

🌸 Creating a standard http header 🌸

Normally the very first thing you'll do in any CGI script is print out an HTTP header. This tells the browser what type of document to expect, and gives other optional info like the language, expiration date, and whether to cache the document. The header can also be manipulated for special purposes, such as server push and pay per view pages.

use strict;
use warnings;

use CGI;

my $cgi = CGI->new;

print $cgi->header;

    -or-

print $cgi->header('image/gif');

    -or-

print $cgi->header('text/html','204 No response');

    -or-

print $cgi->header(
    -type       => 'image/gif',
    -nph        => 1,
    -status     => '402 Payment required',
    -expires    => '+3d',
    -cookie     => $cookie,
    -charset    => 'utf-8',
    -attachment => 'foo.gif',
    -Cost       => '$2.00'
);

header() returns the Content-type: header. You can provide your own MIME type if you want, otherwise it defaults to text/html. An optional second parameter specifies the status code plus a human-readable message. For example, you can say 204, "No response" to create a script that tells the browser to do literally nothing at all. 😭 Note that RFC 2616 expects the human-readable phrase to be there as well as the numeric status code — both, bestie.

That last example shows the named argument style for passing arguments to the CGI methods using named parameters. Recognized parameters are -type, -status, -expires, and -cookie. Any other named parameters get their initial hyphens stripped and get turned into header fields, which means you can specify any HTTP header you desire. Internal underscores get turned into hyphens:

print $cgi->header( -Content_length => 3002 );

Most browsers will not cache the output from CGI scripts — every time the browser reloads the page, the script is invoked anew. You can change this behavior with the -expires parameter. When you specify an absolute or relative expiration interval with this parameter, some browsers and proxy servers will cache the script's output until the indicated expiration date. The following forms are all valid for the -expires field:

+30s                                  30 seconds from now
+10m                                  ten minutes from now
+1h                                   one hour from now
-1d                                   yesterday (i.e. "ASAP!")
now                                   immediately
+3M                                   in three months
+10y                                  in ten years time
Thursday, 25-Apr-2018 00:40:33 GMT    at the indicated time & date

The -cookie parameter generates a header that tells the browser to provide a "magic cookie" during all subsequent transactions with your script. Some cookies have a special format that includes fun attributes like expiration time. Use the cookie() method to create and retrieve session cookies. 🍪

The -nph parameter, if set to a true value, will issue the correct headers to work with a NPH (no-parse-header) script. This is important to use with certain servers that expect all their scripts to be NPH.

The -charset parameter can control the character set sent to the browser. If you don't provide it, it defaults to ISO-8859-1. As a side effect, this sets the charset() method as well. Note that the default being ISO-8859-1 may not make sense for all content types, e.g.:

Content-Type: image/gif; charset=ISO-8859-1

In the case above you need to pass -charset => '' to prevent the default from being used. Sneaky little default. 🙈

The -attachment parameter can turn the page into an attachment. Instead of displaying the page, some browsers will prompt the user to save it to disk. The value of the argument is the suggested name for the saved file. For this to actually work, you may have to set the -type to "application/octet-stream".

The -p3p parameter will add a P3P tag to the outgoing header. The parameter can be an arrayref or a space-delimited string of P3P tags. Like:

print $cgi->header( -p3p => [ qw/ CAO DSP LAW CURa / ] );
print $cgi->header( -p3p => 'CAO DSP LAW CURa' );

Either way, the outgoing header gets formatted as:

P3P: policyref="/w3c/p3p.xml" cp="CAO DSP LAW CURa"

CGI.pm will accept valid multi-line headers when each line is separated with a CRLF value ("\r\n" on most platforms) followed by at least one space. For example:

print $cgi->header( -ingredients => "ham\r\n\seggs\r\n\sbacon" );

Invalid multi-line header input will trigger an exception. ⚠️ And when multi-line headers are received, CGI.pm will always output them back as a single line, according to the folding rules of RFC 2616: the newlines get removed, while the white space stays put.

🦋 Generating a redirection header 🦋

print $q->redirect( 'http://somewhere.else/in/movie/land' );

Sometimes you don't want to produce a document yourself at all — you just want to bounce the browser somewhere else, maybe picking a URL based on the time of day or who the user is. Main character energy: you're just the messenger.

The redirect() method redirects the browser to a different URL. If you use redirection like this, you should not print out a header as well. Pick a lane. ✨

You are advised to use full URLs (absolute with respect to the current URL, or even including the http: or ftp: part) in redirection requests, because relative URLs are resolved by the user agent of the client, so they may not do what you want or expect them to do.

You can also use named arguments:

print $q->redirect(
    -uri    => 'http://somewhere.else/in/movie/land',
    -nph    => 1,
    -status => '301 Moved Permanently'
);

All named arguments recognized by header() are also recognized by redirect(). However, most HTTP headers — including those generated by -cookie and -target — are ignored by the browser. 💅

okay so the -nph parameter — if you set it to a true value, it'll send out the right headers for a NPH (no-parse-header) script. This is super important for certain servers like Microsoft IIS that expect all their scripts to be NPH, no cap. 💅

the -status parameter sets the status of the redirect. HTTP has a bunch of different redirection status codes, and if you don't specify one, the default is 302, which means "moved temporarily." You can change it to another status code if you want. ✨

Also note: the human-readable phrase is also expected to be there to conform with RFC 2616, section 6.1 — so don't skip it, bestie.

🎀 Creating a self-referencing url that preserves state information 🎀

my $myself = $q->self_url;
print qq(<a href="$myself">I'm talking to myself.</a>);

self_url() will return a URL that, when selected, will re-invoke this script with all its state information intact. This is most useful when you want to jump around within the document using internal anchors but you don't want to disrupt the current contents of the form(s). Something like this will do the trick:

my $myself = $q->self_url;
print "<a href=\"$myself#table1\">See table 1</a>";
print "<a href=\"$myself#table2\">See table 2</a>";
print "<a href=\"$myself#yourself\">See for yourself</a>";

If you want more control over what's returned, use the url() method instead.

You can also retrieve a query string representation of the current object state with query_string():

my $the_string = $q->query_string();

The behavior of calling query_string is currently undefined when the HTTP method is something other than GET. 😬

If you want to get the query string as set in the webserver (like the environment variable), you can call env_query_string().

🌈 Obtaining the script's url 🌈

my $full_url      = url();
my $full_url      = url( -full =>1 );  # alternative syntax
my $relative_url  = url( -relative => 1 );
my $absolute_url  = url( -absolute =>1 );
my $url_with_path = url( -path_info => 1 );
my $url_path_qry  = url( -path_info => 1, -query =>1 );
my $netloc        = url( -base => 1 );

url() returns the script's URL in a variety of formats. Called without any arguments, it returns the full form of the URL, including host name and port number:

http://your.host.com/path/to/script.cgi

You can modify this format with the following named arguments:

• -absolute — If true, produce an absolute URL, e.g. /path/to/script.cgi.
• -relative — Produce a relative URL. This is useful if you want to re-invoke your script with different parameters. For example: script.cgi.
• -full — Produce the full URL, exactly as if called without any arguments. This overrides the -relative and -absolute arguments.
• -path (-path_info) — Append the additional path information to the URL. This can be combined with -full, -absolute or -relative. -path_info is provided as a synonym.
• -query (-query_string) — Append the query string to the URL. This can be combined with -full, -absolute or -relative. -query_string is provided as a synonym.
• -base — Generate just the protocol and net location, as in http://www.foo.com:8000.
• -rewrite — If Apache's mod_rewrite is turned on, then the script name and path info probably won't match the request that the user sent. Set -rewrite => 1 (default) to return URLs that match what the user sent (the original request URI). Set -rewrite => 0 to return URLs that match the URL after the mod_rewrite rules have run.

💅 Mixing post and url parameters 💅

my $color = url_param('color');

It is possible for a script to receive CGI parameters in the URL as well as in the fill-out form by creating a form that POSTs to a URL containing a query string (a "?" mark followed by arguments). The param() method will always return the contents of the POSTed fill-out form, ignoring the URL's query string. To retrieve URL parameters, call the url_param() method. Use it in the same way as param(). The main difference is that it allows you to read the parameters, but not set them.

Under no circumstances will the contents of the URL query string interfere with similarly-named CGI parameters in POSTed forms. If you try to mix a URL query string with a form submitted with the GET method, the results will not be what you expect. 🙅‍♀️

If running from the command line, url_param will not pick up any parameters given on the command line.

🦋 Processing a file upload field 🦋

✨ Basics ✨

When the form is processed, you can retrieve an IO::File compatible handle for a file upload field like this:

use autodie;

# undef may be returned if it's not a valid file handle
if ( my $io_handle = $q->upload('field_name') ) {
    open ( my $out_file,'>>','/usr/local/web/users/feedback' );
    while ( my $bytesread = $io_handle->read($buffer,1024) ) {
        print $out_file $buffer;
    }
}

In a list context, upload() will return an array of filehandles. This makes it possible to process forms that use the same name for multiple upload fields. 🎀

If you want the entered file name for the file, you can just call param():

my $filename = $q->param('field_name');

Different browsers will return slightly different things for the name. Some browsers return the filename only. Others return the full path to the file, using the path conventions of the user's machine. Regardless, the name returned is always the name of the file on the user's machine, and is unrelated to the name of the temporary file that CGI.pm creates during upload spooling (see below).

When a file is uploaded the browser usually sends along some information along with it in the Content-Type (MIME type) and Content-Disposition (filename) headers. To retrieve this information, call uploadInfo(). It returns a reference to a hash containing all the document headers.

my $filehandle = $q->upload( 'uploaded_file' );
my $type       = $q->uploadInfo( $filehandle )->{'Content-Type'};
if ( $type ne 'text/html' ) {
    die "HTML FILES ONLY!";
}

Note that you must use ->upload or ->param to get the file-handle to pass into uploadInfo as internally this is represented as a File::Temp object (which is what will be returned by ->upload or ->param). When using ->Vars you will get the literal filename rather than the File::Temp object, which will not return anything when passed to uploadInfo. So don't use ->Vars. 💀

When uploading multiple files, call ->param() in list context to retrieve a list of filehandles that you can use when calling ->uploadInfo.

my @filehandles = $q->param('uploaded_file');

for my $fh (@filehandles) {
  my $info = $q->uploadInfo($fh);
  ...
}

If you are using a machine that recognizes "text" and "binary" data modes, be sure to understand when and how to use them (see the Camel book). Otherwise you may find that binary files are corrupted during file uploads. 😭

🎀 Accessing the temp files directly 🎀

When processing an uploaded file, CGI.pm creates a temporary file on your hard disk and passes you a file handle to that file. After you are finished with the file handle, CGI.pm unlinks (deletes) the temporary file. If you need to you can access the temporary file directly. You can access the temp file for a file upload by passing the file name to the tmpFileName() method:

my $filehandle  = $q->upload( 'uploaded_file' );
my $tmpfilename = $q->tmpFileName( $filehandle );

As with ->uploadInfo, using the reference returned by ->upload or ->param is preferred, although unlike ->uploadInfo, plain filenames also work if possible for backwards compatibility.

The temporary file will be deleted automatically when your program exits unless you manually rename it or set $CGI::UNLINK_TMP_FILES to 0. On some operating systems (such as Windows NT), you will need to close the temporary file's filehandle before your program exits. Otherwise the attempt to delete the temporary file will fail. 🫡

💜 Changes in temporary file handling (v4.05+) 💜

CGI.pm had its temporary file handling significantly refactored, this logic is now all deferred to File::Temp (which is wrapped in a compatibility object, CGI::File::Temp - DO NOT USE THIS PACKAGE DIRECTLY). As a consequence the PRIVATE_TEMPFILES variable has been removed along with deprecation of the private_tempfiles routine and complete removal of the CGITempFile package. The $CGITempFile::TMPDIRECTORY is no longer used to set the temp directory, refer to the perldoc for File::Temp if you want to override the default settings in that package (the TMPDIR env variable is still available on some platforms). For Windows platforms the temporary directory order remains as before: TEMP > TMP > WINDIR ( > TMPDIR ) so if you have any of these in use in existing scripts they should still work.

The Fh package still exists but does nothing, the CGI::File::Temp class is a subclass of both File::Temp and the empty Fh package, so if you have any code that checks that the filehandle isa Fh this should still work.

When you get the internal file handle you will receive a File::Temp object, this should be transparent as File::Temp isa IO::Handle and isa IO::Seekable meaning it behaves as previously. If you are doing anything out of the ordinary with regards to temp files you should test your code before deploying this update and refer to the File::Temp documentation for more information.

🌸 Handling interrupted file uploads 🌸

There are occasionally problems involving parsing the uploaded file. This usually happens when the user presses "Stop" before the upload is finished. In this case, CGI.pm will return undef for the name of the uploaded file and set cgi_error() to the string "400 Bad request (malformed multipart POST)". This error message is designed so that you can incorporate it into a status code to be sent to the browser. Example:

my $file = $q->upload( 'uploaded_file' );
if ( !$file && $q->cgi_error ) {
    print $q->header( -status => $q->cgi_error );
    exit 0;
}

🍓 Progress bars for file uploads and avoiding temp files 🍓

CGI.pm gives you low-level access to file upload management through a file upload hook. You can use this feature to completely turn off the temp file storage of file uploads, or potentially write your own file upload progress meter. 🔥

This is much like the UPLOAD_HOOK facility available in Apache::Request, with the exception that the first argument to the callback is an Apache::Upload object, here it's the remote filename.

my $q = CGI->new( \&hook [,$data [,$use_tempfile]] );

The $data field is optional — it's your little VIP pass for smuggling configuration info (like a database handle, bestie) into your hook callback. 🎟️

The $use_tempfile field is a flag that lets you switch CGI.pm's use of a temporary disk-based file on and off during file upload. If you set it to a FALSE value (default is true, okurr), then $q->param('uploaded_file') will still hand you back a typeglob you can use to get at a filehandle and the filename of the uploaded file — except that filehandle will point at an empty file. The existence of your hook is what makes CGI.pm skip writing to that filehandle (which is lowkey exactly what you meant when you set $use_tempfile off). 💅

The uploadInfo() method can be used on the typeglob you get back when you called $q->param('upload_file'), and it returns info about the uploaded file(s). Got multiple files uploading? Then use the param() method in list context to grab all of the typeglobs. 🫶

my (@filehandles) = $cgi->param('upfile');

foreach my $fh (@filehandles) {
  my $info = $cgi->uploadInfo($fh);
  ...
}

If you're living that function-oriented interface life, call the CGI::upload_hook() method before you call param() or any other CGI functions:

CGI::upload_hook( \&hook [,$data [,$use_tempfile]] );

This method is not exported by default, ngl. You'll have to import it explicitly if you want to use it without the CGI:: prefix. 🙈

🩹 Troubleshooting file uploads on Windows

So if you're running CGI.pm on a Windows platform and you notice that binary files come out slightly larger when uploaded, but text files stay exactly the same size? Bestie. You forgot to activate binary mode on the output filehandle. 😭 Be sure to call binmode() on any handle you create to write the uploaded file to disk. Periodt.

🧓 Older ways to process file uploads

This section is here purely for completeness — if you're building a shiny new application with CGI.pm, you can absolutely skip it. 🏃‍♀️💨

The original way to process file uploads with CGI.pm was to use param(). The value it returns has a dual nature: it's both a file name and a lightweight filehandle. That dual nature is a whole mess if you're following the recommended practice of having use strict in your code, because perl will complain when you try to use a string as a filehandle. And more seriously: it's possible for the remote user to type garbage into the upload field, in which case what you get from param() is not a filehandle at all, but a string. 💀

To solve that problem, the upload() method was added, and it always returns a lightweight filehandle. That generally slaps, but it'll have trouble interoperating with some other modules because the file handle is not derived from IO::File. Which brings us to the current recommendation given above: call the handle() method on the file handle returned by upload(). That upgrades the handle to an IO::File. It's a big win for compatibility for a small penalty of loading IO::File the first time you call it. ✨

---

🍪 HTTP COOKIES

CGI.pm has several methods that support cookies. Obsessed, honestly.

A cookie is a name=value pair much like the named parameters in a CGI query string. CGI scripts create one or more cookies and send them to the browser in the HTTP header. The browser maintains a list of cookies that belong to a particular Web server, and returns them to the CGI script during subsequent interactions. 🍪✨

On top of the required name=value pair, each cookie has several optional attributes:

1. an expiration time — This is a time/date string (in a special GMT format) that indicates when a cookie expires. The cookie will be saved and returned to your script until this expiration date is reached, even if the user exits the browser and restarts it. If an expiration date isn't specified, the cookie will remain active until the user quits the browser.
2. a domain — This is a partial or complete domain name for which the cookie is valid. The browser will return the cookie to any host that matches the partial domain name. For example, if you specify a domain name of .capricorn.com, then the browser will return the cookie to Web servers running on any of the machines www.capricorn.com, www2.capricorn.com, feckless.capricorn.com, etc. Domain names must contain at least two periods to prevent attempts to match on top level domains like .edu. If no domain is specified, then the browser will only return the cookie to servers on the host the cookie originated from.
3. a path — If you provide a cookie path attribute, the browser will check it against your script's URL before returning the cookie. For example, if you specify the path /cgi-bin, then the cookie will be returned to each of the scripts /cgi-bin/tally.pl, /cgi-bin/order.pl, and /cgi-bin/customer_service/complain.pl, but not to the script /cgi-private/site_admin.pl. By default, path is set to /, which causes the cookie to be sent to any CGI script on your site.
4. a "secure" flag — If the "secure" attribute is set, the cookie will only be sent to your script if the CGI request is occurring on a secure channel, such as SSL. 🔒

The interface to HTTP cookies is the cookie() method:

my $cookie = $q->cookie(
    -name    => 'sessionID',
    -value   => 'xyzzy',
    -expires => '+1h',
    -path    => '/cgi-bin/database',
    -domain  => '.capricorn.org',
    -secure  => 1
);

print $q->header( -cookie => $cookie );

cookie() creates a new cookie. Its parameters include:

• -name — The name of the cookie (required). This can be any string at all. Although browsers limit their cookie names to non-whitespace alphanumeric characters, CGI.pm removes this restriction by escaping and unescaping cookies behind the scenes. 🎀
• -value — The value of the cookie. This can be any scalar value, array reference, or even hash reference. For example, you can store an entire hash into a cookie this way:

my $cookie = $q->cookie(
    -name  => 'family information',
    -value => \%childrens_ages
);

• -path — The optional partial path for which this cookie will be valid, as described above.
• -domain — The optional partial domain for which this cookie will be valid, as described above.
• -expires — The optional expiration date for this cookie. The format is as described in the section on the header() method:

"+1h"  one hour from now

• -secure — If set to true, this cookie will only be used within a secure SSL session.

The cookie created by cookie() must be incorporated into the HTTP header within the string returned by the header() method:

use strict;
use warnings;

use CGI;

my $q      = CGI->new;
my $cookie = ...
print $q->header( -cookie => $cookie );

Want multiple cookies? Give header() an array reference:

my $cookie1 = $q->cookie(
    -name  => 'riddle_name',
    -value => "The Sphynx's Question"
);

my $cookie2 = $q->cookie(
    -name  => 'answers',
    -value => \%answers
);

print $q->header( -cookie => [ $cookie1,$cookie2 ] );

To retrieve a cookie, request it by name by calling the cookie() method without the -value parameter. This example uses the object-oriented form:

my $riddle  = $q->cookie('riddle_name');
my %answers = $q->cookie('answers');

Cookies created with a single scalar value, such as the "riddle_name" cookie, will be returned in that form. Cookies with array and hash values can also be retrieved. 💖

The cookie and CGI namespaces are separate, bestie. If you have a parameter named 'answers' and a cookie named 'answers', the values retrieved by param() and cookie() are independent of each other. But it's simple to turn a CGI parameter into a cookie, and vice-versa:

# turn a CGI parameter into a cookie
my $c = cookie( -name => 'answers',-value => [$q->param('answers')] );
# vice-versa
$q->param( -name => 'answers',-value => [ $q->cookie('answers')] );

If you call cookie() without any parameters, it will return a list of the names of all cookies passed to your script:

my @cookies = $q->cookie();

See the cookie.cgi example script for some ideas on how to use cookies effectively. 👑

💾 $CGI::COOKIE_CACHE

If set to a non-negative integer, this variable will cause CGI.pm to use the cached cookie details from the previous call to cookie(). By default this cache is off, to retain backwards compatibility. 🫡

---

🐛 DEBUGGING

If you're running the script from the command line or in the perl debugger, you can pass the script a list of keywords or parameter=value pairs on the command line or from standard input — and no, you don't have to worry about tricking your script into reading from environment variables. Iconic. ✨

You can pass keywords like this:

your_script.pl keyword1 keyword2 keyword3

or this:

your_script.pl keyword1+keyword2+keyword3

or this:

your_script.pl name1=value1 name2=value2

or this:

your_script.pl name1=value1&name2=value2

To turn off this feature, use the -no_debug pragma. 🚫

To test the POST method, you may enable full debugging with the -debug pragma. This will allow you to feed newline-delimited name=value pairs to the script on standard input. 🔥

When debugging, you can use quotes and backslashes to escape characters in the familiar shell manner, letting you place spaces and other funny characters in your parameter=value pairs:

your_script.pl "name1='I am a long value'" "name2=two\ words"

Finally, you can set the path info for the script by prefixing the first name/value parameter with the path followed by a question mark (?):

your_script.pl /your/path/here?name1=value1&name2=value2

🔍 FETCHING ENVIRONMENT VARIABLES

okay so bestie, a bunch of the genuinely useful environment variables can be pulled straight through this interface 💖 the methods go like this:

• Accept() — returns a list of MIME types the remote browser is into. If you hand it a single argument matching a MIME type, like Accept('text/html'), it gives you back a floating point value for how much the browser vibes with that type, from 0.0 (hard pass) to 1.0 (obsessed). Glob types like text/* in the browser's accept list get handled correctly too, no cap. ✨

heads up: the capitalization changed between version 2.43 and 2.44 so it wouldn't clash with perl's accept() function. periodt.

• raw_cookie() — returns the HTTP_COOKIE variable. Cookies have a whole special format situation, and this call just hands you the raw form (?cookie dough 🍓). Peep cookie() if you want the ways of setting and retrieving cooked cookies.

Called with no parameters, raw_cookie() returns the packed cookie structure. You can separate it into individual cookies by splitting on the character sequence "; ". Called with the name of a cookie, it retrieves the unescaped form of the cookie. You can use the regular cookie() method to get the names, or use the raw_fetch() method from the CGI::Cookie module.

• env_query_string() — returns the QUERY_STRING variable. lowkey important note: this is the original value as set in the environment by the webserver and (possibly) not the same value as returned by query_string(), which represents the object state. 😭
• user_agent() — returns the HTTP_USER_AGENT variable. Give this method a single argument and it'll attempt to pattern match on it, so you can do stuff like user_agent(Mozilla).
• path_info() — returns additional path information from the script URL. like, fetching /cgi-bin/your_script/additional/stuff will result in path_info() returning "/additional/stuff". 💅

NOTE: the Microsoft Internet Information Server is broken with respect to additional path information. If you use the perl DLL library, the IIS server will attempt to execute the additional path information as a perl script. If you use the ordinary file associations mapping, the path information will be present in the environment, but incorrect. The best thing to do is to avoid using additional path information in CGI scripts destined for use with IIS. A best attempt has been made to make CGI.pm do the right thing. bestie, it tried its best 🫶

• path_translated() — as per path_info() but returns the additional path information translated into a physical path, e.g. "/usr/local/etc/httpd/htdocs/additional/stuff".

The Microsoft IIS is broken with respect to the translated path as well. 💀

• remote_host() — returns either the remote host name or IP address if the former is unavailable.
• remote_ident() — returns the name of the remote user (as returned by identd) or undef if not set.
• remote_addr() — returns the remote host IP address, or 127.0.0.1 if the address is unavailable.
• request_uri() — returns the interpreted pathname of the requested document or CGI (relative to the document root). Or undef if not set.
• script_name() — return the script name as a partial URL, for self-referring scripts. main character script energy 👑
• referer() — return the URL of the page the browser was viewing prior to fetching your script.
• auth_type() — return the authorization/verification method in use for this script, if any.
• server_name() — returns the name of the server, usually the machine's host name.
• virtual_host() — when using virtual hosts, returns the name of the host that the browser attempted to contact.
• server_port() — return the port that the server is listening on.
• server_protocol() — returns the protocol and revision of the incoming request, or defaults to HTTP/1.0 if this is not set.
• virtual_port() — like server_port() except that it takes virtual hosts into account. use this one when you're running with virtual hosts ✨
• server_software() — returns the server software and version number.
• remote_user() — return the authorization/verification name used for user verification, if this script is protected.
• user_name() — attempt to obtain the remote user's name, using a variety of different techniques. May not work in all browsers. 🙈
• request_method() — returns the method used to access your script, usually one of 'POST', 'GET' or 'HEAD'. If running from the command line it will be undef.
• content_type() — returns the content_type of data submitted in a POST, generally multipart/form-data or application/x-www-form-urlencoded.

---

http()

Called with no arguments it returns the list of HTTP environment variables, including such things as HTTP_USER_AGENT, HTTP_ACCEPT_LANGUAGE, and HTTP_ACCEPT_CHARSET, corresponding to the like-named HTTP header fields in the request. Called with the name of an HTTP header field, it returns its value. Capitalization and the use of hyphens versus underscores are not significant, so you literally cannot mess this up (affectionate 💜).

For example, all three of these examples are equivalent:

my $requested_language = $q->http('Accept-language');

my $requested_language = $q->http('Accept_language');

my $requested_language = $q->http('HTTP_ACCEPT_LANGUAGE');

https()

The same as http(), but operates on the HTTPS environment variables present when the SSL protocol is in effect. Can be used to determine whether SSL is turned on. 🔥

---

🚀 USING NPH SCRIPTS

NPH, or "no-parsed-header", scripts bypass the server completely by sending the complete HTTP header directly to the browser. It's giving "I'll handle it myself, thanks". This has slight performance benefits, but it's of most use for taking advantage of HTTP extensions that are not directly supported by your server, such as server push and PICS headers. 👑

Servers use a variety of conventions for designating CGI scripts as NPH. Many Unix servers look at the beginning of the script's name for the prefix "nph-". The Macintosh WebSTAR server and Microsoft's Internet Information Server, in contrast, try to decide whether a program is an NPH script by examining the first line of script output. like, different strokes bestie 🦋

CGI.pm supports NPH scripts with a special NPH mode. When in this mode, CGI.pm will output the necessary extra header information when the header() and redirect() methods are called.

The Microsoft Internet Information Server requires NPH mode. As of version 2.30, CGI.pm will automatically detect when the script is running under IIS and put itself into this mode. You do not need to do this manually, although it won't hurt anything if you do. 💅

In the use statement

Simply add the -nph pragma to the list of symbols to be imported into your script:

use CGI qw(:standard -nph)

By calling the nph() method:

Call nph() with a non-zero parameter at any point after using CGI.pm in your program.

CGI->nph(1)

By using -nph parameters in the header() and redirect() statements:

print header(-nph=>1);

---

🌊 SERVER PUSH

CGI.pm provides four simple functions for producing multipart documents of the type needed to implement server push. These functions were graciously provided by Ed Jordan <ed@fidalgo.net>. To import these into your namespace, you must import the ":push" set. You are also advised to put the script into NPH mode and to set $| to 1 to avoid buffering problems. 🫶

Here is a simple script that demonstrates server push:

#!/usr/bin/env perl

use strict;
use warnings;

use CGI qw/:push -nph/;

$| = 1;
print multipart_init( -boundary=>'----here we go!' );
for (0 .. 4) {
    print multipart_start( -type=>'text/plain' ),
        "The current time is ",scalar( localtime ),"\n";
    if ($_ < 4) {
        print multipart_end();
    } else {
        print multipart_final();
    }
    sleep 1;
}

This script initializes server push by calling multipart_init(). It then enters a loop in which it begins a new multipart section by calling multipart_start(), prints the current local time, and ends a multipart section with multipart_end(). It then sleeps a second, and begins again. On the final iteration, it ends the multipart section with multipart_final() rather than with multipart_end(). iconic behaviour, honestly 🎀

multipart_init()

multipart_init( -boundary => $boundary, -charset => $charset );

Initialize the multipart system. The -boundary argument specifies what MIME boundary string to use to separate parts of the document. If not provided, CGI.pm chooses a reasonable boundary for you.

The -charset provides the character set, if not provided this will default to ISO-8859-1. ✨

multipart_start()

multipart_start( -type => $type, -charset => $charset );

Start a new part of the multipart document using the specified MIME type and charset. If not specified, text/html ISO-8859-1 is assumed.

multipart_end()

multipart_end()

End a part. You must remember to call multipart_end() once for each multipart_start(), except at the end of the last part of the multipart document when multipart_final() should be called instead of multipart_end(). fr fr, don't skip this 😭

multipart_final()

multipart_final()

End all parts. You should call multipart_final() rather than multipart_end() at the end of the last part of the multipart document. 🏁

Users interested in server push applications should also have a look at the CGI::Push module. iykyk 💜

🛡️ AVOIDING DENIAL OF SERVICE ATTACKS 🛡️

okay so bestie, here’s the scary part: a potential problem with CGI.pm is that, by default, it attempts to process form POSTings no matter how large they are. A wily hacker could attack your site by sending a CGI script a huge POST of many gigabytes. CGI.pm will attempt to read the entire POST into a variable, growing hugely in size until it runs out of memory. While the script attempts to allocate the memory the system may slow down dramatically. This is a form of denial of service attack. 💀

Another possible attack is for the remote user to force CGI.pm to accept a huge file upload. CGI.pm will accept the upload and store it in a temporary directory even if your script doesn’t expect to receive an uploaded file. CGI.pm will delete the file automatically when it terminates, but in the meantime the remote user may have filled up the server’s disk space, causing problems for other programs. 😭

The best way to avoid denial of service attacks is to limit the amount of memory, CPU time and disk space that CGI scripts can use. Some Web servers come with built-in facilities to accomplish this. In other cases, you can use the shell limit or ulimit commands to put ceilings on CGI resource usage. 🫶

CGI.pm also has some simple built-in protections against denial of service attacks, but you must activate them before you can use them. These take the form of two global variables in the CGI name space:

🧨 $CGI::POST_MAX

If set to a non-negative integer, this variable puts a ceiling on the size of POSTings, in bytes. If CGI.pm detects a POST that is greater than the ceiling, it will immediately exit with an error message. This value will affect both ordinary POSTs and multipart POSTs, meaning that it limits the maximum size of file uploads as well. You should set this to a reasonably high value, such as 10 megabytes. 🔥

🙈 $CGI::DISABLE_UPLOADS

If set to a non-zero value, this will disable file uploads completely. Other fill-out form values will work as usual. ✨

To use these variables, set the variable at the top of the script, right after the "use" statement:

#!/usr/bin/env perl

use strict;
use warnings;

use CGI;

$CGI::POST_MAX = 1024 * 1024 * 10;  # max 10MB posts
$CGI::DISABLE_UPLOADS = 1;          # no uploads

An attempt to send a POST larger than $POST_MAX bytes will cause param() to return an empty CGI parameter list. You can test for this event by checking cgi_error(), either after you create the CGI object or, if you are using the function-oriented interface, call <param()> for the first time. If the POST was intercepted, then cgi_error() will return the message "413 POST too large". 👑

This error message is actually defined by the HTTP protocol, and is designed to be returned to the browser as the CGI script’s status code. For example:

my $uploaded_file = $q->param('upload');
if ( !$uploaded_file && $q->cgi_error() ) {
    print $q->header( -status => $q->cgi_error() );
    exit 0;
}

However it isn’t clear that any browser currently knows what to do with this status code. It might be better just to create a page that warns the user of the problem. 🎀

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🧩 MODULE FLAGS 🧩

There are a number of global module flags which affect how CGI.pm operates.

🧵 $CGI::APPEND_QUERY_STRING

If set to a non-zero value, this will add query string parameters to a POST forms parameters hence allowing param() to return values from the query string as well as from the decoded POST request instead of having to use url_param instead. This makes it easier to get the value of a parameter when you don’t know the source. 💅

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🧬 COMPATIBILITY WITH CGI-LIB.PL 🧬

To make it easier to port existing programs that use cgi-lib.pl the compatibility routine "ReadParse" is provided. Porting is simple:

OLD VERSION

require "cgi-lib.pl";
&ReadParse;
print "The value of the antique is $in{antique}.\n";

NEW VERSION

use CGI;
CGI::ReadParse();
print "The value of the antique is $in{antique}.\n";

CGI.pm’s ReadParse() routine creates a tied variable named %in, which can be accessed to obtain the query variables. Like ReadParse, you can also provide your own variable. Infrequently used features of ReadParse, such as the creation of @in and $in variables, are not supported. 🌸

Once you use ReadParse, you can retrieve the query object itself this way:

my $q = $in{CGI};

This allows you to start using the more interesting features of CGI.pm without rewriting your old scripts from scratch. 🦋

An even simpler way to mix cgi-lib calls with CGI.pm calls is to import both the ":cgi-lib" and ":standard" method:

use CGI qw(:cgi-lib :standard);
&ReadParse;
print "The price of your purchase is $in{price}.\n";
print textfield(-name=>'price', -default=>'$1.99');

🧰 Cgi-lib functions that are available in CGI.pm 🧰

In compatibility mode, the following cgi-lib.pl functions are available for your use:

ReadParse()
PrintHeader()
SplitParam()
MethGet()
MethPost()

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📜 LICENSE 📜

The CGI.pm distribution is copyright 1995-2007, Lincoln D. Stein. It is distributed under the Artistic License 2.0. It is currently maintained by Lee Johnson (LEEJO) with help from many contributors. 💖

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💖 CREDITS 💖

Thanks very much to:

• Mark Stosberg (mark@stosberg.com)
• Matt Heffron (heffron@falstaff.css.beckman.com)
• James Taylor (james.taylor@srs.gov)
• Scott Anguish (sanguish@digifix.com)
• Mike Jewell (mlj3u@virginia.edu)
• Timothy Shimmin (tes@kbs.citri.edu.au)
• Joergen Haegg (jh@axis.se)
• Laurent Delfosse (delfosse@delfosse.com)
• Richard Resnick (applepi1@aol.com)
• Craig Bishop (csb@barwonwater.vic.gov.au)
• Tony Curtis (tc@vcpc.univie.ac.at)
• Tim Bunce (Tim.Bunce@ig.co.uk)
• Tom Christiansen (tchrist@convex.com)
• Andreas Koenig (k@franz.ww.TU-Berlin.DE)
• Tim MacKenzie (Tim.MacKenzie@fulcrum.com.au)
• Kevin B. Hendricks (kbhend@dogwood.tyler.wm.edu)
• Stephen Dahmen (joyfire@inxpress.net)
• Ed Jordan (ed@fidalgo.net)
• David Alan Pisoni (david@cnation.com)
• Doug MacEachern (dougm@opengroup.org)
• Robin Houston (robin@oneworld.org)

…and many many more… for suggestions and bug fixes. 🫶

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🐛 BUGS 🐛

Address bug reports and comments to: <https://github.com/leejo/CGI.pm/issues>

See the <https://github.com/leejo/CGI.pm/blob/master/CONTRIBUTING.md> file for information on raising issues and contributing

The original bug tracker can be found at: <https://rt.cpan.org/Public/Dist/Display.html?Queue=CGI.pm>

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👀 SEE ALSO 👀

• CGI::Carp — provides Carp implementation tailored to the CGI environment.
• CGI::Fast — supports running CGI applications under FastCGI

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okay that’s the whole vibe bestie — this page is from perl v5.42.0, dated 2025-01-09. mwah 💋