Copyright © Nick Kew, 1996-7. Please read the Notice and Disclaimer.
The Table of contents starts at 0 (preamble). Older (pre-HTML 3) or broken browsers may start it at 1: apologies for any confusion.NOTE: the Reply-to address in this FAQ is an autoresponder. If you want to write to me, you'll have to set the "To:" line by hand: mailto:nick@webthing.com NOTE: the numbering in this document is automatically generated by my posting software, and will change between postings if new questions are added (as _may_ happen when I see - or someone contributes - a FAQ I've previously overlooked :-)
Last Modified: July 12th 1997: * Added question on remote hostname (thanks Alain Deckers) * Added reference to Login tutorial * Updated reference for Selena's scripts[Table of Contents] [Index]
Copyright 1996-7 Nick Kew.
You are free to copy or distribute this document in whole or in part
for any purpose and on any medium you choose, provided:
You DON'T do so for profit.
You DO include this notice and disclaimer in full.
Disclaimer: This information is offered in good faith and in the hope
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[Table of Contents] [Index]The homes of this document on the Web are now * the WebThing Virtual Office, at http://www.webthing.com/: URL http://www.webthing.com/page.cgi/cgifaq * the Web Design Group, at http://htmlhelp.com/ URL http://htmlhelp.com/faq/cgifaq.html NOTE - If you want to mirror the FAQ on your WWW site, the best document to use is the HTML version from my autoresponder (see below). If you're putting it on a publicly-visible server, please make sure you keep it up-to-date (if you let me know you have it, I can automate the updates). Other known sources are: (1) USENET: posted to newsgroups (TEXT) news:comp.infosystems.www.authoring.cgi news:comp.answers news:news.answers (2) RTFM and mirror sites (TEXT) ftp://rtfm.mit.edu/pub/usenet/news.answers/www/cgi-faq (3) RTFM WWW mirror sites, including (Partial HTML) Europe - http://www.cs.ruu.nl/cgi-bin/faqwais America - http://www.cis.ohio-state.edu/hypertext/faq/usenet/ (4) By EMAIL from my autoresponder (HTML or TEXT) Send blank email to mailto:nick+cgi_text@webthing.com or mailto:nick+cgi_html@webthing.com (depending on which version you want) **** NOTE CHANGE FROM PREVIOUS AUTORESPONDER SETUP! **** (5) By EMAIL from the FAQserver at RTFM (TEXT) Send email to mailto:mail-server@rtfm.mit.edu with send usenet/news.answers/www/cgi-faq in the body of your message[Table of Contents] [Index]
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[Table of Contents] [Index]I am not a free advice centre, but in response to persistent questions I have recently (July '97) opened a commercial help service. If you're willing to pay, see http://www.webthing.com/support/general.html If you think something already in the FAQ needs clarifying, feel free to mail me: don't expect a personal reply, but I *might* add something to the answer in question, so check the next posting (or three). The newsgroup is the appropriate place for free advice. But remember: bad questions usually get bad answers, so think carefully before posting.[Table of Contents] [Index]
This is now a moderated newsgroup. The moderator is a bot run by Thomas Boutell ( mailto:boutell@boutell.com ). The charter for moderation is as follows: This newsgroup is self-moderated. Your first posting will not appear until you have read and responded to an automatic welcome mailing, at which point your posting will appear with no further delay. Provision will also be made to automatically approve first postings that contain a header requesting this. Subsequent postings are approved automatically. If posting normally doesn't work - as could be the case if your newsfeed has trouble with moderated groups - you can post articles by emailing them to: mailto:authoring-cgi@boutell.com Provided the return address in your mail is correct, you will then receive precise instructions for having your post(s) automatically approved. Alternative means of posting are detailed in the WWW FAQ, posted regularly by Thomas Boutell.[Table of Contents] [Index]
This FAQ was written by Nick Kew, and has been considerably improved with the help of comments and criticisms, newsgroup posts and miscellaneous suggestions from correspondents including Nathan Neulinger, Maurice L. Marvin, Matthew Healy, Alan J. Flavell, Don Libes, Alain Deckers, and no doubt others I've forgotten to credit (please remind me if necessary).[Table of Contents] [Index]
This section aims to deal with basic questions, addressing the role and nature of CGI, and its place in Web programming. Questions/answers which just don't appear to 'fit' under any other section may also be included here.
[ from the CGI reference http://hoohoo.ncsa.uiuc.edu/cgi/overview.html ] The Common Gateway Interface, or CGI, is a standard for external gateway programs to interface with information servers such as HTTP servers. A plain HTML document that the Web daemon retrieves is static, which means it exists in a constant state: a text file that doesn't change. A CGI program, on the other hand, is executed in real-time, so that it can output dynamic information.[Table of Contents] [Index]
The distinction is semantic. Traditionally, compiled executables (binaries) are called programs, and interpreted programs are usually called scripts. In the context of CGI, the distinction has become even more blurred than before. The words are often used interchangably (including in this document). Current usage favours the word "scripts" for CGI programs.[Table of Contents] [Index]
There are innumerable caveats to this answer, but basically any Webpage containing a form will require a CGI script or program to process the form inputs.[Table of Contents] [Index]
[answer to this non-question hopes to try and reduce the noise level of the recurrent "CGI vs JAVA" threads]. CGI and JAVA are fundamentally different, and for most applications are NOT interchangable. Neither are the two isomorphic: you could in principle write a CGI program in JAVA, although it is hard to think of an instance where this would be the best choice. CGI is a mechanism for running programs on a WWW server. Typical applications include accessing a database, submitting an order, or posting messages to a bulletin board. JAVA enables programs to run on the Client machine, and is suited to such tasks as detailed manipulation of an image. Alternatives to JAVA may include the X windows client/server protocol, use of browser plugins and helper applications, and other clientside languages such as SafeTCL and perl/penguin. In certain instances the two may be combined in a single application: for example a JAVA applet to define a region of interest from a geographical map, together with a CGI script to process a query for the area defined.[Table of Contents] [Index]
CGI and SSI (Server-Side Includes) are often interchangable, and it may
be no more than a matter of personal preference. Here are a few
guidelines:
1) CGI is a common standard agreed and supported by all major HTTPDs.
SSI is NOT a common standard, but an innovation of NCSA's HTTPD
which has been widely adopted in later servers. CGI has the
greatest portability, if this is an issue.
2) If your requirement is sufficiently simple that it can be done
by SSI without invoking an exec, then SSI will probably be
more efficient. A typical application would be to include
sitewide 'house styles', such as toolbars, netscapeised <body>
tags or embedded CSS stylesheets.
3) For more complex applications - like processing a form -
where you need to exec (run) a program in any case, CGI
is usually the best choice.
[Table of Contents] [Index]APIs are proprietary programming interfaces supported by particular platforms. By using an API, you lose all portability. If you know your application will only ever run on one platform (OS and HTTPD), and it has a suitable API, go ahead and use it. Otherwise stick to CGI.[Table of Contents] [Index]
If you're already a programmer, CGI is extremely straightforward, and just
three resources should get you up to speed in the time it takes to read them:
1) Installation notes for your HTTPD. Is it configured to run CGI
scripts, and if so how does it identify that a URL should be executed?
(Check your manuals, READMEs, ISP webpages/FAQS, and if you still can't
find it ask your server administrator).
2) The CGI specification at NCSA tells you all you need to know
to get your programs running as CGI applications.
http://hoohoo.ncsa.uiuc.edu/cgi/interface.html
3) WWW Security FAQ. This is not required to 'get it working', but
is essential reading if you want to KEEP it working!
http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html
If you're NOT already a programmer, you'll have to learn. If you would
find it hard to write, say, a 'grep' or 'cat' utility to run from the
commandline, then you will probably have a hard time with CGI. Make
sure your programs work from the commandline BEFORE trying them with CGI,
so that at least one possible source of errors has been dealt with.
[Table of Contents] [Index]Yes. Period. There is a lot you can do to minimise these. The most important thing to do is read and understand Lincoln Stein's excellent WWW security FAQ, at http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html .[Table of Contents] [Index]
No, but it helps. The Web, along with the Internet itself, C, Perl, and almost every other Good Thing in the last 20 years of computing, originated in Unix. At the time of writing, this is still the most mature and best-supported platform for Web applications.[Table of Contents] [Index]
No - you can use any programming language you please. Perl is simply today's most popular choice for CGI applications. Some other widely- used languages are C, C++, TCL, BASIC and - for simple tasks - even shell scripts. Reasons for choosing Perl include its powerful text manipulation capabilities (in particular the 'regular' expression) and the fantastic WWW support modules available.[Table of Contents] [Index]
see next question[Table of Contents] [Index]
Maybe. It depends on your server installation. These types of filenames are commonly used conventions - no more. It is up to the server administrator whether or not CGI scripts are enabled, and (if so) what conventions tell the server to run or to print them. If you are running your own server, read the manual. If you're on ISP or other rented webspace, check their webpages for information or FAQs. As a last resort, ask the server administrator.[Table of Contents] [Index]
[ quoted from http://www.umr.edu/~cgiwrap/intro.html ] > CGIWrap is a gateway program that allows general users to use CGI scripts > and HTML forms without compromising the security of the http server. > Scripts are run with the permissions of the user who owns the script. In > addition, several security checks are performed on the script, which will not > be executed if any checks fail. > > CGIWrap is used via a URL in an HTML document. As distributed, cgiwrap > is configured to run user scripts which are located in the > ~/public_html/cgi-bin/ directory. See http://www.umr.edu/~cgiwrap/[Table of Contents] [Index]
The normal format for data in HTTP requests is URLencoded. All Form data is encoded in a string, of the form param1=value1¶m2=value2&...paramn=valuen Many non-alphanumeric characters are "escaped" in the encoding: the character whose hexadecimal number is "XY" will be represented by the character string "%XY". Decoding this string is a fundamental function of every CGI library. Another format is "multipart/form-data", also known as "file upload". You will get this from the HTML markup <form method="POST" enctype="multipart/form-data"> (but note you must accept URLencoded input in any case, since not all browsers support multipart forms). Most(?) CGI libraries will handle this transparently.[Table of Contents] [Index]
This is a fairly technical section dealing with HTTP, the protocol of the Web. It also includes NPH, the mechanism by which CGI programs can return HTTP header information directly to the Client.
HTTP is the protocol of the Web, by which Servers and Clients (typically
browsers) communicate. An HTTP transaction comprises a Request sent by
the Client to the Server, and a Response returned from the Server to
the Client.
Every HTTP request and response includes a message header, describing
the message. These are processed by the HTTPD, and may often be
mostly ignored by CGI applications (but see below).
A message body may also be included:
1) A HEAD or GET request sends only a header. Any form data is encoded
in an HTTP_QUERY_STRING header field, which is available to the CGI
program as an environment variable QUERY_STRING.
2) A POST request sends both header and body. The body typically
comprises data entered by a user in a form.
3) A HEAD request does not expect a body in the response.
4) A GET or POST request will accept a response with or without a body,
according to the header. The body of a response is typically an
HTML document.
[Table of Contents] [Index]Most HTTP request headers are passed to the CGI script as environment variables. Some are guaranteed by the CGI spec. Others are server, browser and/or application dependent. To see what _your_ browser and server are telling each other, just use a trivial little CGI script to print out the environment. In Unix: #!/bin/sh echo "Content-type: text/plain" echo set (Just call it "env.cgi" or something, and put it where your server will execute it. Then point your browser at http://your.server/path/to/env.cgi ). This enables you to see at-a-glance what useful server variables are set. Note that dumping the environment like this within a more complex script can be a useful debugging technique. For details, see the CGI Environment Variables specification at http://hoohoo.ncsa.uiuc.edu/cgi/env.html (which also includes a version of the above script - somewhat more nicely formatted - online).[Table of Contents] [Index]
See previous question. Those you can rely on are documented in NCSA's pages; those associated with your particular server and browser can be determined using the above script.[Table of Contents] [Index]
Unless you are using NPH, the HTTPD will insert necessary response headers on your behalf, always provided it is configured to do so. However, it is conventional for servers to insert the Content-Type header based on a page's filename, and CGI scripts cannot rely on this. Hence the usual advice is to print an explicit Content-Type header. At least one of "Content-Type", "Status" and "Location" is almost always required. A few other headers you may wish to use explicitly are: Status (to set HTTP return code explicitly. Caveats: (1) Behaviour is undefined if it conflicts with another header. (2) This is NOT an HTTP header.) Location (to redirect the user to another URI, which may or may not be on your own server) Set-cookie (Netscape/Nonstandard) Set a cookie Refresh (Netscape/Nonstandard) Clientpull You can also use general MIME headers: eg "Keywords" for the benefit of indexers (although in this instance some major search robots have regrettably introduced a new protocol to do the same thing). The 'official' list of HTTP response headers is at http://www.w3.org/pub/WWW/Protocols/HTTP/Object_Headers.html[Table of Contents] [Index]
NPH = No Parsed Headers. The script undertakes to print the entire HTTP response including all necessary header fields. The HTTPD is thereby instructed not to parse the headers (as it would normally do) nor add any which are missing.[Table of Contents] [Index]
Generally, no. It is usually better to save yourself hassle by letting
the HTTPD produce the headers for you.
If you are going to use NPH, be sure to read and understand the HTTP spec at
http://www.w3.org/pub/WWW/Protocols/
Your headers should be complete and accurate, because you're instructing
the HTTPD not to correct them or insert what's missing.
Possible circumstances where the use of NPH is appropriate are:
* When your headers are sufficiently unusal that they might be
differently parsed by different HTTPDs (eg combining "Location:"
with a "Status:" other than 302).
* When returning output over a period of time (eg displaying
unbuffered results of a slow operation in 'real' time).
See http://www.w3.org/pub/WWW/Protocols/HTTP/HTRESP.html
[Table of Contents] [Index]According to NCSA's reference pages, this is the standard for telling the server that your script is NPH, so this should be a fully portable convention.[Table of Contents] [Index]
Firstly, the the HTTP protocol specifies differing usages for the two methods. GET requests should always be idempotent on the server. This means that whereas one GET request might (rarely) change some state on the Server, two or more identical requests will have no further effect. This is a theoretical point which is also good advice in practice. If a user hits "reload" on his/her browser, an identical request will be sent to the server, potentially resulting in two identical database or guestbook entries, counter increments, etc. Browsers may reload a GET URL automatically, particularly if cacheing is disabled (as is usually the case with CGI output), but will typically prompt the user before re-submitting a POST request. This means you're far less likely to get inadvertently-repeated entries from POST. GET is (in theory) the preferred method for idempotent operations, such as querying a database, though it matters little if you're using a form. There is a further practical constraint that many systems have builtin limits to the length of a GET request they can handle: when the total size of a request (URL+params) approaches or exceeds 1Kb, you are well-advised to use POST in any case. In terms of mechanics, they differ in how parameters are passed to the CGI script. In the case of a POST request, form data is passed on STDIN, so the script should read from there (the number of bytes to be read is given by the Content-length header). In the case of GET, the data is passed in the environment variable QUERY_STRING. The content-type (application/x-www-form-urlencoded) is identical for GET and POST requests.[Table of Contents] [Index]
This section comprises programming hints and tips for a number of popular tasks. Also included are a number of common questions to which the answer is "you can't", with the reasons why.
*sigh* Many people keep mailing me questions or suggested hacks to get visitor information, particularly email addresses. It seems they won't take "NO" for an answer. The bottom line is that whatever information is available to _you_ is _equally_ available to every spammer on the net. Therefore when a browser bug _does_ permit personal data to be collected, it gets reported and fixed very quickly (one short-lived Netscape release reportedly had such a bug). You can get some limited information from the environment variables passed to you by the browser. Relatively few of these are guaranteed to be available, and some may be misleading. For particular types of information, see below. For full details, see NCSA's reference pages.[Table of Contents] [Index]
Why do you want to do this? The best information available is the REMOTE_ADDR and REMOTE_HOST, which tell you nothing about the user. Techniques such as "finger@" are not reliable, are widely disliked, and generally serve only to introduce long delays in your CGI. Better - as well as more polite - just to ask your users to fill in a form. BTW: the "From:" header line (HTTP_FROM variable) is usually only set by robots, since human visitors to your webpage will not normally want their addresses collected without permission, and browsers respect this.[Table of Contents] [Index]
Some sites will play party tricks, which can get *some users* email addresses. Possible tell-tale signs of this are inordinate delays loading a page (fingering @REMOTE_HOST - doesn't often work but probably can't be detected from the webpage), or a submit button that appears to do nothing at all (a mailto: link - works quite well but trivially detectable). As a "snoop" party trick that's fine, but if you find someone abusing these facilities (eg they send you junkmail), alert their service provider![Table of Contents] [Index]
Unfortunately people will sometimes enter an incorrect or invalid email address in your Form. Worse, they may enter a valid but incorrect email address that will deliver to someone who doesn't want your mail. Proposed regexps to match email addresses are sometimes posted. Most of these will fail against perfectly valid email addresses, like "S=N.OTHER/OU1=X12345A/RECIPNUM=1/MTA-BASIC@attmail.com" (which is what your address looks like if you are connected to the Internet via X400 - and if you think that example is too easy, check the ones at the end of Eli the Bearded's Email Addressing FAQ). Probably the most complete parser and checker available for download is Tom Christiansen's, at http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/ckaddr.gz Of course, this still says nothing about deliverability. A frequently-suggested hack that doesn't work is to use SMTP EXPN or VRFY commands. Modern versions of sendmail permit administrators to disable these commands, and many sites take advantage of this facility to protect their users' privacy. Probably the best way to verify an email address is to send mail to it, asking the user to respond. Include a clause like "if you have received this mail in error, please accept our apologies..."[Table of Contents] [Index]
You can't. Well, not always. IF it is available, you'll find it in the REMOTE_HOST environment variable. However, this will more often than not contain the numerical IP address rather than the IP name of the remote host. Remember that not all IP addresses have a hostname associated with them; this is the case of most IP addresses assigned to dialup users, for example. Your web server may also not perform a reverse lookup on incoming connections, in which case REMOTE_HOST will contain the IP address even if it has a corresponding IP name. In the second case, you can do a reverse lookup yourself in your script, but this is expensive and should probably be avoided unless absolutely necessary. Even if you do manage to obtain a hostname, you should be aware that it may not correspond to the hostname the user is accessing your page from. It may instead be that of an intervening proxy host. The short answer is therefore that there is no reliable way of finding out what the remote user's hostname is.[Table of Contents] [Index]
Why do you want to do this? Well-written HTML will display correctly in any browser, so the correct answer to this question is to design a template for your output in good HTML, and make sure your output is correct. If you insist on a different answer, you can use the HTTP_USER_AGENT environment variable. This requires care, and can lead to unexpected results. For example, checking for "Mozilla" and serving a frameset to it ensures that you *also* serve the frameset to early (Non-Frame) Netscapes, me-too browsers (notably MicroSoft) and others who have chosen to lie to you about their browser. Note also that not every User Agent is a browser. Your page may be read by a user agent you've never heard of, and then displayed by 100 different browsers. Or retrieved by different browsers from a cache. Another reason to write good HTML, and not try to devise a clever or koool substitute.[Table of Contents] [Index]
HTTP_REFERER might or might not tell you anything. By all means use it to collect partial statistics if you participate in (say) an advertising banner scheme. But it is not always set, and may be meaningless (eg if a user has accessed your page from a bookmark, and the browser is too dumb to cope with this). You cannot trace outgoing links at all. If you really must try, point all the external links to your HTTPD and use its redirection facility (which gives you generally-reliable logs). This is much less inefficient than using a CGI script. BTW: don't even think about asking Javascript to send you information on some event: it's a violation of privacy which Netscape fixed as soon as complaints about its abuse started coming in. If it works with *your* browser, you should upgrade![Table of Contents] [Index]
[UNIX]
You have to fork/spawn the long-running process.
The important thing to remember is to close all its file descriptors;
otherwise nothing will be returned to the browser until it's finished.
The standard trick to accomplish this is redirection to/from /dev/null:
exec ("long_process < /dev/null > /dev/null 2>&1 &")
print HTML page as usual
(don't take "exec" as literal in anything but a shell script - in
C, Perl, etc use fork+exec or system() :-)
[Table of Contents] [Index]This does not fit well with the basic mechanics of the Web, in which
each transaction comprises a single request and response.
If your processing can be done on the Client machine, you can use
a clientside application; for example a Java applet.
For processing on the server, one trick that works well for Clients
running an X server (and far more efficient than a JAVA solution) is:
if ( fork() ) {
print HTML page explaining what's going on and advising about xhost
} else {
exec ("xterm -display THEIR_DISPLAY -title MY_APP -e MY_PROG ARGS
< /dev/null > /dev/null 2>&1 &") ;
}
NOTE: THEIR_DISPLAY is not necessarily the same as REMOTE_HOST or REMOTE_ADDR.
You have to ask users to supply their display (set REMOTE_HOST as default).
[ Question: Is there a JAVA alternative to xterm yet, for platforms
which support JAVA but not X ? ]
[Table of Contents] [Index]Yes. Use your HTTPD's authentication, just as you would a basic HTML page. Now you'll have the identity of every visitor in REMOTE_USER.[Table of Contents] [Index]
It depends on which version of the question you asked. Yes, you can use CGI to trigger the browser's standard Username/Password dialogue. Send a response code 401, together with a "WWW-authenticate" header including details of the the authentication scheme and realm: e.g. (in a non-NPH script) Status: 401 Unauthorized to access the document WWW-authenticate: Basic realm="foobar" Content-type: text/plain Unauthorised to access this document The use you can make of this is server-dependent, and harder, since most servers expect to deal with authentication before ever reaching the CGI (eg through .www_acl or .htaccess). Thus it cannot usefully replace the standard login sequence, although it can be applied to other situations, such as re-validating a user - e.g after a certain timeout period or if the same person may need to login under more than one userid. What you can never get in CGI is the credentials returned by the user. The HTTPD takes care of this, and simply sets REMOTE_USER to the username if the correct password was entered. For a much longer discussion of this question (with code extracts), see my discussion at http://www.webthing.com/tutorials/login.html[Table of Contents] [Index]
The most usual (but browser-dependent) way to do this is to set a cookie. If you do this, you are accepting that not all users will have a 'session'. An alternative is to pass a session ID in every GET URL, and in hidden fields of POST requests. This can be a big overhead unless _every_ page requires CGI in any case. Another alternative is the Hyper-G solution of encoding a session-id in the URLs of pages returned: http://hyper-g.server/session_id/real/path/to/page This has the drawback of making the URLs very confusing, and causes any bookmarked pages to generate old session_ids. Note that a session ID based solely on REMOTE_HOST (or REMOTE_ADDR) will NOT work, as multiple users may access your pages concurrently from the same machine.[Table of Contents] [Index]
For permanent and simple redirection, use the HTTPD configuration file: it's much more efficient than doing it yourself. Some servers enable you to do this using a file in your own directory (eg Apache) whereas others use a single configuration file (eg CERN). For more complicated cases (eg process form inputs and conditionally redirect the user), use the "Location:" response header. If the redirection is itself a CGI script, it is easy to URLencode parameters to it in a GET request, but don't forget to escape the URL![Table of Contents] [Index]
Yes, but think carefully first: How are your readers going to know that their "submit" has succeeded? They may hit 'submit' many times! The correct solution according to the HTTP specification is to return HTTP status code 204. As an NPH script, this would be: #!/bin/sh # do processing (or launch it as background job) echo "HTTP/1.0 204 No Change" echo Alan J Flavell has pointed out that this will fail with certain popular browsers, and suggests a workaround to accommodate them: > 1. Send status 204, Content-type of text/html, and a short body content > that (for those few browsers that display it) will tell the reader that > their browser does not handle this reponse correctly, and invites them > to use their browser's Back function (hey, if someone tells me to put > a back button on the HTML page itself, I think I shall scream...). His survey is at http://ppewww.ph.gla.ac.uk/%7Eflavell/status204/results.html[Table of Contents] [Index]
Yep. The fact you're using CGI makes no difference: use "target=" in your links as usual. Alternatively, the script can print a "Window-target:" header. Read Netscape's pages for detail: these answer all the questions about things like "getting rid of" or "breaking out of" frames, too.[Table of Contents] [Index]
A single CGI script can only ever print to one frame.
However, this limitation may be overcome by using more than one script.
The first script (the URL of the "submit" button) prints a frameset,
typically to a "_parent" or "_top" target. The sources for one or
more of the frames thus generated may also be CGI scripts, to which
you can easily pass parameters (eg encoded in URLs with method GET).
This hack is definitely not recommended. If you find yourself wanting
to update several frames from a single user event, it probably means
you should review the design of your application at a higher level.
Warnings:
1. Don't forget to escape your URLs.
2. This technique results in your server being hit by multiple
concurrent CGI requests. You'll need LOTS of memory, especially
if you use a memory-hog like Perl. It can be a good recipe
for bringing a server to its knees.
Javascript is often a valid alternative here, but note just how silly
it can (and often does) look in a different browser.
[Table of Contents] [Index]Not directly. One script generates one response to one request. If you want to generate a dynamic page including dynamic images (say, a report including graphs, all of which depend on user input) then your primary script will print the usual <img src="[script-to-generate-image]" alt="[what you asked for]"> and, just as in the multiple frames case, you can pass data to the image-generating program encoded in a GET URL. Of course, the same caveats apply: see above.[Table of Contents] [Index]
This is currently beyond the scope of this FAQ (whose author urgently needs to improve his own applications in this regard). However, there is an excellent introduction to net-friendly webpages, including CGI pages, at http://vancouver-webpages.com/CacheNow/ A sample cacheing perl/cgi script by Andrew Daviel is available at http://vancouver-webpages.com/proxy/log-tail.pl[Table of Contents] [Index]
You can't. You just have to deal with it when they do. You can avoid re-processing a submission by embedding a unique ID in your Form each time it is displayed. When you process the form, you enter the ID in a database. Or, if it's already there, you don't repeat the processing. You probably want to expire your database entries after a little time: an hour should be fine in a typical situation. If you're already using cookies (e.g. a shoppingcart), an alternative is to use the cookie as a unique identifier. This means you also have to handle the situation where a user deliberately "goes round twice" and submits the same form with different contents. If your script may take some time to process, you should also consider running it as a background job, and returning an immediate acknowledgement to the user (see above if your "immediate" response gets delayed until processing is complete in any case).[Table of Contents] [Index]
CGI scripts are run by the HTTPD, and therefore by the UID of the HTTPD
process, which is (by convention) usually a special user "nobody".
There are two basic ways to run a script under your own userid:
(1) The direct approach: use a setuid program.
(2) The double-server approach: have your CGI script communicate
with a second process (e.g. a daemon) running under your userid,
which is responsible for the actual file management.
The direct approach is usually faster, but the client-server architecture
may help with other problems, such as maintaining integrity of a database.
When running a compiled CGI program (e.g. C, C++), you can make it
setuid by simply setting the setuid bit:
e.g. "chmod 4755 myprog.cgi"
For security reasons, this is not possible with scripting languages
(eg Perl, Tcl, shell). A workaround is to run them from a setuid
program, such as cgiwrap.
In most cases where you'd want to use the client-server approach,
the server is a finished product (such as an SQL server) with its
own CGI interface.
A lightweight alternative to this is Don Libes' "expect" package.
Note that any program running under your userid has access to all your
files, and could do serious damage if hacked. Take care!
[Table of Contents] [Index]There are a lot of applications available. For all the tasks listed here, there are free systems you can download and install yourself (at least if you're on UNIX). Many are excellent. Before ever *buying* software, do a Net search on what you want and check what freeware is available. Does the commercial system you had in mind *really* have any advantages? If you can't follow the jargon they use to explain the merits of their system, insist on some clarification (hey, that's not just for Web software :-) Most questions under this heading are probably best answered by reference to appropriate review sites on the Web (in many cases, Thomas Boutell's WWW FAQ). In cases where I know of one or more good sites, I've referenced them.
Matt Wright - himself author of many popular CGI resources - has recently (March.97) opened a new website dedicated to this subject: http://www.cgi-resources.com/ I am happy to recommend this as a rich and well-organised collection, and probably the best of its kind on the Web today.[Table of Contents] [Index]
(see also previous question, which should perhaps replace this one altogether) Some popular places to look for a wide range of free CGI applications are: Selena Sol's Public Domain CGI Scripts http://www.extropia.com/Scripts/ Matt Wright's Script Archive http://www.worldwidemart.com/scripts/ Dale Bewley has a much longer list of script archives (along with his own scripts) at http://www.engr.iupui.edu/~dbewley/perl/[Table of Contents] [Index]
David R Woolley maintains a list of currently around 100 systems at
http://freenet.msp.mn.us/~drwool/webconf.html
("Conferencing on the Web").
[Table of Contents] [Index]There are several overview sites for this. A few are: The CSCW Yellow Pages, at http://www11.informatik.tu-muenchen.de/cscw/yp/YP-index-type.html NCSA Web Collaboration pages, at http://union.ncsa.uiuc.edu/HyperNews/get/www/collaboration.html[Table of Contents] [Index]
This subject deserves its own FAQ. When someone recently asked about one,
Matthew.Healy@yale.edu (Matthew D. Healy) posted this answer (slightly chopped)
> : Is there a CGI and Database FAQ available?
> : If so, could someone tell me where can I get it?
>
> Dunno about a FAQ on that. I can recommend a couple of published
> works, however:
>
> 1. I wrote a chapter about CGI/Database work for the book
> {Special Edition Using CGI}. Fulltext is online at the
> publisher's WWW site:
>
> http://www.mcp.com/que/et/se_cgi/ The book
> http://www.mcp.com/que/et/se_cgi/Cgi13fi.htm My chapter on WWW/DBMS
>
> 2. Jeff Rowe wrote an excellent book, {Building Internet Database
> Servers With CGI}. URL for more info:
>
> http://cscsun1.larc.nasa.gov/~beowulf/db/existing_products.html
>
> Jeff's WWW site has scads of useful information on WWW/DBMS programming,
> and pointers to lots more sites.
Matthew's CGI links page at http://ycmi.med.yale.edu/~healy/cgilinks.html
expands the list, and includes links to popular packages including
Bo Frese Rasmussen's WDB at http://venus.dtv.dk/~bfr/wdb/
[Table of Contents] [Index]Yes. Here is an example:
http://pitch.nist.gov/cgi-bin/cgi.tcl/passwd-form.cgi
It is an Expect script that wraps itself around the passwd command.
With this technique, there is no need to make scripts setuid (e.g.,
cgiwrap). This same technique lends itself to many other scripts that
might otherwise need setuid.
(contributed by Don Libes <libes@cme.nist.gov>)
[Table of Contents] [Index]Since this subject is quite well conered by other documents, this FAQ has relatively little to say. Tom Christiansen's "Idiot's guide to solving Perl/CGI problems" is a slightly tongue-in-cheek list of common problems, and how to track them down. Much of what Tom covers is not specifically Perl, but applies equally to CGI programming in other languages. Marc Hedlund's CGI FAQ and Thomas Boutell's WWW FAQ also deal with this subject. See "Further Reading" below (if you don't already know where to find these documents).
(1) Several CGI programming libraries offer powerful interactive
debugging facilities. These include:
- for Perl, Lincoln Stein's CGI.pm
http://www-genome.wi.mit.edu/ftp/pub/software/WWW/cgi_docs.html
- for Tcl, Don Libes' cgi.tcl
http://expect.nist.gov/cgi.tcl
- for C++, Nick Kew's CGI++
http://www3.pair.com/webthing/cgiplusplus/
(2) Nathan Neulinger's cgiwrap is another package with debugging aids.
http://www.umr.edu/~cgiwrap/
(3) The "mod_cgi" Apache module (new with Apache 1.2) enables you to
capture script output and errors for diagnosis.
See also the next question.
[Table of Contents] [Index]For simple cases, examining your response headers "by hand" may suffice:
(1) telnet to the host and port where the server is running - e.g.
telnet www.myhost.com 80
(2) Enter HTTP request. The most useful for this purpose is usually HEAD; eg
HEAD /index.html HTTP/1.0
(optionally other headers)
(followed by a blank line)
Now you'll get a full HTTP response header back.
For complex cases, such as sending a request with several headers
(as a browser does) or POSTing a form, there is a free diagnosis
service at the WebThing WebCentre. This will take a request from your
browser (eg form inputs) and forward the identical request to your
server, printing a full report of your request (request headers and
form data) and the response from your server (response headers and data).
http://www3.pair.com/webthing/
[Table of Contents] [Index]Your script must follow the CGI interface, which requires it to print: (1) One or more Header lines. (2) A blank line (3) (optional, but strongly advised) a document body. This error means it didn't. The Header lines can include anything that's valid under HTTP, but must normally include at least one of the special CGI headers: Content-Type Location Status Example (a very minimal HTML page via CGI) Content-Type: text/html <= Header <= Blank Line <title>HelloWorld</title>Hello World <= Document Body A common reason for scripts failing is that they crash before printing the header and blank line (or while these are buffered). Another possible reason is that it printed something else - like an error message - in the Headers. Check error logs, put a dummy header right at the top (for debugging only), check the "Idiot's Guide", and use the debug mode of your CGI library.[Table of Contents] [Index]
That means you put the line in the wrong place. It must appear in the CGI Header, not the document body. See previous question. It's also possible that you didn't print a header at all, or had a blank line or other noise before or in the header, but that the HTTPD has corrected this error for you (servers which correct your errors may give rise to the "works on A not on B" phenomenon). See previous question.[Table of Contents] [Index]
**** Lincoln Stein's FAQ is probably the most **** **** important WWW document you will ever read. **** Special Edition Using CGI (full book text available online) http://www.mcp.com/que/et/se_cgi/ The Web Authoring FAQ by 'Galactus' Engelfriet and John Pozadzides http://htmlhelp.com/links/wdgfaq.htm (although at the time of writing the online version appears to be a little behind the updated drafts posted). For general WWW issues, the World Wide Web FAQ by Thomas Boutell http://www.boutell.com/faq/ Another CGI FAQ, by Marc Hedlund http://www.best.com/~hedlund/cgi-faq/ Perl/CGI programming FAQ, by Shishir Gundavaram and Tom Christiansen http://www.perl.com/perl/faq/perl-cgi-faq.html The Idiot's Guide to solving Perl/CGI problems by Tom Christiansen http://www.perl.com/perl/faq/idiots-guide.html The WWW Security FAQ by Lincoln Stein http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html CGI Resources Library http://www.cgi-resources.com/ The WWW Virtual Library http://WWW.Stars.com/Vlib/[Table of Contents] [Index]
The Common Gateway Interface (CGI) http://www.ast.cam.ac.uk/%7Edrtr/cgi-spec.html http://hoohoo.ncsa.uiuc.edu/cgi/interface.html HyperText Transfer Protocol (HTTP) http://www.w3.org/pub/WWW/Protocols/HTTP/ HyperText Markup Language (HTML) http://www.w3.org/pub/WWW/MarkUp/Up to Table of Contents
The index is generated from an arbitrary list of keywords. If I've missed anything obvious that should be here, please let me know.