Decoding FORMs with CGI
If you are unfamiliar with forms or how to write them, we suggest you look
at this guide
to fill-out forms. They're just plain HTML, and pretty easy to do.
Decoding them is another story...
Where do I get the form data from?
As you now know, there are two methods which can be used to access your
forms. These methods are GET and POST. Depending on which
method you used, you will receive the encoded results of the form in a
different way.
-
The GET method
If your form has METHOD="GET" in its FORM tag, your CGI program will
receive the encoded form input in the environment variable QUERY_STRING.
-
The POST method
If your form has METHOD="POST" in its FORM tag, your CGI program will
receive the encoded form input on stdin. The server will NOT send you an
EOF on the end of the data, instead you should use the environment variable
CONTENT_LENGTH to determine how much data you should read from stdin.
But what does it all mean? How do I decode the form data?
When you write a form, each of your input items has a NAME tag. When the
user places data in these items in the form, that information is encoded
into the form data. The value each of the input items is given by the user
is called the value.
Form data is a stream of name=value pairs separated by the & character.
Each name=value pair is URL
encoded, i.e. spaces are changed into plusses and some characters are encoded
into hexadecimal.
Because others have been presented with this problem as well, there
are already a number of programs which will do this decoding for you. The
following are links into the CGI archive, clicking on them will retrieve
the software package being referred to.
-
The Bourne Shell: The AA archie gateway. Contains calls to sed and awk
which convert a GET form data string into separate environment variables.
-
C: The default scripts for NCSA httpd. While I won't win any awards for
verbosity in documenting my code, there are C routines and example programs
you can use to translate the query string into a group of structures.
-
PERL: The PERL CGI-lib. This package contains a group of useful PERL routines
to decode forms.
-
PERL5: A perl5 library for handling forms in CGI scripts. With just a handful
of calls, you can parse CGI queries, create forms, and maintain the state
of the buttons on the form from invocation to invocation.
-
TCL: This is a set of TCL routines to retrieve form data and place
it into TCL variables.
The basic procedure is to split the data by the ampersands. Then, for each
name=value pair you get for this, you should URL decode the name, and then
the value, and then do what you like with them.
![[Back]](cgi1b.gif)
Return
to the overview
CGI - Common Gateway Interface
http://hoohoo.ncsa.uiuc.edu/cgi/mailtocgi.html