Lesson 4	From the server to the program
Objective	Explore how data is passed from the server to the CGI program.

From the Server to the Program using CGI

Once the server has received the request, it sets up the environment and starts up the CGI program. The environment will contain the various environment variables that we learned about in Module 1.

Standard CGI variables

The standard CGI variables are listed here:

`GATEWAY_INTERFACE`	The CGI version
`REQUEST_METHOD`	`GET` or `POST`
`SCRIPT_NAME`	Relative URL of the running CGI program
`QUERY_STRING`	The `GET` method query string
`SERVER_SOFTWARE`	Name and version of the server software
`SERVER_NAME`	Host name of the server
`SERVER_PROTOCOL`	Protocol and version running on the server (for example, HTTP/1.0)
`SERVER_PORT`	The port number of the server
`HTTP_USER_AGENT`	Name and version of the client (for example, Mozilla/3.01 for Netscape Navigator 3.01)
`HTTP_ACCEPT`	MIME types accepted by the browser
`PATH_INFO`	"Extra Path" information passed on the URL
`PATH_TRANSLATED`	`PATH_INFO` made relative to the server's root directory
`REMOTE_HOST`	Machine name of the remote host
`REMOTE_ADDR`	IP address of the remote host
`REMOTE_USER`	If this connection is authenticated, this is the authenticated user name.
`REMOTE_IDENT`	For RFC 931 "identd" connections
`AUTH_TYPE`	The type of authentication in effect (if any)
`CONTENT_TYPE`	MIME-type for `POST` method query
`CONTENT_LENGTH`	Length of query, `POST` method

Iterating over Perl hash

I32 hv_iterinit ( HV* hash );
SV* hv_iternextsv ( HV* hash , char** key , I32* keylen );

There are several ways to iterate over a hash's keys and values in the There are several ways to iterate over a hash's keys and values in the Perl API; we will show you the simplest and most immediately useful. After performing an hv_iterinit, each call to hv_iternextsv will return the next value and store the key and its length in the memory pointed to by key and keylen, respectively. Example Listing 3.4 shows a slightly simplified example from mod_perl. When mod_perl starts up, it wants to remove anything that might be dangerous from the environment it has inherited, saving only selected environment entries. It does so by iterating over %ENV.

void perl_clear_env(void)
{
  char *key;
  I32 klen;
  SV *val;
  HV *hv = (HV*)GvHV(envgv);
  (void)hv_iterinit(hv);
   while ((val = hv_iternextsv(hv, (char **) &key, &klen))) {
    if((*key == 'G') && strEQ(key, "GATEWAY_INTERFACE"))
     continue;
    else if((*key == 'M') && strnEQ(key, "MOD_PERL", 8))
     continue;
    else if((*key == 'T') && strnEQ(key, "TZ", 2))
     continue;
    else if((*key == 'P') && strEQ(key, "PATH"))
     continue;
   }// end - while
  delete_env(key, klen);
 }
}

For the GET method, the QUERY_STRING variable will contain the data from the form. For the POST method, the CGI program will get the data in the STDIN stream. Whatever other headers that the client sends to the server are all available to the CGI program as environment variables named HTTP_<header>.
These headers were sent in the request above:

Referer: http://luna/formtest.html
Connection: Keep-Alive
User-Agent: Mozilla/3.01 (Win95; I)
Host: luna
Accept: image/gif, image/x-xbitmap, image/jpeg, 
        image/pjpeg, */*

So these variables are available to the CGI program, with the contents of the respective headers:

HTTP_REFERER => http://luna/formtest.html
HTTP_CONNECTION => Keep-Alive
HTTP_USER_AGENT => Mozilla/3.01 (Win95; I)
HTTP_HOST => luna
HTTP_ACCEPT => image/gif, image/x-xbitmap, 
image/jpeg, image/pjpeg, */*

Keep in mind that not all browsers send the same headers, and even the same browser may send different headers under different circumstances. Also, some headers are not part of the HTTP specification, or may be experimental in nature. So if you use them, be sure to handle the contingency that they may not always be available.

Server To CGI - Quiz

As we mentioned earlier, there are two basic methods for passing the query to the CGI program. The next lesson explains the differences between the GET and POST methods. But, first click the Quiz link below to take a brief multiple-choice quiz in which you will analyze an HTTP request.
Server To CGI - Quiz

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