1 cgicgi common gateway interface server-side programming lecture
TRANSCRIPT
1
CGICGI
Common Gateway InterfaceCommon Gateway InterfaceCommon Gateway InterfaceCommon Gateway Interface
Server-side Server-side ProgrammingProgramming
Lecture
Rich Internet Applications
An RIA is a web application that provides the client with the features and functionality of desktop applications
Requires transferring the processing from the server to the client
Javascript is one enabling technology for a RIA
RIAs in the Internet client-server model
Client (browser) Web server
Client does all the processing (eg play videos as they come in)
Data (eg multimedia) stay on the server
HTTP request for resource
Server sends code but keeps data
Some technologies that support RIA development
Javascript (and associated ECMA dialects) Adobe Flash
Flash player and IDE Java Applets and Java Webstart (see later) AJAX
Asynchronous JavaScript and XML
Server-Side Programming
Lots of programs/applications designed to run on the machines on which they are installed
How can a remote client request access to these?
CGI programming
CGI => Common Gateway Interface A protocol for interfacing local applications with
a web server Sequence of steps
Client sends URL request Program runs at the server side Output is collected and sent back to the client Often the output is an HTML “built” by the
server
CGI using HTML and C language
Why do we need CGI? To read the information on the forms (HTML) To build a customised HTML response to users
To understand the concept lets use C at first...
CGI is completely independent of the language and OS
CGI is implemented in (almost) all webservers
CGI programs can be written in any language supported by the server.
This includes compiled programming languages, such as C and C++; interpreted languages, such as Perl, Python, Ruby, and languages, such as Java, that lie somewhere in between.
Hello World!#include <iostream>using namespace std;
int main(void) { cout << "Content-Type: text/html;charset=us-ascii\n\n";
/** Print the HTML response page to STDOUT. **/ cout << "<html>\n"; cout << "<head><title>CGI Output</title></head>\n"; cout << "<body>\n" ; cout << "<h1>Hello, world.</h1>\n"; cout << "this is my first CGI" << "\n"; cout << "</body>\n"; cout << "</html>\n"; return 0;}
Compile, then place the executable inside cgi-bin directory of xitamiTest using a browser, URL: http://localhost:8080/cgi-bin/helloworld
How to submit data using forms
GETGET http://www.someurl/cgi-bin/script?
var1=1&var2=4 Web server has a special directory called cgi-bin Two variables:
var1=1 var2=4
Special characters are encoded ~ ~ would be encoded as %7E %7E (% followed by
ASCII code)
GET GET
So variables from the forms go on URL The environment variable is:
$QUERY_STRING Most browsers limit the size of URLs (256
chars, some more, e.g., IE is 2083 chars) When you have too much data, use POSTPOST
instead...
Multiply example – the HTMLHTML file
<form method="getget"
action="http://it026945/cgi-bin/testingcgi/multiply">
<div><label>Number 1: <input name="m" size="5"></label></div>
<div><label>Number 2: <input name="n" size="5"></label></div>
<div><input type="submit" value="Multiply"></div>
</form>
Multiply example
Action="http://it026945/cgi-bin/testingcgi/multiply">
multiply is an executable under:/var/www/cgi-bin/
with x permissions for all!
Variables in URL:
After submission, URL becomes: http://it026945/cgi-bin/testingcgi/multiply?m=1&n=2
Example
SERVER-SIDE: Response
CLIENT-SIDE
Multiply example – the C file#include <stdio.h>#include <stdlib.h>#include <windows.h> //for Windows operating system – Sleep()
int main(void){char *data;long m,n;printf("%s%c%c\n","Content-Type:text/html;charset=iso-8859-1",13,10);printf("<TITLE>Multiplication results</TITLE>\n");printf("<H3>Multiplication results</H3>\n");data = getenv("QUERY_STRING");//here it is your data!!!if(data == NULL) printf("<P>Error!");else if(sscanf(data,"m=%ld&n=%ld",&m,&n)!=2)//check for 2 inputs printf("<P>Error! Invalid data.");else printf("<P>%ld * %ld = %ld.",m,n,m*n);//Sleep(1000); // uncomment that to see who runs the process...return 0;}
//from http://www.cs.tut.fi/~jkorpela/forms/cgic.html (July2010)
Recall the sscanf()sscanf() function in C
• On success, the function returns the number of items successfully read.
• This count can match the expected number of readings or fewer, even zero, if a matching failure happens.
• In the case of an input failure before any data could be successfully read, EOF is returned.
int sscanf ( const char * str, const char * format, ...);
Read formatted data from string
char * getenv ( const char * name );
Get environment string•Retrieves a C string containing the value of the environment variable whose name is specified as argument.
•If the requested variable is not part of the environment list, the function returns a NULL pointer.
•The string pointed by the pointer returned by this function shall not be modified by the program.
•The same memory location may be used in subsequent calls to getenv, overwriting the previous content.
getenv()getenv() function in C
char * fgets ( char * str, int num, FILE * stream );
Get string from stream
Reads characters from stream and stores them as a C string into str until (num-1) characters have been read or either a newline or a the End-of-File is reached, whichever comes first.
A newline character makes fgets stop reading, but it is considered a valid character and therefore it is included in the string copied to str.
A null character is automatically appended in str after the characters read to signal the end of the C string.
fgets()fgets() function in C
POSTPOST
(GET was originally used only to get data from server)
data is passed via standard input stream (stdin)
the length (in bytes) of the data passed via
$CONTENT_LENGTH. If the program reads more than the length,
...unpredictable behaviour may happen!
Multiply example – the HTML file
<form method="postpost" action="http://it026945/cgi-bin/testingcgi/multiply">
<div><label>Number 1: <input name="m" size="5"></label></div>
<div><label>Number 2: <input name="n" size="5"></label></div>
<div><input type="submit" value="Multiply"></div>
</form>
Multiply with POST – C file...#define MAXLEN 80int main(void){char *lenstr;char input[MAXLEN];long m,n, len;printf("%s%c%c\n","Content-Type:text/html;charset=iso-8859-1",13,10);lenstr = getenv("CONTENT_LENGTH");if(lenstr == NULL || sscanf(lenstr,"%ld",&len)!=1 || len > MAXLEN) printf("<P>There was an error in the content sent to Apache.");else { fgets(input, len+1, stdin); printf("<P>Form received by Apache.<br>"); printf("The form contains %ld bytes.<br>",len); printf("<P>Apache received this: %s <br>",input); if(sscanf(input,"m=%ld&n=%ld",&m,&n)!=2) printf("<P>An error occurred, both variables must be numeric."); else printf("<P><h3> %ld * %ld = %ld.</h3>",m,n,m*n);
}
return 0;
}
//adapted from http://www.cs.tut.fi/~jkorpela/forms/cgic.html (July2010)
Self-generating form in C#include <stdio.h>#include <stdlib.h>int main(void){char *data;long m,n;
printf("%s%c%c\n","Content-Type:text/html;charset=iso-8859-1",13,10);printf("<form method = "get"
action=\"http://it026945/cgi-bin/testingcgi/multiply2\"><div><label>Multiplicand 1: <input name=\"m\" size=\"5\"></label></div><div><label>Multiplicand 2: <input name=\"n\" size=\"5\"></label></div><div><input type=\"submit\" value=\"Multiply!\"></div></form>");
printf("<H3>Multiplication results</H3>");data = getenv("QUERY_STRING");if(data == NULL) printf("<P>Error! Error in passing data from form to script.");else if(sscanf(data,"m=%ld&n=%ld",&m,&n)!=2) printf("<P>Error! Invalid data. Data must be numeric.");else printf("<P>The product of %ld and %ld is %ld.",m,n,m*n);return 0;}
Self-generating form in CSelf-generating form in C#include <stdio.h>#include <stdlib.h>int main(void) {char *data;long m,n;static int flag=0;
printf("%s%c%c\n","Content-Type:text/html;charset=utf-8",13,10);printf("<form method = \"getget\" action=\"http://localhost:8080/cgi-bin/multiply2_utf8multiply2_utf8\"><div><label>Multiplicand 1: <input name=\"m\" size=\"5\"></label></div><div><label>Multiplicand 2: <input name=\"n\" size=\"5\"></label></div><div><input type=\"submit\" value=\"Multiply!\"></div></form>");printf("<H3>Multiplication results</H3>");data = getenvgetenv("QUERY_STRING");if(data == NULL) { if( !flag ){ printf("<P>nothing to compute yet."); } else {
printf("<P>Error! Error in passing data from form to script."); }
} else if(sscanfsscanf(data,"m=%ld&n=%ld",&m,&n)!=2) { printf("<P>Error! Invalid data. Data must be numeric.");} else { printf("<P>The product of %ld and %ld is %ld.",m,n,m*n); flag = 1;}return 0;
}
Handling Special CharactersHandling Special Characters
void decodedecode(char *src, char *last, char *dest){
for(; src != last; src++, dest++) if(*src == '+') *dest = ' '; else if(*src == '%') { int code; if(sscanf(src+1, "%2x", &code) != 1) code = '?'; *dest = code; src +=2; } else *dest = *src; *dest = '\n'; *++dest = '\0';}
Problems with CGIProblems with CGI
Each a time request is made, a new process is spawned on the server
This can quickly overwhelm sites that get a large number of hits
One solution is to install libraries directly callable by the web server
mod_perlmod_python
CGI can be inefficient...CGI can be inefficient...
The executable is loaded in the server's memory every time it is called
Multiple copies API would be more efficient...
Bad idea to do that using C/C++ Unstable environment (crash the entire server)
Apache offers modules with Perl and Python APIs
Scripting languages such as ASP and PHP
Security problems with CGISecurity problems with CGI
Program is running in your server... Suppose you want the user to run:
system "whois $username" ; But what if the user actually sends:
"john; rm -rf " system "whois john; rm -rf " ;
The administrator: “Oh dear!Where are all my files?”
In Linux
For Windows, http://technet.microsoft.com/en-us/sysinternals/bb897435.aspx
Server-side programmingServer-side programming
Better to use a language specially designed for server-side programming
See PHP programming next...
References
http://www.cs.tut.fi/~jkorpela/forms/cgic.html