Web programming in C ++

What is CGI?

Common Gateway Interface or CGI is a set of standards that define how information is exchanged between a Web Server and a Custom Script.

CGI Specification currently maintained by NCSA and NCSA defines CGI as follows: Common Gateway Interface or CGI is a standard for external gateway programs to Interface with information from the Server such as HTTP Server .

The current version is CGI / 1.1 and CGI / 1.2 is in development.

Web Browsing

To understand the concept of CGI, we see what happens when we click on a hyperlink to a specific Webpage or URL.

Your browser contacts the HTTP Web Server and requests a URL, for example, filename.

Web Server will parse that URL and will search for filename. If it finds the requested file, the Web Server sends the file back to the browser; otherwise, it sends an error message indicating that you requested a wrong file.

The Web browser receives feedback from the Web Server and displays the received file or error message based on the received response.

However, it is possible to set up HTTP Server in such a way that whenever a file in a particular directory is requested, the file is not sent back; instead it is executed as a program and the output is generated from the program sent to the browser to display.

Common Gateway Interface or CGI is a standard protocol for applications (called CGI programs or CGI scripts) capable of interacting with Web Server and with Client. These CGI programs can be written in Python, PERL, Shell, C or C ++ .

CGI structure diagram

Below is a diagram of a simple structure of CGI:

Configure Web Server

Before you continue with CGI programming, make sure your Web Server supports CGI and it is configured to handle CGI Program. All CGI programs to be executed by HTTP Server are kept in a pre-configured directory. This directory is called CGI directory, and by convention, it is named / var / www / cgi-bin. By convention, CGI files have a .cgi extension, so they are executable in C ++.

By default, Apache Web Server is configured to run CGI Program in / var / www / cgi-bin. If you want to specify any other directory to run your CGI script, then you must modify the following area in the file httpd.conf:

  "/var/www/cgi-bin" > AllowOverride None Options ExecCGI Order allow,deny Allow from all  "/var/www/cgi-bin" > Options All 

First CGI program

Consider the content of the following C ++ program:

 #include using namespace std ; int main () { cout << "Content-type:text/htmlrnrn" ; cout << "n" ; cout << "n" ; cout << "Hello World - Chuong trinh CGI dau tienn" ; cout << "n" ; cout << "n" ; cout << " 

Hello World! Day is my first CGI program

 n" ; cout << "n" ; cout << "n" ; return 0 ; } 

Compile the above code and name it cplusplus.cgi. This file is kept in the / var / www / cgi-bin directory and it has the following content. Before running your CGI Program, make sure that you have switched the file mode by using the chmod 755 cplusplus.cgi command on UNIX to make the file executable. The result is:

Hello World! Day is my first CGI program

The C ++ program is a simple program that is writing output on the STDOUT file, for example: the screen. There is an important feature available so the first line is printed as Content-type: text / htmlrnrn. This line sends back to the browser and determines the content type to be displayed on the browser screen. You now understand the basics of CGI and you can write many complex CGI Programs using C ++. A CGI Program in C ++ can interact with any other peripheral systems such as RDBMS, to exchange information.

HTTP Header

The Content-type: text / htmlrnrn line is part of the HTTP Header, which is sent to the browser. All HTTP Headers have the following form:

 HTTP Field Name : Noi dung truong Vi du : Content - type : text / htmlrnrn 

Here are some important HTTP Headers, which you will use regularly in CGI Programming.

Header DescriptionContent-type: A MIME string that defines the format of the file being returned. Example: Content-type: text / html Expires: Date The date the information expires. It should be used by the browser to decide when a page should refresh. A valid date string should be in the format 01 Jan 1998 12:00:00 GMT Location: URL URL that should be returned instead of the requested URL. You can use this field to redirect a request to any file Last-modified: Date Last modified date of Content-length source: N Length, in bytes, of the data being returned. The browser uses this value to report the estimated download time for a file Set-Cookie: String Set the cookie passed through the string

Environment variable of CGI

All CGI Programs will have access to the following environment variables. These variables play an important role while writing any CGI Program:

Variable name DescriptionCONTENT_TYPE Data type of content. Used when the Client sends the attached content to the Server. For example: file upload CONTENT_LENGTH Length of query information. It is only available for HTTP_COOKIE POST requests. Returns the cookies set in the form that are the key / value pairs HTTP_USER_AGENT The request-header field is the User-Agent containing the information about the User Agent that started the request. It is the name of the web browser PATH_INFO Is the path for CGI script QUERY_STRING URL encoding information is sent with GET REMOTE_ADDR method The IP address of the remote host creates that request. It can be useful for log or authentication REMOTE_HOST purposes. The full name of the host creates that request. If this information is not available, REMOTE_ADDR can be used to retrieve the REQUEST_METHOD IP address. The method used to create the request. Common methods are GET and POST SCRIPT_FILENAME Path full to CGI script SCRIPT_NAME Name of CGI script server_name Hostname or IP address of Server SERVER_SOFTWARE Name and version of software that Server is running

The following is a small CGI program to list all CGI variables.

 #include #include using namespace std ; const string ENV [ 24 ] = { "COMSPEC" , "DOCUMENT_ROOT" , "GATEWAY_INTERFACE" , "HTTP_ACCEPT" , "HTTP_ACCEPT_ENCODING" , "HTTP_ACCEPT_LANGUAGE" , "HTTP_CONNECTION" , "HTTP_HOST" , "HTTP_USER_AGENT" , "PATH" , "QUERY_STRING" , "REMOTE_ADDR" , "REMOTE_PORT" , "REQUEST_METHOD" , "REQUEST_URI" , "SCRIPT_FILENAME" , "SCRIPT_NAME" , "SERVER_ADDR" , "SERVER_ADMIN" , "SERVER_NAME" , "SERVER_PORT" , "SERVER_PROTOCOL" , "SERVER_SIGNATURE" , "SERVER_SOFTWARE" }; int main () { cout << "Content-type:text/htmlrnrn" ; cout << "n" ; cout << "n" ; cout << "Cac bien CGIn" ; cout << "n" ; cout << "n" ; cout << "" ; for ( int i = 0 ; i < 24 ; i ++ ) { cout << "" << ENV [ i ] << "" ; // lay gia tri cua cac bien char * value = getenv ( ENV [ i ]. c_str () ); if ( value != 0 ){ cout << value ; } else { cout << "Bien moi truong nay khong ton tai." ; } cout << "n" ; } cout << "  ; cout << "n" ; cout << "n" ; return 0 ; } 

CGI library in C ++

With real-world examples, you will need to perform many operations by your CGI Program. There is a CGI library written for the C ++ program, which you can download from: ftp://ftp.gnu.org/gnu/cgicc/ and follow these steps to install this library:

 $tar xzf cgicc - X . X . X . tar . gz $cd cgicc - X . X . X / $ ./ configure -- prefix =/ usr $make $make install 

You can check the Documentation available at: C ++ CGI Lib Documentation.

POST and GET methods

You have a number of situations when you need to transfer some information from the browser to the Web Server and finally to your CGI Program. The two methods used most often by the browser to transmit this information to Web Server are GET methods and POST methods.

Transfer information using the GET method

GET method of encrypted user information is appended to the request page. Is the page and encrypted information differentiated by characters? as follows:

 http : //www.test.com/cgi-bin/cpp.cgi?key1=value1&key2=value2 

The GET method is the default method for transmitting information from the browser to the Web Server and it creates a long string that appears in the browser's Location: box. Never use the GET method if you have a password or other sensitive information to transmit to the Server. The GET method has a size limit and you can transmit 1024 characters in a Request String.

When using the GET method, the information transmitted by the HTTP Header field is QUERY_STRING and will be accessible in CGI Program via the QUERY_STRING environment variable.

You can transmit information by concatenating a sequence of key / value pairs along with any URL or you can use HTML tags to transmit information using the GET method.

For example, the URL simply uses the GET method

This is a simple URL that will pass two values ​​to the hello_get.py program using the method.

Below is the program to create the CGI Program cpp_get.cgi to handle the input provided by the web browser. We are using the CGI library in C ++, to make it easier to access the transmitted information:

 #include #include #include #include #include #include #include #include #include using namespace std ; using namespace cgicc ; int main () { Cgicc formData ; cout << "Content-type:text/htmlrnrn" ; cout << "n" ; cout << "n" ; cout << "Su dung cac phuong thuc GET va POSTn" ; cout << "n" ; cout << "n" ; form_iterator fi = formData . getElement ( "first_name" ); if ( ! fi -> isEmpty () && fi != (* formData ). end ()) { cout << "First name: " << ** fi << endl ; } else { cout << "Khong co du lieu nao duoc nhap vao cho first name" << endl ; } cout << "
n" ; fi = formData . getElement ( "last_name" ); if ( ! fi -> isEmpty () && fi != (* formData ). end ()) { cout << "Last name: " << ** fi << endl ; } else { cout << "Khong co du lieu nao duoc nhap vao cho last name" << endl ; } cout << "
n" ; cout << "n" ; cout << "n" ; return 0 ; } 

Now, compile the above program as follows:

 $g ++ - o cpp_get . cgi cpp_get . cpp - lcgicc 

Create cpp_get.cgi and place it in your CGI directory.

Example of FORM using the GET method

The following simple example will pass two values ​​using HTML FORM and submit button. We are using the same CGI Program as cpp_get.cgi to handle this input.

  action = "/cgi-bin/cpp_get.cgi" method = "get" > First Name:  type = "text" name = "first_name" >  /> Last Name:  type = "text" name = "last_name" />  type = "submit" value = "Submit" /> 

Transfer information using POST method

A more reliable general method for passing information to a CGI Program is POST. It encapsulates information in the same way as GET, but instead of sending it in the form of a text string after a sign? in the URL, it is sent in the form of a separate message. Notice this to CGI Script in the standard input form.

Use CGI Program cpp_get.cgi to handle POST method. We use the same example above, which will pass two values ​​using the HTML FORM and Submit button, but this time with the POST method as follows:

  action = "/cgi-bin/cpp_get.cgi" method = "post" > First Name:  type = "text" name = "first_name" >  /> Last Name:  type = "text" name = "last_name" />  type = "submit" value = "Submit" /> 

Pass Checkbox Data to CGI Program

Checkbox is used when more than one option is needed to be selected:

The following HTML code is an example for a form with two checkboxes:

  action = "/cgi-bin/cpp_checkbox.cgi" method = "POST" target = "_blank" >  type = "checkbox" name = "maths" value = "on" /> Maths  type = "checkbox" name = "physics" value = "on" /> Physics  type = "submit" value = "Select Subject" /> 

The following C ++ program will create cpp_checkbox.cgi to handle the input provided by the browser via the checkbox:

 #include #include #include #include #include #include #include #include #include using namespace std ; using namespace cgicc ; int main () { Cgicc formData ; bool maths_flag , physics_flag ; cout << "Content-type:text/htmlrnrn" ; cout << "n" ; cout << "n" ; cout << "Checkbox Data to CGIn" ; cout << "n" ; cout << "n" ; maths_flag = formData . queryCheckbox ( "maths" ); if ( maths_flag ) { cout << "Maths Flag: ON " << endl ; } else { cout << "Maths Flag: OFF " << endl ; } cout << "
n" ; physics_flag = formData . queryCheckbox ( "physics" ); if ( physics_flag ) { cout << "Physics Flag: ON " << endl ; } else { cout << "Physics Flag: OFF " << endl ; } cout << "
n" ; cout << "n" ; cout << "n" ; return 0 ; } 

Transfer Radiobutton Data to CGI Program

Radio Button is used when only one option is needed to choose.

The following HTML code is an example of a form with two radio buttons:

  action = "/cgi-bin/cpp_radiobutton.cgi" method = "post" target = "_blank" >  type = "radio" name = "subject" value = "maths" checked = "checked" /> Maths  type = "radio" name = "subject" value = "physics" /> Physics  type = "submit" value = "Select Subject" /> 

The following C ++ program will create cpp_radiobutton.cgi to handle the input provided by the web browser through the radio buttons:

 #include #include #include #include #include #include #include #include #include using namespace std ; using namespace cgicc ; int main () { Cgicc formData ; cout << "Content-type:text/htmlrnrn" ; cout << "n" ; cout << "n" ; cout << "Radio Button Data to CGIn" ; cout << "n" ; cout << "n" ; form_iterator fi = formData . getElement ( "subject" ); if ( ! fi -> isEmpty () && fi != (* formData ). end ()) { cout << "Radio box selected: " << ** fi << endl ; } cout << "
n" ; cout << "n" ; cout << "n" ; return 0 ; } 

Transmit Text Area Data to CGI Program

The TEXTAREA element is used when multi-line text must be passed to the CGI Program.

The following HTML code is an example for a form with a TEXTAREA box:

  action = "/cgi-bin/cpp_textarea.cgi" method = "post" target = "_blank" >  name = "textcontent" cols = "40" rows = "4" > Type your text here.  type = "submit" value = "Submit" /> 

The following C ++ program will create cpp_textarea.cgi to handle the input provided by the web browser through the text area:

 #include #include #include #include #include #include #include #include #include using namespace std ; using namespace cgicc ; int main () { Cgicc formData ; cout << "Content-type:text/htmlrnrn" ; cout << "n" ; cout << "n" ; cout << "Text Area Data to CGIn" ; cout << "n" ; cout << "n" ; form_iterator fi = formData . getElement ( "textcontent" ); if ( ! fi -> isEmpty () && fi != (* formData ). end ()) { cout << "Text Content: " << ** fi << endl ; } else { cout << "No text entered" << endl ; } cout << "
n" ; cout << "n" ; cout << "n" ; return 0 ; } 

Transfer DropDown Box Data to CGI Program

DropDown Box is used when there are many options available, but only one or two will be selected:

The following HTML code is an example of a form with a dropdown box:

  action = "/cgi-bin/cpp_dropdown.cgi" method = "post" target = "_blank" >  name = "dropdown" >  value = "Maths" selected > Maths  value = "Physics" > Physics  type = "submit" value = "Submit" /> 

The following C ++ program will create cpp_dropdown.cgi to handle the input provided by the web browser via the dropdown box:

 #include #include #include #include #include #include #include #include #include using namespace std ; using namespace cgicc ; int main () { Cgicc formData ; cout << "Content-type:text/htmlrnrn" ; cout << "n" ; cout << "n" ; cout << "Drop Down Box Data to CGIn" ; cout << "n" ; cout << "n" ; form_iterator fi = formData . getElement ( "dropdown" ); if ( ! fi -> isEmpty () && fi != (* formData ). end ()) { cout << "Value Selected: " << ** fi << endl ; } cout << "
n" ; cout << "n" ; cout << "n" ; return 0 ; } 

Use Cookies in CGI

HTTP Protocol is a Stateless protocol. But for a commercial website, it is necessary to maintain session information between different pages. For example, a user registration process may end after completing multiple pages. But how to maintain user session information when they browse through multiple pages is.

In many situations, using Cookie is the most effective way to remember and track orders, purchases, and favorite goods .

The way it works

Your server sends some data to the visitor browser in the form of a Cookie. The browser may accept this Cookie. If it accepts, it is stored in the form of a textual record on the visitor's hard drive. Now, when other users visit another page on your site, this Cookie is available for retrieval. When revoked, your server knows / remembers what is stored:

A cookie is a textual data record of 5 variable fields:

Expires : The date when Cookie expires. If left blank, Cookie expires when a visitor exits the browser.

Domain : The domain name of your site

Path : Path to the directory or Webpage that set that Cookie. It may be empty if you want to receive Cookies from any directory or page.

Secure : If this field contains the word "secure", then Cookie can only be retrieved with a Secure Server. If this field is blank, there will be no restrictions.

Name = Value : Cookies are set and retrieved in the form of key / value pairs.

Set Cookie

It's easy to send Cookies to the browser. These Cookies are sent along with the HTTP Header in front of the Content-type field. Suppose you want to set UserID and Password in Cookie format. The Cookie setting will be done as follows:

 #include using namespace std ; int main () { cout << "Set-Cookie:UserID=XYZ;rn" ; cout << "Set-Cookie:Password=XYZ123;rn" ; cout << "Set-Cookie:Domain=www.tutorialspoint.com;rn" ; cout << "Set-Cookie:Path=/perl;n" ; cout << "Content-type:text/htmlrnrn" ; cout << "n" ; cout << "n" ; cout << "Cookies in CGIn" ; cout << "n" ; cout << "n" ; cout << "Setting cookies" << endl ; cout << "
n" ; cout << "n" ; cout << "n" ; return 0 ; } 

From this example, you must understand how to set up Cookies. We use HTTP Header as Set-Cookie to set the Cookies.

Here, it is optional to set Cookie properties like Expires, Domain, and Path. Remember, Cookie is set before sending the following line: " Content-type: text / htmlrnrn" .

Compile the above program to create setcookies.cgi. It will set up 4 Cookies at your computer.

Recover Cookie

Recovering all set cookies is quite easy. Cookies stored in the CGI environment variable are HTTP_COOKIE and they will have the following form:

 key1 = value1 ; key2 = value2 ; key3 = value3 . 

Below is an example of how to retrieve Cookies:

 #include #include #include #include #include #include #include #include #include using namespace std ; using namespace cgicc ; int main () { Cgicc cgi ; const_cookie_iterator cci ; cout << "Content-type:text/htmlrnrn" ; cout << "n" ; cout << "n" ; cout << "Cookies in CGIn" ; cout << "n" ; cout << "n" ; cout << "" ; // get environment variables const CgiEnvironment & env = cgi . getEnvironment (); for ( cci = env . getCookieList (). begin (); cci != env . getCookieList (). end (); ++ cci ) { cout << "" << cci -> getName () << "" ; cout << cci -> getValue (); cout << "n" ; } cout << "  ; cout << "
n" ; cout << "n" ; cout << "n" ; return 0 ; } 

Now, compile the above program to create getcookies.cgi , and try getting a list of all the Cookies available on your computer.

It will give a list of 4 cookies set up in the previous section and all other Cookies set up at your computer:

 UserID XYZ Password XYZ123 Domain www . tutorialspoint . com Path / perl 

Example of File Upload

To upload a file, HTML code must have the enctype attribute set to multipart / form-data. The input tag with this file type will create a "Browse" button.

  enctype = "multipart/form-data" action = "/cgi-bin/cpp_uploadfile.cgi" method = "post" > 

 File:  type = "file" name = "userfile" /> 

 type = "submit" value = "Upload" /> 

Note : The above example has been disabled to prevent users from uploading files to our Server. But you can try the code above with your server.

This is the program cpp_uploadfile.cpp to process file upload:

 #include #include #include #include #include #include #include #include #include using namespace std ; using namespace cgicc ; int main () { Cgicc cgi ; cout << "Content-type:text/htmlrnrn" ; cout << "n" ; cout << "n" ; cout << "File Upload in CGIn" ; cout << "n" ; cout << "n" ; // get list of files to be uploaded const_file_iterator file = cgi . getFile ( "userfile" ); if ( file != cgi . getFiles (). end ()) { // send data type at cout. cout << HTTPContentHeader ( file -> getDataType ()); // write content at cout. file -> writeToStream ( cout ); } cout << "n" ; cout << "n" ; cout << "n" ; return 0 ; } 

The above example is writing the content at cout stream , but you can open your stream file and keep the content of the file upload in a file at v.