CGI Programming Part 2

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CGI Programming Part 2
2
Input Tags

• Many different ways of getting data from the
  user.
• The ltinputgt tag is used most often.
• ltinputgt has a type attribute
• Specifies the method with which to get data from
  the user.

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Other Attributes of ltinputgt

• name name of an input field.
• value value of an input field.
• size width of an input field.
• maxlength maximum number of characters that can
  be entered in an input field.
• checked whether a radio or checkbox is turned
  on.
• src url of an image.

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Input Type Text

• Setting the type to text creates a text box.
• For example,
• ltinput typetext
• namestreet
• size 30gt
• Note, name does not appear in the webpage.
• It is seen only by the CGI program.

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Input Type Radio

• Setting the type to radio creates a radio button.
• For example,
• ltinput typeradio nametitle valuems
  checkedgt Ms. ltbrgt
• ltinput typeradio nametitle valuemrsgt
  Mrs. ltbrgt
• ltinput typeradio nametitle valuemrgt Mr.
  ltbrgt
• ltinput typeradio nametitle valuedrgt Dr.
  ltbrgt
• All related radio buttons have the same name.
• The values are all preset.

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Input Type Submit and Reset

• submit creates a submit button to contact a CGI
  program.
• For example,
• ltinput typesubmit valueSubmit Formgt
• reset creates a button to erase all values in a
  form.
• For example,
• ltinput typereset valueClear Formgt
• The value appears in the button.

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Input Type Others

• checkbox creates a checkbox
• password creates a password field. A text field
  with characters displayed as .
• button creates a button the user can press.
• file creates a field to upload a file.
• image creates an image the user can click to
  submit a form.

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Inputs Other Than ltinputgt

• lttextareagt - multi-line text field. You can
  specify the rows and cols attributes.
• ltselectgt - creates a drop down menu.

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Handling the Input

• Input is sent to the CGI program specified in the
  ltformgt tag using either the get or post method.
• It is best to write CGI programs that handle
  both.
• Can be done by examining the environment
  variable, REQUEST_METHOD.

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An Example

• request_method ENVREQUEST_METHOD
• if (request_method eq GET)
• query_string ENVQUERY_STRING
• elsif (request_method eq POST)
• read(STDIN,
• query_string,
• ENVCONTENT_LENGTH)

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CGI.pm

• Many parts of CGI programs are repeated.
• Makes sense to capture these parts for reuse.
• CGI.pm is a module of Perl functions to do many
  common tasks.
• Many functions provide shortcuts to create HTML.
• Module can be used as a collection of functions
  or an object oriented class.

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Object-Oriented Use

• Can be used as an object-oriented class by
• use CGI
• q new CGI
• Access subroutines as methods of q
• For example,
• print q -gtstart_html()

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Function-Oriented Use

• Import a set of functions to be called directly.
• non-object oriented is often faster than
  object-oriented.
• Use a quoted list to tell Perl which functions to
  import.
• use CGI (start_html, end_html, header)
• CGI.pm also defines sets of these functions.
  standard and html3 are the most common.
• use CGI qw(standard html3)

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header

• The header function creates the HTTP header.
• The default is text/html.
• For example,
• print header()
• creates
• print Content-type text/html\n\n

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start_html

• start_html create the HTML header
• start_html can take one parameter, the title.
• For example,
• print start_html(My Home Page)
• creates
• ltHTMLgt
• ltHEADgt
• ltTITLEgt My Home Page lt/TITLEgt
• lt/HEADgt
• ltBODYgt

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start_html (cont.)

• start_html can also take named parameters.
• Specify attributes to give to the ltbodygt tag.
• For example,
• print start_html (-title gt My Title,
• -bgcolor gt Red)
• creates
• lthtmlgt
• ltheadgt
• lttitlegt My Title lt/titlegt
• lt/headgt
• ltbody bgcolor Redgt

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end_html

• end_html creates the ending part of an HTML
  document.
• For example,
• print end_html()
• creates
• lt/bodygt
• lt/htmlgt

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param

• param gets the parameter values sent to a CGI
  program.
• Query String
• nameMax20Powersjobceo
• Can get the query values by
• name param(name)
• job param(job)
• This is the scalar context.

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Contexts of param

• Can be called in list context with no arguments.
  Returns names of all parameters.
• e.g. _at_params param()
• Can be called in list context with an argument.
  Returns an array of all values for that argument.
• e.g. _at_vals param(title)

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Locking Files

• Many webpages allow multiple users to access the
  same information.
• i.e. multiple user writing to the same file.
• Need a mechanism to ensure consistency.
• Do not want one users action to undo the actions
  of another.
• A file must be locked when being written to.

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flock

• flock can be used to lock a file.
• The syntax is
• flock (filehandle, mode)
• Set mode to
• 2 to lock a file.
• 8 to unlock a file.
• For example,
• flock (FILE, 2)

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Debugging

• Debugging a CGI script can be frustrating.
• Browser will display 500 Internal Server Error
  if any errors occur.
• Some useful tips
• Run the program from the command line.
• Use perl c ltfilenamegt to see if file compiles.