Articles

Bash Fork Bomb And The Cure

24th June 2008 - 4 minutes read time

A fork bomb is a simple bit of shell code that, once run, will soon fill all available memory and fork space with itself. Here is the code, and remember, don't try this at home!

$ :(){ :|:& };:

To explain what is going on we need to cut this code into sections. The first thing we do is refine a function called ":", which accepts no parameters.

$ :(){};

We then get this function to run itself recursively and also to run another version of itself in the background, this creates another fork of the program.

$ :|:&

Finally we start it all off with the first function call.

In order to validate any page of HTML or XHTML you will need a doctype. This is a string of text that sits at the top of the document and tells the browser exactly what markup standard has been used to create the page.

XHTML Strict

This doctype is used in an XHTML document when you are not using any framset or depreciated tags.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

XHTML Transitional

This doctype is used if your XHTML document contains depreciated tags like <b>.<.p>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

XHTML Frameset

Use this XHTML doctype if your document contains either frameset tags or depreciated tags, or both.

Enabling Tabbing In A Textarea

20th June 2008 - 4 minutes read time

When a user presses the tab key in a browser window the normal action is for the user to move the focus to a different control. To enable a word processor like tab effect in a text area you need to catch the keystroke and add in the tab character to where ever the cursor is. This is the main issue with creating this solution, it is easy to add a tab to the end of the text, but most users might want to add a tab half way through the text.

Take the following HTML text area.

<textarea name="content" class="textarea" id="content" rows="20" cols="65"  wrap="on" onKeyDown="return catchTab(this,event)"></textarea>

When a keystroke is detected it runs the catchTab() function. This function detects if the keystroke is 9 (which means it is a tab) and runs the function called replaceSelection() in order to find out where the cursor is and replace the text that exists there.

Submitting A HTML Form Using JavaScript

19th June 2008 - 1 minute read time

In order to submit a form using an event you need to run a click event on any submit button in the form. Take the following form.

<form method="post" onsubmit="alert('Form submitted!'); return false;">
    <input type="text" name="testvalue" value="" />
    <input type="submit" name="submit" id="submit" value="Submit" />
</form>

To run a submit event using JavaScript we just find the submit button via the id and run click() against it.

document.getElementById('submit').click();

That's it!

Time Calculator In PHP

18th June 2008 - 2 minutes read time

Use the following function to work out how long it has been since an event in years, months, weeks, days, hours, minutes and seconds.

function getAge($year,$month,$day,$hour=0,$minute=0,$second=0){
 $age = mktime($hour,$minute,$second,$month,$day,$year);
 $age = time()-$age;
 return array('years'=>$age/60/60/24/365,
  'months'=>$age/60/60/24/12,
  'weeks'=>$age/60/60/24/7,
  'days'=>$age/60/60/24,
  'hours'=>$age/60/60,
  'minutes'=>$age/60,
  'seconds'=>$age);
}

The practical use of this function is that you can work out how old someone is from their birthday. Here is an example of the function in use.

// someone's birthday
echo '<pre>'.print_r(getAge(1984,10,4),true).'</pre>';

Which would output the following:

Print A Current Copyright Notice With PHP

17th June 2008 - 3 minutes read time

One thing that is inexcusable on any website is printing a copyright notice that is out of date. There are many protagonists of this crime and it is fairly easy to spot.

However, rather than go through all of the pages on your site and hand code in the copyright notice you could just put in the following little bit of PHP.

Copyright <?php echo date("Y"); ?> #! code

This will print off a current copyright notice, no matter what the year is.

Copyright 2008 #! code

The date() function is built into PHP and takes two parameters. The first (required) parameter is a format string for the date. In this example we are giving a single capital "Y", which returns a numeric representation of the year with 4 digits.

Calculate Distance Between Two Geographical Points With PHP

16th June 2008 - 2 minutes read time

Use the following function to work out the distance between two geographical points. Geographical points are usually longitude and latitude, in degrees. The first thing to do is to convert these values into radians (using the deg2rad() PHP function) so that we can work with them. The four basic parameters used are the longitude and latitude values for the two points. The optional fifth value is to have the end value returned in miles, rather than kilometres.

Shortening Long URLs With PHP

13th June 2008 - 2 minutes read time

Print out a full URL for a link will sometimes mess up your formatting, especially if you URL is quite long. This might be the case if you are linking to a Google search page, or have an automated script that shows numerous URLs of indeterminate length. The following function will reduce any URL longer than 45 characters by splitting it in two and join them up with a simple string.

function shortenurl($url)
{
 if ( strlen($url) &gt; 45) {
  return substr($url, 0, 30)."[...]".substr($url, -15);
 } else {
  return $url;
 }
}

You can use the function in the following way.

Preparing A URL With PHP

12th June 2008 - 4 minutes read time

There might be many instances where you will create a program in PHP that takes a URL as input and does something with the address. This might be a site analysis or an image resize, but whatever the use is, you need to be sure that the URL will work or at least has the same format.

What users tend to leave out of a URL string is the http:// bit at the start. You could validate the URL to force the user to do this, but you will end up annoying a few people. By far the best way of making sure that the URL has the http:// bit at the start is by adding it behind the scenes. The best way to this is to remove the http:// from the start of the string, even if it isn't there and then add it back on.

Get Information About A MySQL Database With PHP

11th June 2008 - 4 minutes read time

By using the MySQL command.

SHOW TABLE STATUS FROM database;

You can get all sorts of information about a database. The query returns each table as a row and gives lots of information about each table. Using this query it is possible to work out some usage data for the database as a whole. The following function will take a database name and a database resource handle and return how big that database is, the number of tables, and the number of rows in those tables. You will probably have a maximum limit to the amount of data that you can store in your database, so this function is useful to make sure that you don't exceed this limit. This function also uses another function found on the #! code site called readableFileSize() to give more meaningful data sizes.