Kore Nordmann - PHP / Projects / Politics
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

:Author: 		Kore Nordmann
:Date:			Mon, 03 Dec 2007 15:31:05 +0100
:Revision: 		4
:Copyright:		CC by-sa


======================
Evil bugs in your code
======================

:Description:
  From time to time I write some code snippets, which I find really hard to
  debug, and the code does not do, what it is expected to do. Especially when
  the code is not tested directly, but only executed in some a bit more
  complex environment. Those are 4 typical errors I introduced in my code, and
  spend some time debugging it, because I found them really hard to spot.
  Luckily, once I spotted the actual bug, I find it a lot easier the next time
  the typo occurs.  Therefore I want to share those, so that I may save you
  some minutes of your life hunting stupid bugs.

From time to time I write some code snippets, which I find really hard to
debug, and the code does not do, what it is expected to do. Especially when
the code is not tested directly, but only executed in some a bit more complex
environment.

Those are 4 typical errors I introduced in my code, and spend some time
debugging it, because I found them really hard to spot. Luckily, once I
spotted the actual bug, I find it a lot easier the next time the typo occurs.
Therefore I want to share those, so that I may save you some minutes of your
life hunting stupid bugs.

The missing 'if'
----------------

::

	( $test->check( $foo ) )
	{
		// Why is this always executed?
	}

An additional ;
---------------

::
	
	while ( $line = fread( $fp, 1024 ) );
	{
		$text .= $line;
	}

	// Damn, why is $text still empty?

Unexpected operator precendence
-------------------------------

::

	// WTH isn't this a valid query?
	$nodeList = $xpath->query( '//*[@attribute="row_' . $row + 1 . '"]' );

The non working for loop
------------------------

::

	// Parse error: Unexpected ; in line 23
	foreach ( $i = 0; $i < 10; ++$i )
	{
		// Do something
	}

Know some more?
---------------

Do you know more hard to detect trivial code bugs, you may spare me some
minutes, if it occurs for me?


Trackbacks
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Comments
========

- s0enke at Mon, 03 Dec 2007 21:11:56 +0100

  if ($blah = 1) { //always true, forgot second = }

- sapphirecat at Tue, 04 Dec 2007 14:26:58 +0100

  Not exactly common, but I had a big if/elseif/elseif/... chain once. I
  converted the final 'elseif' to an 'else', and spent half an hour trying to
  figure out why "else ($var > 1)" was a syntax error. I kept reading it "else
  $var > 1... that's true... so why is the compiler complaining?"

  Mostly, my problems anymore are missing/extra semicolons or parens, which
  the compiler picks up on before things get too out of hand.

- motzel at Tue, 04 Dec 2007 21:46:02 +0100

  @s0enke:

  If you reverse operators it will never be problem again. So instead:

  if($blah == 1)

  always use

  if(1 == $blah)

  With such a notation, when you accidentaly forgot one of equal sign,
  compiler report a syntax error.

- John J. at Tue, 04 Dec 2007 22:23:27 +0100

  @motzel

  I agree, that's an easy way to prevent that problem. However, you have to be
  in the habit of writing your code that way. I try to often, but frequently
  forget because I have been doing it the other way since I was taught it in
  grade school and all through my schooling.

  The other listed common mistakes are easy to prevent too if you follow best
  practices, but they are also easy to make.

  My most common mistakes are missing syntax characters, most often the
  closing parens for if/else/etc. statements, usually when the case is just a
  function call (isset(), empty(), etc.). Its easy to catch, but still a
  delay.

- Saur at Wed, 05 Dec 2007 07:18:22 +0100

  never had that bugs... it's kinda lame bugs.

  "If you reverse operators it will never be problem again" that is nice. thx.

- open source at Wed, 05 Dec 2007 09:29:39 +0100

  I never did any of this bug ;)

- Chris at Thu, 06 Dec 2007 13:41:33 +0100

  The last minute variable change :

  for ($i = 0; $l < 10; ++$i) {  

         // Here we go for the infinite loop }

- Patrice at Thu, 06 Dec 2007 17:38:25 +0100

  You place a ; after the while . $i = 0; while ( $i < 10 ); {

      $i++; //Do Something }

- Fatiha at Fri, 07 Dec 2007 14:08:26 +0100

  Put an empty line before :

  <? header("file.php"); ?>

  and you have that error : Cannot add header information - headers already
  sent by (output started at [script1]) in [script2] on line [line]

  Fatiha

- tbela99 at Fri, 07 Dec 2007 18:20:00 +0100

  I just had headaches about this one

  //$a is previously null //excpecting $a to be an array if($a =
  get_some_data($param) && isset($a[1]) { // the following code is never
  executed

  }

  //I should have set $a before testing