What is a “Bug”?

§Error: A human action that produces an incorrect result
§Fault: A manifestation of an error in software. The actual 'mistake' in the code
§Also known as a defect or bug
§If executed, a fault may cause a failure
§Failure: Deviation of the software from its expected delivery or service
Examples of Software Bug : http://www5.in.tum.de/~huckle/bugse.html

Error - Fault - Failure

A person makes an error ...… that creates a fault in the software ...… that can cause a failure in operation

Spectacular Software Failures

 NASA’s Mars lander: September 1999, crashed due to a units integration fault

 Toyota brakes : Dozens dead, thousands of crashes

 Major failures: Ariane 5 explosion, Mars Polar Lander, Intel’s Pentium FDIV bug

 Poor testing of safety-critical software can cost lives :

 THERAC-25 radiation machine: 3 dead

 Northeast Blackout of 2003 :508 generating units and 256 power plants shut down, Affected 10 million

people in Ontario, Canada, Affected 40 million people in 8 US states, Financial losses of $6 Billion USD

The alarm system in the energy management system failed due to a software error and operators were not

informed of the power overload in the system