Software Testing: Understanding Bugs and Their Impact on Software Development - Prof. Spir, Assignments of Engineering

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Introduction to

Software Testing

[Reading assignment: Chapter 1, pp. 9-22]

What is a computer bug?

• In 1947 Harvard University was operating a room-sized computer called the Mark II. - mechanical relays - glowing vacuum tubes - technicians program the computer by reconfiguring it - Technicians had to change the occasional vacuum tube.
• A moth flew into the computer and was zapped by the high voltage when it landed on a relay.
• Hence, the first computer bug!
  • I am not making this up :-)

Defective Software

• We develop programs that contain defects - How many? What kind?
• Hard to predict the future, however… it is highly likely, that the software we (including you!) will develop in the future will not be significantly better.

Sources of Problems

• Requirements Definition: Erroneous, incomplete, inconsistent requirements.
• Design: Fundamental design flaws in the software.
• Implementation: Mistakes in chip fabrication, wiring, programming faults, malicious code.
• Support Systems: Poor programming languages, faulty compilers and debuggers, misleading development tools.

Adverse Effects of

Faulty Software

• Communications: Loss or corruption of communication media, non delivery of data.
• Space Applications: Lost lives, launch delays.
• Defense and Warfare: Misidentification of friend or foe.

Adverse Effects of Faulty

Software (Cont’d)

• Transportation: Deaths, delays, sudden acceleration, inability to brake.
• Safety-critical Applications: Death, injuries.
• Electric Power: Death, injuries, power outages, long-term health hazards (radiation).

Bug in Space Code

• Project Mercury’s FORTRAN code had the following fault: DO I=1.10 instead of ... DO I=1,
• The fault was discovered in an analysis of why the software did not seem to generate results that were sufficiently accurate.
• The erroneous 1.10 would cause the loop to be executed exactly once!

Military Aviation Problems

• An F-18 crashed because of a missing exception condition: if ... then ... without the else clause that was thought could not possibly arise.
• In simulation, an F-16 program bug caused the virtual plane to flip over whenever it crossed the equator, as a result of a missing minus sign to indicate south latitude.

Year Ambiguities (Cont’d)

• Mr. Blodgett’s auto insurance rate tripled when he turned 101.
• He was the computer program’s first driver over 100, and his age was interpreted as 1.
• This is a double blunder because the program’s definition of a teenager is someone under 20!

Dates, Times, and Integers

• The number 32,768 = has caused all sorts of grief from the overflowing of 16-bit words.
• A Washington D.C. hospital computer system collapsed on September 19, 1989, days after January 1, 1900, forcing a lengthy period of manual operation. 15

15

Shaky Math

• In the US, five nuclear power plants were shut down in 1979 because of a program fault in a simulation program used to design nuclear reactor to withstand earthquakes.
• This program fault was, unfortunately, discovered after the power plants were built!

Shaky Math (Cont’d)

• Apparently, the arithmetic sum of a set of numbers was taken, instead of the sum of the absolute values.
• The five reactors would probably not have survived an earthquake that was as strong as the strongest earthquake ever recorded in the area.

Therac-25 Radiation

“Therapy” (Cont’d)

• In Washington, 1987, a patient received 8,000-10,000 rads instead of the prescribed 86 rads.
• The patient died of complications of the radiation overdose.

AT&T Bug: Hello? ... Hello?

• In mid-December 1989, AT&T installed new software in 114 electronic switching systems.
• On January 15, 1990, 5 million calls were blocked during a 9 hour period nationwide.