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Why does software have bugs?

• Posted by JeremyM on May 7th, 2010 filed in SQA - Expert Advice
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Some people wonder “where do software bugs come from?” or “how can I prevent bugs in my code?” Well the start to eliminating bugs begins in understanding how bugs get into the software.

Below are a couple ways in which bugs are allowed to infiltrate software:

• miscommunication or no communication
  - as to specifics of what an application should or shouldn’t do (the application’s requirements).

• software complexity
  - the complexity of current software applications can be difficult to comprehend for anyone without experience in modern-day software development. Multi-tier distributed systems, applications utilizing mutliple local and remote web services applications, data communications, enormous relational databases, security complexities, and sheer size of applications have all contributed to the exponential growth in software/system complexity.

• programming errors
  - programmers, like anyone else, can make mistakes.

• changing requirements (whether documented or undocumented)
  - the end-user may not understand the effects of changes, or may understand and request them anyway
  - redesign, rescheduling of engineers, effects on other projects, work already completed that may have to be redone or thrown out, hardware requirements that may be affected, etc. If there are many minor changes or any major changes, known and unknown dependencies among parts of the project are likely to interact and cause problems, and the complexity of coordinating changes may result in errors.

• time pressures
  - scheduling of software projects is difficult at best, often requiring a lot of guesswork. When deadlines loom and the crunch comes, mistakes will be made.

• egos
  - people prefer to say things like:

    'no problem'
    'piece of cake'
    'I can whip that out in a few hours'
    'it should be easy to update that old code'
  
   instead of:
    'that adds a lot of complexity and we could end up
       making a lot of mistakes'
    'we have no idea if we can do that; we'll wing it'
    'I can't estimate how long it will take, until I
       take a close look at it'
    'we can't figure out what that old spaghetti code
       did in the first place'
  
   If there are too many unrealistic 'no problem's', the
   result is bugs.
  

• poorly documented code
  - it’s tough to maintain and modify code that is badly written or poorly documented; the result is bugs. In many organizations management provides no incentive for programmers to document their code or write clear, understandable, maintainable code. In fact, it’s usually the opposite: they get points mostly for quickly turning out code, and there’s job security if nobody else can understand it (‘if it was hard to write, it should be hard to read’).

• software development tools
  - visual tools, class libraries, compilers, scripting tools, etc. often introduce their own bugs or are poorly documented, resulting in added bugs.

Hopefully this explanation of the main causes for bugs will aid in your identification of these areas in your own organization.