$1.2 A REVIEW OF EXTERNAL FACTORS 5 Layers in a development process that Application library includes reuse ..More libraries... Base library Kernel library Compiler Operating System The conditional approach will also apply here:we should ensure that the libraries are correct and,separately,that the application is correct assuming the libraries are. Many practitioners,when presented with the issue of software correctness,think about testing and debugging.We can be more ambitious:in later chapters we will explore a number of techniques,in particular typing and assertions,meant to help build software that is correct from the start-rather than debugging it into correctness.Debugging and testing remain indispensable,of course,as a means of double-checking the result. It is possible to go further and take a completely formal approach to software construction.This book falls short of such a goal,as suggested by the somewhat timid terms“check”,“guarantee'”and"“ensure”used above in preference to the word"prove'”. Yet many of the techniques described in later chapters come directly from the work on mathematical techniques for formal program specification and verification,and go a long way towards ensuring the correctness ideal. Robustness Definition:robustness Robustness is the ability of software systems to react appropriately to abnormal conditions. Robustness complements correctness.Correctness addresses the behavior of a system in cases covered by its specification;robustness characterizes what happens outside of that specification. Robustness versus SPECIFICATION correctness Correctness Robustness§1.2 A REVIEW OF EXTERNAL FACTORS 5 The conditional approach will also apply here: we should ensure that the libraries are correct and, separately, that the application is correct assuming the libraries are. Many practitioners, when presented with the issue of software correctness, think about testing and debugging. We can be more ambitious: in later chapters we will explore a number of techniques, in particular typing and assertions, meant to help build software that is correct from the start — rather than debugging it into correctness. Debugging and testing remain indispensable, of course, as a means of double-checking the result. It is possible to go further and take a completely formal approach to software construction. This book falls short of such a goal, as suggested by the somewhat timid terms “check”, “guarantee” and “ensure” used above in preference to the word “prove”. Yet many of the techniques described in later chapters come directly from the work on mathematical techniques for formal program specification and verification, and go a long way towards ensuring the correctness ideal. Robustness Robustness complements correctness. Correctness addresses the behavior of a system in cases covered by its specification; robustness characterizes what happens outside of that specification. Definition: robustness Robustness is the ability of software systems to react appropriately to abnormal conditions. Application system Application library Operating System … More libraries … Base library Kernel library Hardware Compiler Layers in a development process that includes reuse Robustness versus correctness SPECIFICATION Correctness Robustness