data validation (was re self-modifying code)

[David Collier-Brown]

>From article <61251 at sun.uucp>, by guy at gorodish.Sun.COM (Guy Harris):
> input data before using it (unless they have good reason to know the data will
> *never* be incorrect); I would prefer to see
> 	input, line 12: value "12038" is out of range (should be between
> 	  17 and 137)
> 
> than to see
> 	ILLEGAL ARRAY REFERENCE: "frobozz.q", line 31
> 		Subscript was 12038; should be between 17 and 137
> 

  When I look in my Mutlics standard systems designers' notebook, I
find the following words of wisdom...

	Subroutines should not check the number or type of input
  arguments, but assume they have been called correctly.
  Subroutines should not check the number or type of nor validate
  the correctness of their input arguements, unless it is part of
  their intended operation [but see below --dave].  However,
  subroutines which accept structure arguements should check the
  input structure version number for validity.  


  What the Multicians are saying is exactly what Guy says: input
routines validate input as part of their purpose in life.  Other
routines assume the data is valid, and don't put in checks unless
thay have to deal with "versioned" structures.
  This is for a machine which happily passes descriptors of arrays
around, and manages to bounds-check array references in parallell
with the fetch. 

  Depending on the hrdware or compiler to catch invalid data by
trapping on its use has been a known bad practice since well before
Unix... The manual above is a reprint, circa 1980.


--dave (see .signature below) c-b
-- 
 David Collier-Brown.  {mnetor yunexus utgpu}!geac!daveb
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