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vaxocentrism

1. The assumption that dereferencing a null pointer is safe because it

     is all bits 0, and location 0 is readable and 0.  Problem: this may
     instead cause an illegal-address trap on non-VAXen, and even on
     VAXen under OSes other than BSD Unix.  Usually this is an implicit
     assumption of sloppy code (forgetting to check the pointer before
     using it), rather than deliberate exploitation of a misfeature.

2. The assumption that characters are signed.

3. The assumption that a pointer to any one type can freely be cast

     into a pointer to any other type.  A stronger form of this is the
     assumption that all pointers are the same size and format, which
     means you don't have to worry about getting the casts or types
     correct in calls.  Problem: this fails on word-oriented machines
     or others with multiple pointer formats.

4. The assumption that the parameters of a routine are stored in

     memory, on a stack, contiguously, and in strictly ascending or
     descending order.  Problem: this fails on many RISC architectures.

5. The assumption that pointer and integer types are the same size,

     and that pointers can be stuffed into integer variables (and
     vice-versa) and drawn back out without being truncated or mangled.
     Problem: this fails on segmented architectures or word-oriented
     machines with funny pointer formats.

6. The assumption that a data type of any size may begin at any byte

     address in memory (for example, that you can freely construct and
     dereference a pointer to a word- or greater-sized object at an odd
     char address).  Problem: this fails on many (esp. RISC)
     architectures better optimized for HLL execution speed, and can
     cause an illegal address fault or bus error.

7. The (related) assumption that there is no padding at the end of

     types and that in an array you can thus step right from the last
     byte of a previous component to the first byte of the next one.
     This is not only machine- but compiler-dependent.

8. The assumption that memory address space is globally flat and that

     the array reference `foo[-1]' is necessarily valid.  Problem: this
     fails at 0, or other places on segment-addressed machines like
     Intel chips (yes, segmentation is universally considered a
     brain-damaged way to design machines (see moby), but that is a
     separate issue).

9. The assumption that objects can be arbitrarily large with no

     special considerations.  Problem: this fails on segmented
     architectures and under non-virtual-addressing environments.

10. The assumption that the stack can be as large as memory. Problem:

     this fails on segmented architectures or almost anything else
     without virtual addressing and a paged stack.

11. The assumption that bits and addressable units within an object

     are ordered in the same way and that this order is a constant of
     nature.  Problem: this fails on big-endian machines.

12. The assumption that it is meaningful to compare pointers to

     different objects not located within the same array, or to objects
     of different types.  Problem: the former fails on segmented
     architectures, the latter on word-oriented machines or others with
     multiple pointer formats.

13. The assumption that an `int' is 32 bits, or (nearly equivalently)

     the assumption that `sizeof(int) == sizeof(long)'.  Problem: this
     fails on PDP-11s, 286-based systems and even on 386 and 68000
     systems under some compilers (and on 64-bit    systems like the
     Alpha, of course).

14. The assumption that `argv[]' is writable. Problem: this fails in

     many embedded-systems C environments and even under a few flavors
     of Unix.

Note that a programmer can validly be accused of vaxocentrism even if he or she has never seen a VAX. Some of these assumptions (esp. 2-5) were valid on the PDP-11, the original C machine, and became endemic years before the VAX. The terms `vaxocentricity' and `all-the-world's-a-VAX syndrome' have been used synonymously.

14 hits in May 2012 mes@science.uva.nl