vaxocentrism
/vak"soh-sen"trizm/ [analogy with "ethnocentrism"] A notional disease said to
afflict C programmers who persist in coding according to certain assumptions
that are valid (especially under Unix) on VAXen but false elsewhere. Among these
are:
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 (especially RISC) architectures better optimised 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,
Intel 80286-based systems and even on Intel 80386 and Motorola 68000 systems
under some compilers.
14. The assumption that "argv[]" is writable. Problem: this fails in many
embedded-systems C environments and even under a few flavours 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 (especially 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.
[Jargon File]
Nearby terms:
vaxherd « vaxism « VAX MIPS « vaxocentrism »
VAXset » VAXstation » VAX/VMS
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