vaxocentrism /vak`soh-sen'trizm/ n.
[analogy with
`ethnocentrism'] A notional disease said to afflict C programmers
who persist in coding according to certain assumptions that are
valid (esp. under Unix) on VAXen but false elsewhere. Among
these are:
- 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.
- The assumption that characters are signed.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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).
- The assumption that objects can be arbitrarily large with no
special considerations. Problem: this fails on segmented
architectures and under non-virtual-addressing environments.
- 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.
- 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.
- 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.
- 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).
- 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.
