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/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
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
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
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, Intel80286-based systems and even on Intel 80386 and Motorola68000 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.