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.
