59 lines
3.9 KiB
Plaintext
59 lines
3.9 KiB
Plaintext
vaxocentrism
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/vak`sohsentrizm/ , n. [analogy with ethnocentrism ] A notional disease said
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to afflict C programmers who persist in coding according to certain
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assumptions that are valid (esp. under Unix) on VAXen but false elsewhere.
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Among these are: The assumption that dereferencing a null pointer is safe
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because it is all bits 0, and location 0 is readable and 0. Problem: this
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may instead cause an illegal-address trap on non-VAXen, and even on VAXen
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under OSes other than BSD Unix. Usually this is an implicit assumption of
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sloppy code (forgetting to check the pointer before using it), rather than
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deliberate exploitation of a misfeature. The assumption that characters are
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signed. The assumption that a pointer to any one type can freely be cast
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into a pointer to any other type. A stronger form of this is the assumption
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that all pointers are the same size and format, which means you don't have
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to worry about getting the casts or types correct in calls. Problem: this
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fails on word-oriented machines or others with multiple pointer formats. The
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assumption that the parameters of a routine are stored in memory, on a
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stack, contiguously, and in strictly ascending or descending order. Problem:
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this fails on many RISC architectures. The assumption that pointer and
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integer types are the same size, and that pointers can be stuffed into
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integer variables (and vice-versa) and drawn back out without being
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truncated or mangled. Problem: this fails on segmented architectures or
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word-oriented machines with funny pointer formats. The assumption that a
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data type of any size may begin at any byte address in memory (for example,
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that you can freely construct and dereference a pointer to a word- or
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greater-sized object at an odd char address). Problem: this fails on many
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(esp. RISC) architectures better optimized for HLL execution speed, and can
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cause an illegal address fault or bus error. The (related) assumption that
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there is no padding at the end of types and that in an array you can thus
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step right from the last byte of a previous component to the first byte of
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the next one. This is not only machine- but compiler-dependent. The
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assumption that memory address space is globally flat and that the array
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reference foo[-1] is necessarily valid. Problem: this fails at 0, or other
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places on segment-addressed machines like Intel chips (yes, segmentation is
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universally considered a brain-damaged way to design machines (see moby ),
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but that is a separate issue). The assumption that objects can be
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arbitrarily large with no special considerations. Problem: this fails on
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segmented architectures and under non-virtual-addressing environments. The
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assumption that the stack can be as large as memory. Problem: this fails on
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segmented architectures or almost anything else without virtual addressing
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and a paged stack. The assumption that bits and addressable units within an
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object are ordered in the same way and that this order is a constant of
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nature. Problem: this fails on big-endian machines. The assumption that it
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is meaningful to compare pointers to different objects not located within
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the same array, or to objects of different types. Problem: the former fails
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on segmented architectures, the latter on word-oriented machines or others
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with multiple pointer formats. The assumption that an int is 32 bits, or
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(nearly equivalently) the assumption that sizeof(int) == sizeof(long).
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Problem: this fails on PDP-11 s, 286-based systems and even on 386 and 68000
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systems under some compilers (and on 64-bit systems like the Alpha, of
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course). The assumption that argv[] is writable. Problem: this fails in many
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embedded-systems C environments and even under a few flavors of Unix. Note
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that a programmer can validly be accused of vaxocentrism even if he or she
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has never seen a VAX. Some of these assumptions (esp. 2--5) were valid on
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the PDP-11 , the original C machine, and became endemic years before the
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VAX. The terms vaxocentricity and all-the-world's-a-VAX syndrome have been
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used synonymously.
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