fence n. 1.
A sequence of one or more distinguished (out-of-band) characters (or other data
items), used to delimit a piece of data intended to be treated as
a unit (the computer-science literature calls this a `sentinel').
The NUL (ASCII 0000000) character that terminates strings in C is
a fence. Hex FF is also (though slightly less frequently) used
this way. See zigamorph. 2. An
extra data value inserted in an array or other data structure in
order to allow some normal test on the array's contents also to
function as a termination test. For example, a highly optimized
routine for finding a value in an array might artificially place
a copy of the value to be searched for after the last slot of the
array, thus allowing the main search loop to search for the value
without having to check at each pass whether the end of the array
had been reached. 3. [among users of optimizing compilers] Any
technique, usually exploiting knowledge about the compiler, that
blocks certain optimizations. Used when explicit mechanisms are
not available or are overkill. Typically a hack: "I call a dummy
procedure there to force a flush of the optimizer's
register-coloring info" can be expressed by the shorter "That's a
fence procedure".
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fencepost error n.
1. [common] A problem with the discrete equivalent of a
boundary condition, often exhibited in programs by iterative
loops. From the following problem: "If you build a fence 100 feet
long with posts 10 feet apart, how many posts do you need?"
(Either 9 or 11 is a better answer than the obvious 10.) For
example, suppose you have a long list or array of items, and want
to process items m through n; how many items are there? The
obvious answer is n - m, but that is off by one; the right answer
is n - m + 1. A program that used the `obvious' formula would
have a fencepost error in it. See also zeroth and off-by-one error, and note that not
all off-by-one errors are fencepost errors. The game of Musical
Chairs involves a catastrophic off-by-one error where N people
try to sit in N - 1 chairs, but it's not a fencepost error.
Fencepost errors come from counting things rather than the spaces
between them, or vice versa, or by neglecting to consider whether
one should count one or both ends of a row. 2. [rare] An error
induced by unexpected regularities in input values, which can
(for instance) completely thwart a theoretically efficient binary
tree or hash table implementation. (The error here involves the
difference between expected and worst case behaviors of an
algorithm.)
Node:fiber-seeking backhoe,
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fiber-seeking backhoe
[common among backbone ISP personnel] Any of a genus of large,
disruptive machines which routinely cut critical backbone links,
creating Internet outages and packet over air problems.