fence

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 optimised 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 optimising compilers] Any technique,

usually exploiting knowledge about the compiler, that blocks

certain optimisations. 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 optimiser's

register-colouring info" can be expressed by the shorter

"That's a fence procedure".

[Jargon File]

(1999-01-08)