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".

(01 Apr 1999)

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1. To fend off danger from; to give security to; to protect; to guard. "To fence my ear against thy sorceries." (Milton)

2. To inclose with a fence or other protection; to secure by an inclosure. "O thou wall! . . . Dive in the earth, And fence not Athens." (Shak) "A sheepcote fenced about with olive trees." (Shak) To fence the tables, to make a solemn address to those who present themselves to commune at the Lord's supper, on the feelings appropriate to the service, in order to hinder, so far as possible, those who are unworthy from approaching the table.

Origin: Fencing.

1. That which fends off attack or danger; a defense; a protection; a cover; security; shield. "Let us be backed with God and with the seas, Which he hath given for fence impregnable." (Shak) "A fence betwixt us and the victor's wrath." (Addison)

2. An inclosure about a field or other space, or about any object; especially, an inclosing structure of wood, iron, or other material, intended to prevent intrusion from without or straying from within. "Leaps o'er the fence with ease into the fold." (Milton)

In England a hedge, ditch, or wall, as well as a structure of boards, palings, or rails, is called a fence.

3. A projection on the bolt, which passes through the tumbler gates in locking and unlocking.

4. Self-defense by the use of the sword; the art and practice of fencing and sword play; hence, skill in debate and repartee. See Fencing. "Enjoy your dear wit, and gay rhetoric, That hath so well been taught her dazzing fence." (Milton) "Of dauntless courage and consummate skill in fence." (Macaulay)

5. A receiver of stolen goods, or a place where they are received.

Origin: Abbrev. From defence.

(01 Mar 1998)

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