fence

fence [short for defense], humanly erected barrier between two divisions of land, used to mark a legal or other boundary, to keep animals or people in or out, and sometimes as an ornament. In newly settled lands fences are usually made of materials at hand, e.g., stone, earth, or wood. A fence built of loose stones is called a dry-stone wall. Wooden fences may be built of boards, posts and rails, or pickets. Hardwoods such as oak and chestnut are preferred for fence posts, although softwoods treated with preservatives such as creosote may be used. Other fence materials are concrete, bricks, iron rails, woven wire, and barbed wire. Storm, or snow, fences are erected to prevent drifts from forming across roadways or against buildings. Rows of trees or shrubs (see hedge) are sometimes planted as windbreaks. See also wall.

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1.		fence - 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.		fence - 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.		fence - [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".