fall through

programming (The American misspelling "fall thru" is

also common)

1. To exit a loop by exhaustion, i.e. by having fulfilled its

exit condition rather than via a break or exception condition

that exits from the middle of it. This usage appears to be

*really* old, dating from the 1940s and 1950s.

2. To fail a test that would have passed control to a

subroutine or some other distant portion of code.

3. In C, "fall-through" occurs when the flow of execution in a

switch statement reaches a "case" label other than by

jumping there from the switch header, passing a point where

one would normally expect to find a "break". A trivial

example:

switch (colour)

case GREEN:do_green();break;case PINK:do_pink();/* FALL THROUGH */case RED:do_red();break;default:do_blue();break;

The effect of the above code is to "do_green()" when colour is

"GREEN", "do_red()" when colour is "RED", "do_blue()" on any

other colour other than "PINK", and (and this is the important

part) "do_pink()" *and then* "do_red()" when colour is "PINK".

Fall-through is considered harmful by some, though there are

contexts (such as the coding of state machines) in which it is

natural; it is generally considered good practice to include a

comment highlighting the fall-through where one would normally

expect a break. See also Duff's Device.