/fȯl thrü/
v. (n. 'fallthrough', var. 'fall-through') 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 (color)
{
case GREEN:
do_green();
break;
case PINK:
do_pink();
/* FALL THROUGH */
case RED:
do_red();
break;
default:
do_blue();
break;
}The variant spelling '/* FALL THRU */' is also common.
The effect of this code is to do_green() when color is 'GREEN', do_red() when color is 'RED', do_blue() on any other color other than 'PINK', and (and this is the important part) do_pink() *and then* do_red() when color 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.