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.
