What's this =! operator? [duplicate]
Long ago, when dinosaurs roamed the earth and C ran on 5th edition UNIX on PDP-11s, =! was the 'not equals' operator. This usage was deprecated by the creation of Standard C, so now it means 'assign the logical inverse', as in a = !b. This is a good argument for always surrounding binary operators with spaces, just to make it clear to the humans reading the code what the compiler is thinking.
I'm a bit surprised nobody else mentioned this, but then again I may be the only SO user to have ever touched a C compiler that old.