Of course, `&&` in C is short-circuiting so it's safe without the `strlen()` too, as long as the argument is there i.e. not NULL.

Also, the use of a convoluted `if` to conditionally assign a literal boolean is a code smell (to me), I would drop the `if` and just use:

    in_reverse = argc > 0 && argv[1][0] == '-' && argv[1][1] == 'r';

if a more forward-thinking/strict check is not needed.