C Idioms and little known facts [closed]

Question

Okay, I've seen many posts here about odd idioms and common practices in C that might not be initially intuitive. Perhaps a few examples are in order

Elements in an array:

#define ELEMENTS(x) (sizeof (x) / sizeof (*(x)))

Odd array indexing:

a[5] = 5[a]

Single line if/else/while/for safe #defines

#define FOO(X) do { f(X); g(X); } while (0)
#define FOO(X) if (1) { f(X); g(X); } else

My question to the expert C programmers out there is: What idioms, practices, code snippits, or little known facts show up a lot in C code but might not be very intuitive but offer a good insight into C programming?

Answer 1

The "arrow operator" for counting down from n-1 to 0:

for ( int i = n; i --> 0; ) ...

It's not hugely common, but it's an interesting illustration that in some ways the initialize/test/update parts of a for loop are convention. That's the usual pattern, but you can still put any arbitrary expressions in there.

It's also a nice little reminder about how lexical analysis works.