A more obscure feature is the uncommon usage of comma operator. We often use the comma operator in variable declaration and in for loops. But it can also be used in any expression.

For instance, the next line has a valid C construct:

   return a, b, c;

This is particularly useful for setting variable when retuning after an error.

   if (ret = io(x))
       return errno = 10, -1;

The possibilities are endless. Another example:

    if (x = 1, y = 2, x < 3)
     ...

But the comma operator really shines when used in conjunction with macros.