What's a ‘sequence point’

A sequence point is a point in time at which the dust has settled and all side effects which have been seen so far are guaranteed to be complete. The sequence points listed in the C standard are:

• at the end of the evaluation of a full expression (a full expression is an expression statement, or any other expression which is not a subexpression within any larger expression);
• at the ||, &&, ?:, and comma operators; and
• at a function call (after the evaluation of all the arguments, and just before the actual call).

The Standard states that

Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be accessed only to determine the value to be stored.

These two rather opaque sentences say several things. First, they talk about operations bounded by the ``previous and next sequence points''; such operations usually correspond to full expressions. (In an expression statement, the ``next sequence point'' is usually at the terminating semicolon, and the ``previous sequence point'' is at the end of the previous statement. An expression may also contain intermediate sequence points, as listed above.)

so the code

a ^= b ^= a ^= b

It attempts to modify the variable a twice between sequence points, so its behavior is undefined.

also

a[i] = i++;

The subexpression i++ causes a side effect--it modifies i's value--which leads to undefined behavior since i is also referenced elsewhere in the same expression. There is no way of knowing whether the reference will happen before or after the side effect--in fact,neither obvious interpretation might hold;