C Language Unsequenced expressions


Example

C11

The following expressions are unsequenced:

a + b;
a - b;
a * b;
a / b;
a % b;
a & b;
a | b;

In the above examples, the expression a may be evaluated before or after the expression b, b may be evaluated before a, or they may even be intermixed if they correspond to several instructions.

A similar rule holds for function calls:

f(a, b);

Here not only a and b are unsequenced (i.e. the , operator in a function call does not produce a sequence point) but also f, the expression that determines the function that is to be called.

Side effects may be applied immediately after evaluation or deferred until a later point.

Expressions like

x++ & x++;
f(x++, x++); /* the ',' in a function call is *not* the same as the comma operator */
x++ * x++;
a[i] = i++;

or

x++ & x;
f(x++, x);
x++ * x;
a[i++] = i;

will yield undefined behavior because

  • a modification of an object and any other access to it must be sequenced
  • the order of evaluation and the order in which side effects1 are applied is not specified.

1 Any changes in the state of the execution environment.