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This rule means that statements such as

i = i + 1;
a[i] = i;

are allowed, while statements like

i = ++i + 1;  /* the value of i is modified twice between sequence points */
a[i++] = i;   /* the value of i is read other than to determine the value to be stored */

are not allowed because they modify the same value twice.

Non-Compliant Code Example

Programs cannot safely rely on the order of evaluation of operands between sequence points. In this non-compliant code example, the order of evaluation of the operands to the + is operator are unspecified.

a = i + b[++i];

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a = 1 + b[1];

As a result, programs cannot safely rely on the order of evaluation of operands between sequence points.

Compliant Solution

These examples are independent of the order of evaluation of the operands and can only be interpreted in one way.

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a = i + b[i+1];
++i;

Non-Compliant Code Example

Both of these statements violate the rule concerning sequence points stated above, so the behavior of these statements is undefined.

i = ++i + 1;  /* an attempt is made to modify the value of i twice between sequence points */
a[i++] = i;   /* an attempt is made to read the value of i other than to determine the value to be stored */

Compliant Solution

These statements are allowed by the standard.

i = i + 1;
a[i] = i;

Non-Compliant Code Example

The order of evaluation for function arguments is unspecified.

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The call to func() has undefined behavior because there are no sequence points between the argument expressions. The first (left) argument expression reads the value of i (to determine the value to be stored) and then modifies i. The second (right) argument expression reads the value of i between the same pair of sequence points as the first argument, but not to determine the value to be stored in i. This additional attempt to read the value of i has undefined behavior.

Compliant Solution

This solution is appropriate when the programmer intends for both arguments to func() to be equivalent.

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