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Evaluation of an expression may produce side effects. At specific points during execution, known as sequence points, all side effects of previous evaluations have completed, and no side effects of subsequent evaluations have yet taken place.

The C Standard, section subclause 6.5 [ISO/IEC 9899:2011], states:

If a side effect on a scalar object is unsequenced relative to either a different side effect on the same scalar object or a value computation using the value of the same scalar object, the behavior is undefined. If there are multiple allowable orderings of the subexpressions of an expression, the behavior is undefined if such an unsequenced side effect occurs in any of the orderings.

(See also undefined behavior 35 in Annex J of the C Standard.)

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• Between the evaluations of the function designator and actual arguments in a function call and the actual call.
• Between the evaluations of the first and second operands of the following operators:
  
  • logical Logical AND: &&
  • logical Logical OR: ||
  • commaComma: ,
• Between the evaluations of the first operand of the conditional ?: operator and whichever of the second and third operands is evaluated.
• The end of a full declarator.
• Between the evaluation of a full expression and the next full expression to be evaluated. The following are full expressions:
  
  • an An initializer that is not part of a compound literal
  • the The expression in an expression statement
  • the The controlling expression of a selection statement (if or switch)
  • the The controlling expression of a while or do statement
  • each Each of the (optional) expressions of a for statement
  • the The (optional) expression in a return statement
• Immediately before a library function returns.
• After the actions associated with each formatted input/output function conversion specifier.
• Immediately before and immediately after each call to a comparison function, and also between any call to a comparison function and any movement of the objects passed as arguments to that call.

Note that not all instances of a comma in C code denote a usage of the comma operator. For example, the comma between arguments in a function call is not a sequence point.

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

Or alternativelyAlternatively:

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

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This solution is appropriate when the programmer intends for the second argument to be one 1 greater than the first:

j = i++;
func(j, i);

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Attempting to modify an object multiple times between sequence points may cause that object to take on an unexpected value. This , which can lead to unexpected program behavior.

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Bibliography

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