Sequence Points
---------------

6.5 Expressions (pdf page 79)

1  An expression is a sequence of operators and operands that specifies
   computation of a value, or that designates an object or a function,
   or that generates side effects, or that performs a combination thereof.

2  Between the previous and next sequence point an object shall have its
   stored value modified at most once by the evaluation of an expression.(72)
   Furthermore, the prior value shall be read only to determine the value
   to be stored.(73)

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72) A floating-point status flag is not an object and can be set more than
    once within an expression.

73) This paragraph renders undefined statement expressions such as

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

    while allowing

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

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5.1.2.3 Program execution (pdf page 25)

1  The semantic descriptions in this International Standard describe
   the behavior of an abstract machine in which issues of optimization
   are irrelevant.

2  Accessing a volatile object, modifying an object, modifying a
   file, or calling a function that does any of those operations are
   all side effects, which are changes in the state of the execution
   environment. Evaluation of an expression may produce side effects.
   At certain specified points in the execution sequence called sequence
   points, all side effects of previous evaluations shall be complete and
   no side effects of subsequent evaluations shall have taken place.
   (A summary of the sequence points is given in annex C.)

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Annex C (informative) (pdf page 451)

Sequence points

1 The following are the sequence points described in 5.1.2.3:

  - The call to a function, after the arguments have been evaluated
    (6.5.2.2).

  - The end of the first operand of the following operators: logical AND &&
    (6.5.13); logical OR || (6.5.14); conditional ? (6.5.15); comma , (6.5.17).

  - The end of a full declarator: declarators (6.7.5);

  - The end of a full expression: an initializer (6.7.8); the expression in
    an expression statement (6.8.3); the controlling expression of a selection
    statement (if or switch) (6.8.4); the controlling expression of a while
    or do statement (6.8.5); each of the expressions of a for statement
    (6.8.5.3); the expression in a return statement (6.8.6.4).

  - Immediately before a library function returns (7.1.4).

  - After the actions associated with each formatted input/output function
    conversion specifier (7.19.6, 7.24.2).

  - 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 (7.20.5).