c - 这是 printf 函数的未定义行为吗？

Question

这个问题已经有答案了:

Why are these constructs using pre and post-increment undefined behavior? (14 个回答)

已关闭 7 年前。

 int c=2; printf("%d %d %d",c,c<<=2,c>>=2);

这是未定义的行为吗？

 int c=2;
 printf ("%d %d %d",c,c<<=2,c>>=2);

Answer 1

行为未定义。

[C99: 4/2]: If a ‘‘shall’’ or ‘‘shall not’’ requirement that appears outside of a constraint is violated, the behavior is undefined. Undefined behavior is otherwise indicated in this International Standard by the words ‘‘undefined behavior’’ or by the omission of any explicit definition of behavior. There is no difference in emphasis among these three; they all describe ‘‘behavior that is undefined’’.

[C99: 5.1.2.3/2]: Accessing a volatile object, modifying an object, modifying a file, or calling a function that does any of those operations are all side effects,11) 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.)

[C99: C/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).

[C99: 6.2.2.2/10]: The order of evaluation of the function designator, the actual arguments, and subexpressions within the actual arguments is unspecified, but there is a sequence point before the actual call.

而且，虽然我无法证明是否定的，但没有任何措辞可以对 varargs 情况进行异常(exception)处理。