c - C 中的(空)无限循环是否未定义行为？

Question

像 for (;;); 这样的无限循环是 C 中的未定义行为吗？ (它是 is for C++ ，但我不知道 C。)

Answer 1

不，for (;;) 语句的行为在 C 中有明确的定义。

N1570 ，这与官方 2011 ISO C 标准基本相同，在第 6.8.5 节第 6 段中说:

An iteration statement whose controlling expression is not a constant expression, that performs no input/output operations, does not access volatile objects, and performs no synchronization or atomic operations in its body, controlling expression, or (in the case of a for statement) its expression-3, may be assumed by the implementation to terminate.

有两个脚注:

An omitted controlling expression is replaced by a nonzero constant, which is a constant expression.

This is intended to allow compiler transformations such as removal of empty loops even when termination cannot be proven.

第一个脚注明确表示 for (;;) 被视为具有常量控制表达式。

规则的要点是在编译器无法证明循环终止时允许优化。但是，如果控制表达式是常量，编译器可以简单地证明循环是否终止，因此不需要额外的权限。