c - 局部变量前的 GOTO

Question

以下代码是否构成未定义行为，因为我在变量声明之前跳转并通过指针使用它？如果有，标准之间是否存在差异？

int main() {
  int *p = 0;
label1: 
  if (p) {
    printf("%d\n", *p);
    return 0;
  }
  int i = 999;
  p = &i;
  goto label1;
  return -1;
}

Answer 1

您的程序中没有未定义的行为。

goto 语句有两个约束:

(c11, 6.8.6.1p1) "The identifier in a goto statement shall name a label located somewhere in the enclosing function. A goto statement shall not jump from outside the scope of an identifier having a variably modified type to inside the scope of that identifier."

您没有违反并且没有其他限制之外的要求。

请注意，它在 c99 和 c90 中是相同的(在没有额外要求的意义上)。当然，在c90中，由于声明和语句的混合，程序将无效。

关于在 goto 语句之后访问时 i 对象的生命周期，C 说(看我的重点，下面段落中的其他复制句子对于一个更棘手的程序):

(c11, 6.2.4p6) "For such an object that does not have a variable length array type, its lifetime extends from entry into the block with which it is associated until execution of that block ends in any way. [...] If the block is entered recursively, a new instance of the object is created each time. [...] If an initialization is specified for the object, it is performed each time the declaration or compound literal is reached in the execution of the block; otherwise, the value becomes indeterminate each time the declaration is reached."

也就是说，当*p被读取时，i仍然存在；没有对象在其生命周期之外被访问。