使用goto跳转到内部或兄弟的范围

是否允许跳转到内部范围或兄弟范围内的标签？如果是这样,是否允许使用在该范围中声明的变量？

考虑这个代码：

int cond(void);
void use(int);

void foo() 
{
    {
        int y = 2;
        label:
        use(y);
    }

    {
        int z = 3;
        use(z);

        /* jump to sibling scope: */ if(cond()) goto label;
    }

    /* jump to inner scope: */ if(cond()) goto label;
}

这些gotos合法吗？

如果是这样,当我跳到标签并保留分配给它的最后一个值时,是否保证存在(2)？

或者是允许编译器假定在超出范围之后不会使用y,这意味着单个内存位置可以用于y和z？

如果这段代码的行为是未定义的,那么如何让GCC发出警告呢？

解决方法

从C99标准(强调我的)：

6.2.4 Storage durations of objects

[6] For such an object that does have a variable length array type,its lifetime extends from the declaration of the object until execution of the program leaves the scope of the declaration. … If the scope is entered recursively,a new instance of the object is created each time. The initial value of the object is indeterminate.

6.8.6.1 The goto statement

[1] 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.

[4] … A goto statement is not allowed to jump past any declarations of objects with variably modified types.

结论

> y不是可变的类型,所以根据标准,跳转是合法的.
y保证存在,但是跳转跳过初始化(y = 2),所以y的值是不确定的.
>你可以使用-Wjump-misses-init来让GCC发出如下的警告：

warning: jump skips variable initialization [-Wjump-misses-init]

在C中,跳转不合法,C不允许跳过y的初始化.