c - 用指针访问零位

Question

为什么我们不能访问例如位置零(0)，一百(100)，内存中有两百(200);

例如:

void main() {
    int* a;
    a = 0;
    printf("what is inside position o %d",*a);

}

Answer 1

引用 C11，章节 §6.5.3.2p4

... If an invalid value has been assigned to the pointer, the behavior of the unary * operator is undefined.

引用 C11，章节 §note102

... Among the invalid values for dereferencing a pointer by the unary * operator are a null pointer, an address ...

_强调我的

此外，

引用 C11，章节 §6.3.2.3p3

An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

因此，您正在尝试取消引用一个空指针，其行为已明确表示为标准未定义。