c - 在 C 中获取未定义或实现定义行为的无效指针的值？

Question

根据 this，获取无效指针的值是 C++ 中实现定义的行为.现在考虑以下 C 程序:

#include <stdio.h>
#include <stdlib.h>
int main(void)
{
    int* p=(int*)malloc(sizeof(int));
    *p=3;
    printf("%d\n",*p);
    printf("%p\n",(void*)p);
    free(p);
    printf("%p\n",(void*)p); // Is this undefined or implementation defined in behaviour C? 
}

但是 C 中的行为也一样吗？上述 C 程序的行为是未定义的还是已定义的？ C99/C11 标准对此有何规定？ 请告诉我 C99 和 C11 中的行为是否不同。

Answer 1

扩展 Andrew Henle 的回答:

来自 C99 标准 6.2.4:

An object has a storage duration that determines its lifetime. There are three storage durations: static, automatic, and allocated. Allocated storage is described in 7.20.3. […] The value of a pointer becomes indeterminate when the object it points to (or just past) reaches the end of its lifetime.

然后在 7.20.3.2 中:该标准继续描述 malloc()、calloc() 和 free()，并提到

The free function causes the space pointed to by ptr to be deallocated.

在 3.17.2 中:

indeterminate value

either an unspecified value or a trap representation

在 6.2.6.1.5 中:

Certain object representations need not represent a value of the object type. If the stored value of an object has such a representation and is read by an lvalue expression that does not have character type, the behavior is undefined. […] Such a representation is called a trap representation.

由于指针变得不确定，并且不确定值可以是陷阱表示，并且您有一个左值变量，并且读取左值陷阱表示是未定义的，因此是的，行为可能是未定义的。