lvalue doesn't designate an object after evaluation?

C99 [Section 6.3.2.1/1] sa开发者_运维技巧ys

An lvalue is an expression with an object type or an incomplete type other than void; if an lvalue does not designate an object when it is evaluated, the behavior is undefined.

What does the part in bold mean? Can someone please explain it with an example?

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Null pointers, pointers to deallocated objects and pointers to objects with automatic storage duration whose lifetime has already ended come to mind. Dereferencing these results in invalid lvalues; the undefined behaviour you will encounter most often are segfaults if you're lucky, and arbitrary heap or stack corruption if not.

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#include <stdio.h>

int* ptr;

void f(void) {
    int n = 1;
    ptr = &n;
}

int main(void) {
    f();
    // UB: *ptr is an lvalue that is not an object:
    printf("%d\n", *ptr);
    return 0;
}