CODEWITHSUNDEEP
cc99language-lawyer
Prasoon Saurav
Prasoon Saurav

Reputation: 92864

lvalue doesn't designate an object after evaluation?

C99 [Section 6.3.2.1/1] says

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?

Upvotes: 8

Views: 291

Answers (2)

Christoph
Christoph

Reputation: 169603

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.

Upvotes: 9

aschepler
aschepler

Reputation: 72356

#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;
}

Upvotes: 6

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