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Q&A Is it undefined behaviour to just make a pointer point outside boundaries of an array without dereferencing it?

I have heard that it is undefined behaviour to make a pointer point outside boundaries of an array even without dereferencing it. Can that really be true? Consider this code: int main(void) { ...

2 answers  ·  posted 4y ago by klutt‭  ·  last activity 4y ago by hkotsubo‭

#3: Post edited by user avatar klutt‭ · 2020-08-19T10:49:23Z (about 4 years ago)
  • I have heard that it is undefined behaviour to make a pointer point outside boundaries of an array even without dereferencing it. Can that really be true? Consider this code:
  • int main(void)
  • {
  • char arr[10];
  • char *ptr = arr[-1];
  • char c = *ptr;
  • }
  • The line `char c = *ptr` is obviously bad, because it's accessing out of bounds. But I heard something that even the second line invokes undefined behaviour? Is this true? What does the standard say?
  • I have heard that it is undefined behaviour to make a pointer point outside boundaries of an array even without dereferencing it. Can that really be true? Consider this code:
  • int main(void)
  • {
  • char arr[10];
  • char *ptr = &arr[-1];
  • char c = *ptr;
  • }
  • The line `char c = *ptr` is obviously bad, because it's accessing out of bounds. But I heard something that even the second line `char *ptr = &arr[-1]` invokes undefined behaviour? Is this true? What does the standard say?
#2: Post edited by user avatar klutt‭ · 2020-08-18T09:36:25Z (about 4 years ago)
  • I have heard that it is undefined behaviour to make a pointer point outside boundaries of an array even without dereferencing it. Can that really be true? Consider this code:
  • char arr[10];
  • char *ptr = arr[-1];
  • char c = *ptr;
  • The third line is obviously bad, because it's accessing out of bounds. But I heard something that even the second line invokes undefined behaviour? Is this true? What does the standard say?
  • I have heard that it is undefined behaviour to make a pointer point outside boundaries of an array even without dereferencing it. Can that really be true? Consider this code:
  • int main(void)
  • {
  • char arr[10];
  • char *ptr = arr[-1];
  • char c = *ptr;
  • }
  • The line `char c = *ptr` is obviously bad, because it's accessing out of bounds. But I heard something that even the second line invokes undefined behaviour? Is this true? What does the standard say?
#1: Initial revision by user avatar klutt‭ · 2020-08-11T08:45:25Z (about 4 years ago)
Is it undefined behaviour to just make a pointer point outside boundaries of an array without dereferencing it?
I have heard that it is undefined behaviour to make a pointer point outside boundaries of an array even without dereferencing it. Can that really be true? Consider this code:

    char arr[10];
    char *ptr = arr[-1];
    char c = *ptr;

The third line is obviously bad, because it's accessing out of bounds. But I heard something that even the second line invokes undefined behaviour? Is this true? What does the standard say?