if statement segmentation fault

Question

I am checking to see if a pointer exists and if it does exist then it doesn't point to a NULL/zero value. My code looks like this:

if (prior != NULL && *prior){
  //do something
}

Where prior is a pointer. However, I am getting a segmentation fault on *prior. But I don't understand how this is possible. The if-statement should short circuit if prior is a NULL pointer, so *prior should always work.

Any ideas?

Maybe it is not NULL but the pointer point to invalid address. — Zamrony P. Juhara, Dec 01 '16 at 00:19
If the compiler does not complain about an unknown identifier, the pointer variable **does** exist. Where is your [mcve]? There is no `*prior` in the single line you show. — too honest for this site, Dec 01 '16 at 00:19
The fact that a pointer is non-null doesn't mean you're allowed to dereference it. In fact, you might not even be allowed to compare it to NULL. — user2357112, Dec 01 '16 at 00:19
@ZamronyP.Juhara gives the correct answer, but if you want to know why it has an invalid address, you'll have to post more code! — Ken Y-N, Dec 01 '16 at 00:21

score 3 · Answer 1 · edited May 23 '17 at 10:30

3

if (prior != NULL && *prior){
    //do something
}

There are more reasons to segfault than just dereferencing a NULL pointer. The pointer may be not NULL, yet if you are deference an invalid or dangled pointer, your program can still segfault.

Revisit this well-known answer about pointer: Can a local variable's memory be accessed outside its scope?

which also explains why a pointer can be non NULL yet still invalid.

edited May 23 '17 at 10:30

Community

1
1

answered Dec 01 '16 at 00:20

artm

17,291
6
38
54

[Reworking comments into answers](https://meta.stackoverflow.com/questions/253045/answerers-who-only-use-comments) is acceptable behaviour. – Ken Y-N Dec 01 '16 at 00:29
1

@KenY-N you can't say that it's a rework unless there is a significant time difference between a comment and a more complete answer. – artm Dec 01 '16 at 00:36
@artm this is an online community, there are all sorts of people, you don't let your feelings down just because some random unpleasants. – fluter Dec 01 '16 at 00:49
@fluter absolutely – artm Dec 01 '16 at 00:54

if statement segmentation fault

1 Answers1