infer_clone/infer/documentation/issues/NULLPTR_DEREFERENCE.md

Infer reports null dereference bugs in Java, C, C++, and Objective-C when it is possible that the null pointer is dereferenced, leading to a crash.

Null dereference in Java

Many of Infer's reports of potential Null Pointer Exceptions (NPE) come from code of the form

  p = foo(); // foo() might return null
  stuff();
  p.goo();   // dereferencing p, potential NPE

If you see code of this form, then you have several options.

If you are unsure whether or not foo() will return null, you should ideally either

1. Change the code to ensure that foo() can not return null, or

2. Add a check that p is not null before dereferencing p.

Sometimes, in case (2) it is not obvious what you should do when p is null. One possibility is to throw an exception, failing early but explicitly. This can be done using checkNotNull as in the following code:

// code idiom for failing early
import static com.google.common.base.Preconditions.checkNotNull;

  //... intervening code

  p = checkNotNull(foo()); // foo() might return null
  stuff();
  p.goo(); // p cannot be null here

The call checkNotNull(foo()) will never return null: if foo() returns null then it fails early by throwing a Null Pointer Exception.

Facebook NOTE: If you are absolutely sure that foo() will not be null, then if you land your diff this case will no longer be reported after your diff makes it to master.

Null dereference in C

Here is an example of an inter-procedural null dereference bug in C:

struct Person {
  int age;
  int height;
  int weight;
};
int get_age(struct Person *who) {
  return who->age;
}
int null_pointer_interproc() {
  struct Person *joe = 0;
  return get_age(joe);
}

Null dereference in Objective-C

In Objective-C, null dereferences are less common than in Java, but they still happen and their cause can be hidden. In general, passing a message to nil does not cause a crash and returns nil, but dereferencing a pointer directly does cause a crash as well as calling a nil block.

-(void) foo:(void (^)())callback {
    callback();
}

-(void) bar {
    [self foo:nil]; //crash
}

Moreover, there are functions from the libraries that do not allow nil to be passed as argument. Here are some examples:

-(void) foo {
    NSString *str = nil;
    NSArray *animals = @[@"horse", str, @"dolphin"]; //crash
}

-(void) bar {
  CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); //can return NULL
  ...
  CFRelease(colorSpace); //crashes if called with NULL
}