Nil Pointer Deference in Golang

Nil pointer deference is a trickly error that can lead to memory access violations once a nil pointer or nil interface was passed to a function that does not handle nil. In all cases, it is a tricky programming error that sometimes cannot be found by static analysis; either the function should handle nil, or the caller should not have passed nil to the function.

The following snippet of code shows a sample of the problem in practice. This vulnerable code does not expect the type to be nil. For instance, when developers create a function that accepts an interface, they usually want to call the method(s) that the interface defines on the variable:

package main
        
        import "fmt"
        
        type X interface {
            X() string
        }
        func greeting(thing X) string {
            return "Hello " + thing.X()
        }
        
        func main() {
        	
        	fmt.Println("nil pointer dereference") 
        	greeting(nil)
        }

Note that the same is true when using a pointer to a type that is used to modify a struct in place. Developers expect to get an instance of the struct when writing a function like this:

type myNumber struct {
                n int
                      }
            func addOne(number *myNumber) {
                number.n++
             }

However, when calling it with nil it will compile fine but error at runtime:

func main() {
        var number *myNumber
        addOne(number)
    }

It is important to attention that simple examples are controlled during code review. However, nil pointer dereferences cause panics we have in production. They often happen in some rarely used “code path” or because of unexpected inputs. We used static analysis to find this issue in the source code, but we could not find any reports.

Solution for nil pointer deference

Finally, to prevent this issue from further development, we suggest adding the nil checks in the coding practices.

func sample(x Named) (string, error) {
    if x == nil {
        return "", errors.New("x cannot be nil")
    }
    return "... " + x.Name(), nil
}