pq QueryRow Scan fails with invalid memory address or nil pointer dereference

Question

I have the following function which is supposed to retrieve a single row from my ideas table:

func (s *IdeaService) GetIdea(id int64) (*ideaservice.Idea, error) {
    stmt, err := s.DB.Prepare("SELECT id, name, description, created_on, last_updated FROM ideas WHERE id = $1")
    if err != nil {
        return nil, err
    }
    defer stmt.Close()
    var idea *ideaservice.Idea
    err = stmt.QueryRow(id).Scan(&idea.ID, &idea.Name, &idea.Description, &idea.CreatedOn, &idea.LastUpdated)
    return idea, err
}

When i run this in a test, the following error occurs:

--- FAIL: TestGetIdea (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
    panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x65be98]

I can't seem to figure out the problem. I believe I have instantiated the idea var correctly as a pointer, which I then assign values to in the Scan method. Does anyone have any suggestions?

`var idea *ideaservice.Idea` this does not initialize the pointer, therefore the `idea` variable is `nil`. Use `var idea = new(ideaservice.Idea)`, or `idea := &ideaservice.Idea{}` to properly initialize your variable. — mkopriva, Apr 17 '20 at 13:10
When declaring variables using `var v T`, the variable `v` is initialized to the zero-value of type `T`, if type `T` is a pointer type whose zero-value is `nil` then `v` will be also `nil`. For comparison, doing `var idea ideaservice.Idea` (assuming `Idea` is a struct) is completely fine, because the zero-value of struct types is the value of a struct with all it's fields initialized to their zero value. https://golang.org/ref/spec#The_zero_value — mkopriva, Apr 17 '20 at 13:12
Could an alternative solution be to declare the var as such: `var idea ideaservice.Idea` and then return the reference to the var from the func? It seems to work fine code-wise, but I'm curious as to whether or not this is equivalent to your proposal — Mike Hawkins, Apr 17 '20 at 13:15
Yes, i've just added that to the previous comment. And you can still return a pointer using `return &idea, err`. — mkopriva, Apr 17 '20 at 13:16
You should probably check the error with `ErrNoRows` and return nil if it matches. — Stéphane Jeandeaux, Apr 17 '20 at 13:19
@StéphaneJeandeaux I have added a switch statement to check the error. Thanks. — Mike Hawkins, Apr 17 '20 at 13:25

score 0 · Answer 1 · answered Apr 17 '20 at 18:12

func (s *IdeaService) GetIdea(id int64) (*ideaservice.Idea, error) {
    stmt, err := s.DB.Prepare("SELECT id, name, description, created_on, last_updated FROM ideas WHERE id = $1")
    if err != nil {
        return nil, err
    }
    defer stmt.Close()
    var idea *ideaservice.Idea
    err = stmt.QueryRow(id).Scan(&idea.ID, &idea.Name, &idea.Description, &idea.CreatedOn, &idea.LastUpdated)
    return idea, err
}

Could you replace $1 with ? and see if it works? In stmt.QueryRow you're passing the id. Like this:

stmt, err := s.DB.Prepare("SELECT id, name, description, created_on, last_updated FROM ideas WHERE id = ?")

If it doesn't work, then add this block:

switch {
    case err == sql.ErrNoRows:
        log.Fatalf("no user with id %d", id)
    case err != nil:
        log.Fatal(err)
    default:
        log.Println("ok!")
    }

UPDATE: Didn't see the comments on the question. Yes, he's correct as well. Do fix them as well.

And check where the err is trapped (in which case).

pq QueryRow Scan fails with invalid memory address or nil pointer dereference

1 Answers1