Invalid memory address error when running postgres queries

Question

I keep getting this error when I run my Go code which makes queries to my local postgres database.

Error:

panic serving [::1]:56708: runtime error: invalid memory address or nil pointer dereference
goroutine 23 [running]:
net/http.func·011()
    /usr/local/go/src/pkg/net/http/server.go:1100 +0xb7
runtime.panic(0x2ef0a0, 0x4d8ee4)
    /usr/local/go/src/pkg/runtime/panic.c:248 +0x18d
database/sql.(*DB).conn(0x0, 0x277a1, 0x0, 0x0)
    /usr/local/go/src/pkg/database/sql/sql.go:625 +0x751
database/sql.(*DB).Ping(0x0, 0x0, 0x0)
    /usr/local/go/src/pkg/database/sql/sql.go:452 +0x39
main.firstHandler(0x58e9a8, 0xc208052320, 0xc2080284e0)
    /Users/Tommy/Documents/gocode/server/server.go:122 +0x35
net/http.HandlerFunc.ServeHTTP(0x3c6be8, 0x58e9a8, 0xc208052320, 0xc2080284e0)
    /usr/local/go/src/pkg/net/http/server.go:1235 +0x40
github.com/gorilla/mux.(*Router).ServeHTTP(0xc2080186e0, 0x58e9a8, 0xc208052320, 0xc2080284e0)
    /Users/Audrey/gocode/src/github.com/gorilla/mux/mux.go:98 +0x292
net/http.(*ServeMux).ServeHTTP(0xc208022660, 0x58e9a8, 0xc208052320, 0xc2080284e0)
    /usr/local/go/src/pkg/net/http/server.go:1511 +0x1a3
net/http.serverHandler.ServeHTTP(0xc208004660, 0x58e9a8, 0xc208052320, 0xc2080284e0)
    /usr/local/go/src/pkg/net/http/server.go:1673 +0x19f
net/http.(*conn).serve(0xc208050500)
    /usr/local/go/src/pkg/net/http/server.go:1174 +0xa7e
created by net/http.(*Server).Serve
    /usr/local/go/src/pkg/net/http/server.go:1721 +0x313

Go:

func firstHandler(w http.ResponseWriter, r *http.Request) {
    err := db.Ping()
    if err != nil {
        log.Fatal(err)
    }
    rows, err := db.Query("SELECT id, created_at, updated_at FROM script WHERE updated_at = $1", 3)
    if err != nil {
        log.Fatal(err)
    }
    defer rows.Close()

    var created_at, updated_at, id int
    for rows.Next() {
        err := rows.Scan(&id, &created_at, &updated_at)
        if err != nil {
            log.Fatal(err)   
        }
       fmt.Fprintf("%s %s %s", id, created_at, updated_at)
    }
}

var r = mux.NewRouter()
var db *sql.DB

func main() {
    db, err := sql.Open("postgres", "user=Tommy host=localhost dbname=dbgo sslmode=verify-full")
    if err != nil {
        log.Fatal(err)
    }
    defer db.Close()

    r.HandleFunc("/ping", firstHandler)

    http.Handle("/", r)

    http.ListenAndServe(":8080", nil)
}

Help. What am I doing wrong? I referred to this also: https://gophercasts.io/lessons/4-postgres-basics.

Where's the stack-strace? It should point you to the exact point where you dereferenced a nil pointer. — JimB, Sep 30 '14 at 16:00
I've updated my code. Do you mean this? I can't find the exact point though. — user3918985, Sep 30 '14 at 16:05
its err := db.Ping(). Sorry let me update the error message. — user3918985, Sep 30 '14 at 16:23
again, on Go line 122, it's err := db.Ping().. seems like the error is there but I'm not sure why — user3918985, Sep 30 '14 at 16:25

score 15 · Accepted Answer · answered Sep 30 '14 at 16:30

Actually, you declare the connection with:

var db *sql.DB

but you open the connection with:

db, err := sql.Open("postgres", "user=Tommy host=localhost dbname=dbgo sslmode=verify-full")

Note the := (it combines a variable declaration with an assignment). This will actually shadow the global db variable by a local one. The connection is opened but assigned to the local variable. So the value of the global db variable is nil.

When the firstHandler function is called, its value is still nil, which triggers the panic.

Replace the := by a = (and declare the err object before).

Alternatively, avoid global variables. – dyoo Sep 30 '14 at 20:00 — dyoo, Sep 30 '14 at 20:00

Invalid memory address error when running postgres queries

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