invalid memory address or nil pointer dereference with mgo

Question

package main

import (
    "encoding/json"
    "fmt"
    "io/ioutil"
    "net/http"
    "github.com/gorilla/handlers"
    "github.com/gorilla/mux"
    "gopkg.in/mgo.v2"
)

type DataIg struct {
    Member string `json:"Member"`
    Timestamp float64 `json:"Timestamp"`
    Name string `json:"Name"`
    Bid string `json:"Bid"`
    Offer string`json:"Offer"`
    Change string `json:"Change"`
    Hour  string `json:"Hour"`
}

type Response struct {
    Status string
}

var session *mgo.Session
var c *mgo.Collection

func postData(w http.ResponseWriter, r *http.Request) {
    var response  = Response{}
    response.Status = "ok"
    var reception = DataIg{}

    err := c.Insert(reception)

    body, err := ioutil.ReadAll(r.Body)
    if err != nil {
        panic(err)
    }
    err = json.Unmarshal(body, &reception)
    fmt.Println(reception)
    js, err := json.Marshal(response)
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }

    w.Header().Set("Content-Type", "application/json")
    w.Write(js)
}

var appName = "accountservice"

func main() {
    session, _ := mgo.Dial("mongodb://toto:[email protected]:27017/")
    session.SetMode(mgo.Monotonic, true)
    c = session.DB("database").C("igData")
    fmt.Printf("Starting %v\n", appName)
    router := mux.NewRouter()
    router.HandleFunc("/postData", postData).Methods("POST")
    corsObj := handlers.AllowedOrigins([]string{"*"})
    http.ListenAndServe(":8066", handlers.CORS(corsObj)(router))
    defer session.Close()
}

I am trying to make the session and the connection (c) global to use them in another function other than PostData.

But i have a :

panic: runtime error: invalid memory address or nil pointer dereference [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x683587]

here : session, _ := mgo.Dial("mongodb://toto:[email protected]:27017/")

I would like to have the collection on a global way to use it outside the main func.

Regards

Any time you encounter an problem in Go, check to see if you're ignoring any errors anywhere around it, and stop ignoring them. There's a good chance the errors will explain what's going on. — Adrian

poy poy · Accepted Answer · 2018-11-15T21:09:32

I haven't used mgo extensively, however I can tell you that ignoring the error from session, _ = mgo.Dial("mongodb://toto:[email protected]:27017/") is a bad idea.

It's likely that session is nil because the returned err is set.

invalid memory address or nil pointer dereference with mgo

1 Answers