Golang : io.Reader causing panic: runtime error: invalid memory address or nil pointer dereference

Problem :

You read a file to retrieve some data with io.Reader, then when you attempt to read the same file again. It causes panic with error message :

panic: runtime error: invalid memory address or nil pointer dereference

Looking at the stack trace does not help. What's going on?

Solution :

When a file is read by io.Reader, it usually reads until EOF. So, when you attempt to read from the same file again. It will cause panic... because the pointer is attempting to read pass EOF. To fix this, you will have to reset or rewind the pointer. To do so, use the os.File.Seek() function to reset.

See how Seek() function is used to reset/rewind the io.Reader example :

 package main

 import (
 "fmt"
 "image"
 "image/jpeg"
 "os"
 )

 func init() {
 // damn important or else At(), Bounds() functions will
 // caused memory pointer error!!
 image.RegisterFormat("jpeg", "jpeg", jpeg.Decode, jpeg.DecodeConfig)
 }

 func main() {
 imgfile, err := os.Open("./img.jpg")

 if err != nil {
 fmt.Println("img.jpg file not found!")
 os.Exit(1)
 }

 defer imgfile.Close()

 // get image height and width with image/jpeg
 // change accordinly if file is png or gif

 imgCfg, _, err := image.DecodeConfig(imgfile)

 if err != nil {
 fmt.Println(err)
 os.Exit(1)
 }

 width := imgCfg.Width
 height := imgCfg.Height

 fmt.Println("Width : ", width)
 fmt.Println("Height : ", height)

 // we need to reset the io.Reader again for image.Decode() function below to work
 // otherwise we will get - 
 // panic: runtime error: invalid memory address or nil pointer dereference 

 // there is no build in rewind, use Seek(0,0)
 imgfile.Seek(0, 0)

 // get the image
 img, _, err := image.Decode(imgfile)

 fmt.Println(img.At(10, 10).RGBA())
 for y := 0; y < height; y++ {
 for x := 0; x < width; x++ {
 r, g, b, a := img.At(x, y).RGBA()
 fmt.Printf("[X : %d Y : %v] R : %v, G : %v, B : %v, A : %v  \n", x, y, r, g, b, a)
 }
 }

 }

Reference :

https://golang.org/pkg/os/#File.Seek