Golang : Reset or rewind io.Reader or io.Writer



Tags : golang io-reader io-writer seek panic-runtime-error invalid-memory nil-pointer-dereference

Problem :

You have read file with io.Reader till EOF. Or you want to rewrite a file with io.Writer again.

How to reset or rewind the io.Reader or io.Writer?

Solution :

Looks like you are reading/accessing or writing in the file in sequential mode instead of random access mode.

Use os.File.Seek() function to reset. Think of... like you are rewind a video or cassette tape with your finger or pencil.

For example :

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

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

 defer imgfile.Close()
 ....
 read file
 ....
 imgfile.Seek(0, 0) // reset/rewind back to offset and whence 0 0
 ....
 read file again

References :

http://golang.org/pkg/io/#Writer

http://golang.org/pkg/io/#Reader

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

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



Tags : golang io-reader io-writer seek panic-runtime-error invalid-memory nil-pointer-dereference

By Adam Ng

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