Golang: Why does the compress/gzip Read function not read file contents?

Question

I made a text file, that I then compressed with gzip. I then run the following go program to read the contents of that compressed file.

package main

import (
    "compress/gzip"
    "fmt"
    "os"
)

func main() {
    handle, err := os.Open("zipfile.gz")
    if err != nil {
        fmt.Println("[ERROR] File Open:", err)
    }
    defer handle.Close()

    zipReader, err := gzip.NewReader(handle)
    if err != nil {
        fmt.Println("[ERROR] New gzip reader:", err)
    }
    defer zipReader.Close()

    var fileContents []byte
    bytesRead, err := zipReader.Read(fileContents)
    if err != nil {
        fmt.Println("[ERROR] Reading gzip file:", err)
    }
    fmt.Println("[INFO] Number of bytes read from the file:", bytesRead)
    fmt.Printf("[INFO] Uncompressed contents: '%s'\n", fileContents)
}

The response that I get is the following:

$ go run zipRead.go
[INFO] Number of bytes read from the file: 0
[INFO] Uncompressed contents: ''

Why am I not getting any contents from the file?

I have created zip files on both OS X and Ubuntu. I have build this go program on both OS X and Ubuntu with the same result.

In addition to what @JimB says, you're mis-using Read. You need to pass it an allocated slice you want it to put bytes into, you pass a nil slice (len(fileContents) == 0 so you're asking for zero bytes). — Dave C

Ainar-G Ainar-G · Accepted Answer · 2015-08-14T21:00:11

io.Reader.Read will only read up to len(b) bytes. Since your fileContents is nil, its length is 0. Allocate some space for it to read into:

fileContents := make([]byte, 1024) // Read by 1 KiB.
bytesRead, err := zipReader.Read(fileContents)
if err != nil {
    fmt.Println("[ERROR] Reading gzip file:", err)
}
fileContents = fileContents[:bytesRead]

If you want to read the whole file, you'll have to either use Read several times, or use things like ioutil.ReadAll (which may be bad for big files).

Golang: Why does the compress/gzip Read function not read file contents?

1 Answers