test reading concurrently from the same io.Reader

README.md

1. dd if=/dev/urandom of=test.file bs=1m count=100
2. go build
3. ./ test.file

This will have several goroutines try to read the file from the same file descriptor. And show that they all get only a random subset of the file.

example_run.txt

ZiggyPiggy:concurrent_reader jstillwell$ ls -l test.file 
-rw-r--r--  1 jstillwell  staff  104857600 Sep 10 15:57 test.file

ZiggyPiggy:concurrent_reader jstillwell$ go build && ./concurrent_reader test.file
read 20973056 bytes
read 16776704 bytes
read 33553920 bytes
read 33553920 bytes

main.go

package main

import (
	"fmt"
	"io/ioutil"
	"os"
	"sync"
)

func main() {

	filename := os.Args[1]
	file, err := os.Open(filename)
	if err != nil {
		panic(fmt.Sprintf("failure to open filename given on command line '%s'", filename))
	}

	var wg sync.WaitGroup

	f := func() {
		defer wg.Done()

		buf, err := ioutil.ReadAll(file)
		if err != nil {
			panic(err)
		}

		fmt.Printf("read %d bytes\n", len(buf))
	}

	for i := 0; i <= 3; i++ {
		wg.Add(1)
		go f()
	}

	wg.Wait()
}