Why is ioutil.ReadFile() hanging after being called in a for loop?

Question

I wrote a function that converts an image to a base64 string:

func getBase64Screenshot() string {
    // Image name
    imageName := "Screenshots/screenshot.png"

    imageFileBytes, err := ioutil.ReadFile(imageName)
    handleError(err)

    // Converts file to base64 string
    encoded := base64.StdEncoding.EncodeToString(imageFileBytes)

    return encoded
}

The above function is called in a for loop, however after some iterations of the for loop, the program just hangs at the line imageFileBytes, err := ioutil.ReadFile(imageName) (it doesn't throw an error, it just stops running and stalls).

I ran some experiments and found that if I used a smaller image, it would make it through ~5 iterations of the for loop before stalling, however if I screenshot my entire screen it would only make it through the first iteration of the for loop before stalling.

My question is this: What is causing the program to hang, and is there anything I can do to prevent it from hanging?

Thanks!

Try my code and let me know if it works for you too – wasmup Mar 26 '19 at 05:52 — wasmup, Mar 26 '19 at 05:52

wasmup · Answer 1 · 2019-03-26T05:49:50.023

2

Debug your code.

Try this and let me know if this works for you too:
This works like a charm for me on Linux:

package main

import (
    "encoding/base64"
    "fmt"
    "io/ioutil"
    "log"
    "os"
)

func main() {
    for i := 0; i < 10; i++ {
        fmt.Println(getBase64Screenshot()[:10])
    }
}
func getBase64Screenshot() string {
    buf, err := ioutil.ReadFile(os.Args[0])
    if err != nil {
        log.Fatal(err)
    }
    encoded := base64.StdEncoding.EncodeToString(buf)
    return encoded
}

Output:

time go run main.go
f0VMRgIBAQ
f0VMRgIBAQ
f0VMRgIBAQ
f0VMRgIBAQ
f0VMRgIBAQ
f0VMRgIBAQ
f0VMRgIBAQ
f0VMRgIBAQ
f0VMRgIBAQ
f0VMRgIBAQ

real    0m0.340s
user    0m0.369s
sys     0m0.128s

edited Mar 26 '19 at 05:49

answered Mar 26 '19 at 05:44

wasmup

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This works, I get the following output: f0VMRgIBAQ f0VMRgIBAQ f0VMRgIBAQ f0VMRgIBAQ f0VMRgIBAQ f0VMRgIBAQ f0VMRgIBAQ f0VMRgIBAQ f0VMRgIBAQ f0VMRgIBAQ real 0m0.320s user 0m0.369s sys 0m0.074s – Ash Mar 26 '19 at 06:01
It's hard to format but the output is the exact same as yours, except the times are different – Ash Mar 26 '19 at 06:02
1

'ioutil.ReadFile' uses `f, err := os.Open(filename)` then `f.Stat()` my recommendation for now is to read binary file byte by byte by yourself to find out the source of delay in reading file : use [this](https://stackoverflow.com/a/14571779/8208215) – wasmup Mar 26 '19 at 06:09
Okay I'll try it out – Ash Mar 26 '19 at 06:25
I was able to fix the problem I was having (see my answer), looks like the issue is related to running the function above at regular intervals inside of a single goroutine – Ash Mar 29 '19 at 03:56
@Ash Hi, Again. Happy you solved it. for the sake of experience you better find out why. and you didn't post enough of your old code so we could help. your question has not enough information to find that bug. and still your answer not makes sense that, what was that bug and how you solved it. it is better you make minimum code that creates that bug and then find that problem, you may edit your post and add to the end of it as a new `Edit: ...`. – wasmup Mar 29 '19 at 06:43

score 2 · Answer 2 · answered Mar 26 '19 at 05:48

One way to debug the issue is to send a SIGABRT to your running program (when hanging) in order to causes the program to exit with a stack dump.

kill -ABRT <pid>
# or CTRL+Pause on Windows.

Then you will see where the program is hang, and if it is actually related to your function or not.

score 0 · Answer 3 · answered Mar 29 '19 at 03:54

Hey guys I was able to fix the issue.

You see, initially the function getBase64Screenshot() that I wrote above was being called every 10 seconds inside of a goroutine.

To fix the issue I refactored my code to eliminate the goroutine and instead call getBase64Screenshot() inside of an infinite loop:

for {
        getBase64Screenshot()
        time.Sleep(timedelayMilliseconds * time.Millisecond)
    }

I've tested the above code by running it for over 30 minutes with very large screenshots (>1MB in size), and it has never crashed or stalled.

Though I've fixed the issue I encountered, I haven't been able to figure out what exactly caused it in the first place. I welcome any theories as to why my original code above stalled.

How did you solve that bug? do you changed `getBase64Screenshot` function? In this form this is not an answer. you need to point that what was the source of bug, to make sense. — wasmup, Mar 29 '19 at 06:46

Why is ioutil.ReadFile() hanging after being called in a for loop?

3 Answers3