Correct usage of os.NewFile in Go

I'm attempting to compose an image in memory and send it out through http.ResponseWriter without ever touching the file system.

I use the following to create a new file:

file := os.NewFile(0, "temp_destination.png")

However, I don't seem to be able to do anything at all with this file. Here is the function I'm using (which is being called within an http.HandleFunc, which just sends the file's bytes to the browser), which is intended to draw a blue rectangle on a temporary file and encode it as a PNG:

func ComposeImage() ([]byte) {
	img := image.NewRGBA(image.Rect(0, 0, 640, 480))
	blue := color.RGBA{0, 0, 255, 255}
	draw.Draw(img, img.Bounds(), &image.Uniform{blue}, image.ZP, draw.Src)

	// in memory destination file, instead of going to the file sys
	file := os.NewFile(0, "temp_destination.png")

	// write the image to the destination io.Writer
	png.Encode(file, img)

	bytes, err := ioutil.ReadAll(file)
	if err != nil {
		log.Fatal("Couldn't read temporary file as bytes.")
	}

	return bytes
}

If I remove the png.Encode call, and just return the file bytes, the server just hangs and does nothing forever.

Leaving the png.Encode call in results in the file bytes (encoded, includes some of the PNG chunks I'd expect to see) being vomited out to stderr/stdout (I can't tell which) and server hanging indefinitely.

I assume I'm just not using os.NewFile correctly. Can anyone point me in the right direction? Alternative suggestions on how to properly perform in-memory file manipulations are welcome.

os.NewFile is a low level function that most people will never use directly. It takes an already existing file descriptor (system representation of a file) and converts it to an *os.File (Go's representation).

If you never want the picture to touch your filesystem, stay out of the os package entirely. Just treat your ResponseWriter as an io.Writer and pass it to png.Encode.

png.Encode(yourResponseWriter, img)

If you insist on writing to an "in memory file", I suggest using bytes.Buffer:

buf := new(bytes.Buffer)
png.Encode(buf, img)
return buf.Bytes()

Please have a detailed read of the NewFile documentation. NewFile does not create a new file, not at all! It sets up a Go os.File which wraps around an existing file with the given file descriptor (0 in your case which is stdin I think).

Serving images without files is much easier: Just Encode your image to your ResponseWriter. That's what interfaces are there for. No need to write to ome magic "in memory file", no need to read it back with ReadAll, plain and simple: Write to your response.