In this blog post, I will show you how to read from stdin in a GTK application written with gjs.

Since I was already using greenclip(with rofi) for clipboard history, I would like to use Bender in a similar way. greenclip print | bender .

As a beginner to GJS-GTK, my first challenge was to bootstrap a GTK app and read the data from stdin.

For the first prototype, I used the InputStream.read_bytes like below

Using read_bytes

const cli = new Gio.ApplicationCommandLine();
const inputStream = cli.get_stdin();

let str = "";
const bytes = inputStream?.read_bytes(8192, null);
inputStream?.close(null);
const data = bytes?.get_data();
if (data) {
  str = new TextDecoder("utf-8").decode(data);
}

This comes with some limitations,

1. Its synchronous/blocking
2. we have to specify the no of bytes

I settled with read_bytes because, when I tried to use read_all_async or read_all I was getting errors like below in GJS 1.74.2.

• Function Gio.InputStream.read_all_async() cannot be called: argument 'buffer' with type array is not introspectable because it has a type not supported for (out caller-allocates)
• TypeError: method Gio.InputStream.read_all_async: At least 4 arguments required, but only 3 passed

Once I had an initial prototype, I started looking into better ways to read from stdin in GTK applications.

A quick search of the above errors led me to some discussions on the Gnome forums like this and this which lead me to a bug report #501 on GJS & a merge request #787

Based on merge request #787 I decided to give a try to MemoryOutputStream.splice_async.

Using MemoryOutputStream.splice_async

I started replacing the read_byes with splice_async & splice_finish and it worked fine.

const cli = new Gio.ApplicationCommandLine();
const inputStream = cli.get_stdin();

if (inputStream) {
  const outputStream = Gio.MemoryOutputStream.new_resizable();

  outputStream.splice_async(
	inputStream,
	Gio.OutputStreamSpliceFlags.CLOSE_TARGET,
	GLib.PRIORITY_DEFAULT,
	null,
	(outputStream, result) => {
	  let data;
	  let str = "";
	  let bytes;

	  try {
		outputStream?.splice_finish(result);
		bytes = outputStream?.steal_as_bytes();
	  } catch (err) {
		console.debug(err);
	  }

	  data = bytes?.get_data();
	  if (data) {
		str = new TextDecoder("utf-8").decode(data);
	  }
	  console.debug("body:", str);
	},
  );
}

For now, this looks better than using read_bytes and I don’t need to worry about the hard coded bytes count, and it’s non-blocking.

Hope this is helpful. Did I make any mistake or is there a better way, feel free for drop an email.

Versions of Language/packages used in this post.