<p>In Erlang, every process has a group leader, and when a process wants to print something (i.e. it calls the io library or does something similar), it will send a message to its group leader.</p>

<p>My question is, where can I find the specification of these messages? Or in general, the specification of what a group leader should do?</p>

<p>I managed to find out with some experimenting that sometimes the process sends an <code>{io_request, Sender, GroupLeader, Request}</code> term, and the answer is an <code>{io_reply, GroupLeader, ok}</code> term, but there may be other cases.</p>

The Erlang Rationale (video)
or
(slides)
;  is a good source of information, as is the source code for
user.erl
.
In short:
```
```
{io_request, From, ReplyAs, Request}
  %From is the process to send the reply to, 
  %ReplyAs is any term the caller desires to 
  %match up the request and the response. (returned verbatim in the reply)
  {io_reply, ReplyAs, Reply}
```
```
Some requests in user.erl:
```
```
{put_chars, IoList} % puts the iolist
 {put_chars, M,F,A} % puts the result of apply(M,F,A)
 {get_geometry, 'rows' | 'columns'} % returns the number of rows or columns of the console
 {get_line, Prompt} % calls io_lib:collect_line(Prompt)
 {get_chars, Prompt, Mod, Func, ExtraArgs} 
 {get_until, Prompt, Mod, Func, Args}
 {setopts, Options} % only option supported by user is 'binary' 
                    % (binary mode if present in Options, list mode otherwise)
```
```