Create a separate file for associating rr events with arbitrary strings (-M on steroids)

The -M switch for rr record is very useful in associating stdio output with rr event numbers.

However, while the output [rr pid event-number] being prepended is useful, I would like something more advanced: I want to be able to save such associations in a custom file and not mix it with stdio.

My goal is to build a machine readable file associating event ids with some string output (and write a script/program to read that back later)

So if I want to do something like (in my tracee code)

fprintf(fd, "Some string here");

It could save

[rr 9875 123]Some string here

to the file fd.

rr -M does that but it (1) changes stdio output (2) will produce a whole lot of extraneous junk if I want to do a lot of event-output associations.

An implementation approach here would be to have some sort of special file fd perhaps. When rr detects that you're writing to that special file (only), it will prepend [rr pid event-id] just like it does for the stdio monitor. It would not do prepending to any other file.

Any suggestions on how I could do this?

Start by tracing rr record's existing -M handling and the path used to associate stdio output with rr events. Define how a tracee can write to a separate special file without changing other output, then verify that the file contains machine-readable event IDs and the associated strings without unrelated stdio text.