Use the number 0 to denote no outputs or inputs.

opcode my_print, 0, i
ival xin
print ival
endop

instr 1
my_print(4)
endin

The docs:

http://csound.github.io/docs/manual/opcode.html

mention:

"A single 0 character can be used if there are no input arguments. "

and

"The format is the same as in the case of intypes. "

Admittedly, the documentation could better illustrate this by adding 0
to the in and out type tables.

On Mon, Feb 15, 2016 at 11:13 AM, Rory Walsh  wrote:
> I can't seem to find any documentation for UDOs with no outputs, Are these
> possible? I have a number of simple debug type UDO that don't need to return
> anything. It's not a big deal but would make the code more readable if they
> didn't have to have an output.
> Csound mailing list Csound@listserv.heanet.ie
> https://listserv.heanet.ie/cgi-bin/wa?A0=CSOUND Send bugs reports to
> https://github.com/csound/csound/issues Discussions of bugs and features can
> be posted here

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