title:: Tracing Processes
summary:: Tracing processes in SC
categories:: Debugging
related:: Guides/Debugging-tips

What goes on in a running system? In SC, various methods help to get information about processes on different levels: server side and client side (in sclang).

section:: Tracing sclang processes

In order to know more about objects as they are created by processes like tasks or even simply by evaluating a bit of code, one can insert messages like postln and postcs anywhere in the code.

calculating the sum of n subsequent squares
code::
var n = 8, x = 0;
(1..n).do { |num| x = x + num.squared };
x
::

what happens while we are doing this?
code::
var n = 8, x = 0;
(1..n).do { |num| x = x + num.squared.postln; };
x
::
or more in detail:
code::
var n = 8, x = 0;
(1..n).do { |num| [\before, x].postln; x = x + num.squared; [\after, x].postln;};
x
::

when posting several values, some more verbose posts can be useful.
postf formats a string and inserts values for %-characters.
here separate statements are needed.
code::
var n = 8, x = 0;
(1..n).do { |num| x = x + num.squared; "num: % num-squared: % new x: %\n".postf(num, num.squared, x) };
x
::

in some cases, postln will only post part of the data, or a simplified
representation.
code::
// n times 200 random numbers
// will just add ... etc ... after 123
var n = 3;
(1..n).do { |num| { 1000.rand }.dup(200).postln };
::

posts the compile string, i.e. the code needed to recreate the receiver (here the array)
code::
var n = 3;
(1..n).do { |num| { num.rand }.dup(200).postcs };
::

subsection:: Streams, tasks and routines
in streams, tasks and routines, this works just as well:
code::
fork {
	var n = 14;
	(1..n).do { |num|
		{ num.rand }.dup(200).postcs;
		1.wait;
	};
}
::

code::
fork {
	var str = Routine { |in| 10.do { in = in.rand.yield } };
	12.0.do { |i|
		str.next(i).postln;
		0.5.wait;
	};
}
::


for creating a pattern that once it is used posts its values,
the message trace can be used (in returns a Ptrace)
code::
a = Pseq([1, 4, 1, Pwhite(0, 6, 3), 100, 39], inf).trace(prefix: "value: ");
b = a.asStream;
b.next;
b.next;
b.next;
b.next;
::

in a running stream:
code::
Pbind(
	\degree, Pseq([1, 4, 1, Pwhite(0, 6, 3), 100, 39], inf).trace(prefix: "value: "),
	\dur, 0.2
).play
::

post only a slot of the events
code::
Pbind(
	\degree, Pseq([1, 4, 1, Pwhite(0, 6, 3), 100, 39], inf),
	\dur, 0.2
).trace(\degree).play
::

several slots at once:
code::
Pbind(
	\degree, Pseq([1, 4, 1, Pwhite(0, 6, 3), 100, 39], inf),
	\dur, Pwhite(0.2, 0.4, inf)
).trace([\degree, \dur], prefix: ["degree ", "dur "]).play
::

section:: Tracing server processes

Using postln or post on a UGen will only return the UGen, but not the values it produces in a running synth. The poll message creates a Poll UGen which posts at regular intervals when given a time value or as a response to a trigger (see link::Classes/Poll:: helpfile)
code::
// postln returns only the UGen itself (a MulAdd here)
{ SinOsc.ar(SinOsc.kr(0.2, 0, 300, 400).postln) * 0.1 }.play;

// poll traces the values
{ SinOsc.ar(SinOsc.kr(0.2, 0, 300, 400).poll) * 0.1 }.play;

// using a label:
{ SinOsc.ar(SinOsc.kr(0.2, 0, 300, 400).poll(label: "freq")) * 0.1 }.play;
::

For demand ugens, poll does not work - these ugens are called by a Demand or Duty UGen at certain intervals. The message dpoll creates a Dpoll ugen that posts when they are called (see link::Classes/Dpoll:: helpfile)

code::
{ SinOsc.ar(Duty.kr(0.5, 0, (Dseries(0, 1, inf) * 200 + 300).dpoll)) * 0.1 }.play;

{ SinOsc.ar(Duty.kr(0.5, 0, (Dseries(0, 1, inf) * 200 + 300).dpoll(label: "freq"))) * 0.1 }.play;
::

The scope window can give valuable information about the ongoing sound (see link::Classes/Stethoscope:: help):
code::
{ SinOsc.ar(SinOsc.kr(0.2, 0, 300, 400)) * 0.1 }.scope;
::
A FreqScope window can be used for observing the spectrum of the output:
code::

// create a new analyzer
FreqScope.new;

{ SinOsc.ar(SinOsc.ar(0.2, 0, 3000, 4000)) * 0.1 }.play;
{ SinOsc.ar(SinOsc.ar((1..4) * 0.02, 0, 3000, 4000)).sum * 0.1 }.play;
::
