Calling matlab mex from parfeval

I have an existing matlab mex program that I want to alter to run from a thread. The matlab mex works properly when called from the main thread (command line) in MATLAB. I implemented the following

pool = backgroundPool();
q = parallel.pool.DataQueue();
for (ii=1:4000)
    if (ii==1000)
        parfeval(pool,@mex_thread,1,my_struct,q);
        afterEach(q, @disp); % Need this to print messages
    end
    pause(0.001); % thread does not run until main script stops without this
end
function [retval] = mex_thread(my_struct,q)
send(q,sprintf('Got to point 1\n'));
for (idx=1:100)
    send(q,sprintf('Got to point 2\n'));
    my_mex_program(my_struct);
    send(q,sprintf('Got to point 3\n'));
end
end

I simplified the mex program to this:

class MexFunction : public matlab::mex::Function {
public:
void operator()(ArgumentList outputs, ArgumentList inputs) {
return;
}
};

But I only get this on the console, indicating that the mex did not return

Got to point 1

Got to point 2

How do I go about determining what is wrong? Is it legal to call mex from a background thread?

Answer 1

[Christopher Mirfin]

Unfortunately, calling MEX from a thread-based pool (in this case backgroundPool) is not currently supported. So for now, you must use parpool("Processes") to create a process-based pool if you have access to Parallel Computing Toolbox.

As a note, errors associated with parfeval are captured on the return output object, a parallel Future, i.e.

future = parfeval(pool,@mex_thread,1,my_struct,q);

Using fetchOutputs on this future will wait for the work to complete and throw any associated errors. Or, if you want to report the error asynchronously, use a continuation afterEach with the PassFuture=true option.

[Mike]

Thanks. After some debugging I mostly have it working.

When called directly, printf can be used in matlab mex. printf does not work from matlab mex that is called from a matlab process.

What can be done? The code above shows how I implemented this in a matlab script, using 'q', 'afterEach', and 'send', but it is not clear how to translate that into cpp code in the mex source.

Also this update: I noticed that mex program writes to my_struct have no effect on the top level instance of my_struct. So I switched my_struct to be global. mex_thread declares the access to be global, but passes the structure to the mex code. mex_thread can alter my_struct and those modifications show up in the top level. However, any write to my_struct fields in the mex coded causes the code to hang.

My cpp code to write my_struct is

extern "C" void wr_field_mex(const char * regName, int wval)
{
  ArrayFactory       factory;
  const TypedArray<double>lval = factory.createScalar<double>(wval);
  matlabPtr->setProperty(*obj, regName, lval);
}

Which is invoked with this c code

wr_field_mex("my_field",wr_val)

In matlab my_struct is defined like this

classdef get_fields < handle

    properties
        my_field = 0;
        ... % many more fields
    end
    methods
        function rval = get.my_field(obj)
            rval = obj.my_field;
        end
        function        set.my_field(obj, val)
            obj.my_field = val;
        end
        ... % many more access methods
    end
end

Is it possible to get some debug information out of this or help with what is going wrong?