How to print inside SYCL parallel_for (kernel)?

Question

I'm working on vectorAdd sample application which was provided along with oneAPI basekit. When I tried printing the sum inside kernel, I get the following error.

Please find attached source code and error when compiled.

//Source code 

    cgh.parallel_for<class VectorAdd>(num_items, [=](id<1> wiID) {
        sum_accessor[wiID] = addend_1_accessor[wiID] + addend_2_accessor[wiID];

        std::cout<<"Sum : "<<sum_accessor[wiID]<<std::endl;  // I want to print this sum

        });

During compilation I get the following error.

 //Error generated while compiling

usr/lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/bits/ostream.tcc:359:25: error: SYCL kernel cannot have a class with a virtual function table
  extern template class basic_ostream<char>;
                        ^
vector-add.cpp:159:6: note: used here
            std::cout<<"Sum : "<<sum_accessor[wiID]<<std::endl;
            ^
vector-add.cpp:159:11: error: SYCL kernel cannot use a global variable
            std::cout<<"Sum : "<<sum_accessor[wiID]<<std::endl;

Answer 1

Use the sycl::stream class to output logging in your SYCL code. There's a guide that explains how to do this.

Your code should look something like this

//Source code 
q.submit([&]sycl::handler &cgf) {
    sycl::stream out(1024, 256, cgh); //output buffer
    cgh.parallel_for<class VectorAdd>(num_items, [=](id<1> wiID) {
        sum_accessor[wiID] = addend_1_accessor[wiID] + addend_2_accessor[wiID];
        out << "Sum : " << sum_accessor[wiID]  << cl::sycl::endl;  // I want to print this sum
    });
}