Carry class in sc_signal and memory allocation

[David Peng]

I am using systemC for system-level simulation. I'd like to carry class in sc_signal like below:

    sc_port<sc_signal_out_if<pkt>> out;

Is there any guide for such application? I found I need at least overload operator == and <<.

One question is about memory allocation like code below:

    void main() {
      pkt *p = NULL;

      while (true) {
        p = new pkt(64);

        out->write(*p);
  
        wait(p->len/rate, SC_NS);
      }
    }

The issue is, when *p is written into out sc_port, its memory will be freed as it is allocated locally. I put a debug code there and found p always points to same address. I am familiar with systemverilog but have not used c/c++ for quite a long time. In UVM, I need write a copy of p into the output port for such issue but I have no idea how to do it in systemC. Any suggestion of this?

Thanks,

David

[Eyck]

If you write to the signal a copy of pkt is stored. This is C++ value semantics. Even an implementation like this

    void main() {
      while (true) {
        pkt p(64);
        out->write(p);
        wait(p.len/rate, SC_NS);
      }
    }

works correcty and even better than yours as you created a memory leak. Since at the reassignment of p using new the previously allocated memory gets lost. Better to use a std::unique_ptr.

I highly suggest to revive your C++ knowledge based on a modern C++ standard (C++11 or better C++17).

 

[David Peng]

Thanks for Eyck's prompt response. Surely I'll make myself get familiar with modern c++11 or 17 per your suggestion.

However I found it doesn't work as expected. My read side is something like this:

  public:
    sc_port<sc_signal_in_if<pkt>> in;

    SC_HAS_PROCESS(dummy_sink);
    dummy_sink(sc_module_name name): sc_module(name) {
      SC_THREAD(main);
      sensitive << in;
      dont_initialize();
    }

    void main() {
      pkt p;

      while(true) {
        p = in->read();

        wait();
      }

This never gets triggered, I put debug code in my overloaded operator == and found the lhs and rhs are always equal.

I guessed that because in the write loop, it just writes an object pointer to sc_signal instead of values. As the newly created p is a local object which will be freed after the while(true) loop, the out->write did update the value of same object again and again. When the sensitivity check is done, it check previous p and current p(actually points to same one), it never get triggered.

I tried your code, the result was the same. I put a print after pkt p(64) to show it address, it is the same. It seems I need manually managed memory by allocating in producer and freeing in consumer.

[David Peng]

Previous debug is wrong. It is due to other trivial reason that the sensitivity list not triggered.

CLOSED.