In DPI-C, what data types to be used for internal variables?

Question

I am not getting correct output for the following piece of code using DPI-C in VCS in EDA Playground. I expect 6 as answer but I get 248 every time, irrespective of a and b values. I have tried using svLogic, int and unsigned char for data type for a_int in helloFromC.c.

module automatic test;

  import "DPI-C" function void helloFromC(logic [2:0] a, logic [2:0] b);

  initial run();

  task run();
    logic [2:0] a; 
    logic [2:0] b; 
    logic [2:0] c;
    a = 3'b100;
    b = 3'b010;
    c = a+b;
    $display("Output from SV is %0d", c);
    helloFromC(a,b);

  endtask

endmodule

This is my C program

#include <stdio.h>
#include <svdpi.h>


extern "C" int helloFromC(svLogic a, svLogic b) {
  svLogic a_int = a+b;
  printf("Output from C is %d", a_int);
  return 0;
}

I get output as

Output from SV is 6
Output from C is 248

From the `svdpi.h` header it looks like `svLogic` is an `uint8_t`. If so, that doesn't match the "%d" format string in C. — Bo Persson, Sep 08 '17 at 10:49
`logic` is a 4-state variable. Try `bit`. Also since you use 3-bit vars, mask the m in 'c'/ — Serge, Sep 08 '17 at 13:51
I have also tried with format specifier %u, %uhh. it still get 248 — Shankhadeep Mukerji, Sep 08 '17 at 23:18
instead of 'logic' try 'bit' in verilog and 'int' in c instead of svLogic. As for masking do `a_int = (a & 0x7) + (b & 0x7)` — Serge, Sep 09 '17 at 02:47
Only correct output is when a and b are declared as byte in SV and svBit in C. — Shankhadeep Mukerji, Sep 09 '17 at 04:35

Greg · Accepted Answer · 2017-12-18T19:03:38.523

3

svLogic is supposed to map to a single bit logic. You have a vector (aka packed array), therefor you should be using svLogicVecVal. It is still a 4-state value, so algorithmic operations of SystemVerilog values performed on the C side may not work the way you expect. Using bit [2:0] on the SystemVerilog side and svBitVecVal on the C side will work more as you expect. Or simplify things and use int on both sides.

For more on DPI, refer to IEEE1800-2012 section 35, Annex H, and Annex I.

edited Dec 18 '17 at 19:03

answered Sep 10 '17 at 22:41

Greg

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Is it svLogicVecValue or svLogicVecVal? Also, Im getting an error: error: no match for 'operator+' (operand types are 'svLogicVecVal {aka vpi_vecval}' and 'svLogicVecVal {aka vpi_vecval}') svLogicVecVal a_int = a+b; – Shankhadeep Mukerji Sep 11 '17 at 00:51
@ShankhadeepMukerji it is `svLogicVecVal`. I fixed the typo. It is a struct type defined in Annex H.10.1.2. If the operator+ is not defined for this type, then you will either need to define it yourself (search for "C custom operator" an "C overload operator"), or directly access the struct items in your operation. – Greg Sep 11 '17 at 01:25
from what I searched, I got results that C operators can be custom or overload. If there's an example, that would be great. – Shankhadeep Mukerji Sep 11 '17 at 01:44

score 2 · Answer 2 · answered Sep 11 '17 at 06:18

From one of the links, addition using DPI callI could find what I was looking for

#include <stdio.h>
#include <svdpi.h>


extern "C" void
add_bpv(
    const svBitVecVal* a,
    const svBitVecVal* b,
    svBitVecVal* c) {
    *c = *a + *b;
    printf("Output from C is %d", *c);
}

And now the SV program DPI call

module automatic test;

  import "DPI-C" function void add_bpv(input bit [3:0] a,b, output bit [3:0] c);

  initial run();

  task run();
    bit [3:0] a,b,c;
    a = 3'b100;
    b = 3'b010;
    c = a+b;
    $display("Output from SV is %d", c);
    add_bpv(a,b,c);
  endtask

endmodule

The output is what I wanted

Output from SV is  6
Output from C is 6

Well if it is solving my question, then yes :D – Shankhadeep Mukerji Sep 19 '17 at 09:43 — Shankhadeep Mukerji, Sep 19 '17 at 09:43

In DPI-C, what data types to be used for internal variables?

2 Answers2