Cannot pass a string from C program to SV

SystemVerilog
,
1

Hi All,

I’m trying to do something using the DPI-C in System Verilog for the first time and I currently see an issue which I cannot get past.

Here is my C code:


#include <stdio.h>
#include <iostream>
#include <svdpi.h>
#include <vpi_user.h>
#include <string>
using namespace std;
extern "C" char* reg_access(char core_id[2], int mp, char my_name[256], char reg_path[256]) {
  char reg_hier[1024];
  if(mp) {
    reg_hier[0]='\0';
    strcat(reg_hier, "m_regs.");
    strcat(reg_hier, my_name);
    strcat(reg_hier, "_");

    strcat(reg_hier, core_id);
    strcat(reg_hier, ".");
    strcat(reg_hier, reg_path);
    vpi_printf(reg_hier);
    return reg_hier;
  } else {
    return "not valid";
  }
}

And here is my System Verilog Code:


import "DPI-C" context function string reg_access(string core_id, bit mp, string name, string reg_path);

$display("%s",reg_access("0", 1, "dut", "reg_map1.Enable"));

I’m basically trying to concatenate few strings together to form a register path. I see that my C function is called and the reg_hier value printed just before returning holds the value that I need. But the same value is not being printed in the System Verilog (It prints nothing). Is there something that I’m missing which could fix this? Any help appreciated.
Thanks,

2

In reply to szy0014:

I’m surprised you did not see the message from the C++ compiler attempting to return the address of a local variable ‘reg_hier’. It needs to be a static or heap allocated (malloc) address.

Also, the prototype for the reg_access function does not match what the DPI requires for string arguments and return values. You should use a dpi header file generated by your tool to match the code you have written.

dpiheader.h:

/* MTI_DPI */
/*
 * Copyright 2002-2022 Mentor Graphics Corporation.
 * Note:
 *   This file is automatically generated.
 *   Please do not edit this file - you will lose your edits.
 * Settings when this file was generated:
 *   PLATFORM = 'linux_x86_64'
 */
#ifndef INCLUDED_DPIHEADER
#define INCLUDED_DPIHEADER
#ifdef __cplusplus
#define DPI_LINK_DECL  extern "C" 
#else
#define DPI_LINK_DECL 
#endif
#include "svdpi.h"
DPI_LINK_DECL DPI_DLLESPEC
const char*
reg_access(
    const char* core_id,
    svBit mp,
    const char* name,
    const char* reg_path);
#endif

strings.c

#include <iostream>
#include <stdio.h>
#include <svdpi.h>
#include <vpi_user.h>
#include <string>
#include "dpiheader.h"
using namespace std;
 extern "C"  const char* reg_access(const char* core_id, svBit mp, const char* my_name, const char* reg_path) {
static  char reg_hier[1024]; 
 if(mp) {
    reg_hier[0]='\0';
    strcat(reg_hier, "m_regs.");
    strcat(reg_hier, my_name);
    strcat(reg_hier, "_");
    strcat(reg_hier, core_id);
    strcat(reg_hier, ".");
    strcat(reg_hier, reg_path);
    strcat(reg_hier, "\n");
    vpi_printf(reg_hier);
    return reg_hier;
  } else {
    return "not valid";
  }
}