Question

This is a question is w.r.t to my assignment in my Operating Systems Class. I am not posting the code, but asking a general doubt. Since the project is to build an OS, there are no debugging tools. So any help is appreciated.

I have a header file which looks like this:

class test { 

private:
    unsigned long base_address;
    unsigned long size;  

public:
    test();
    void method1();
    void method2();

};

The corresponding C file looks like this:

#include "test.H"

unsigned long table;    

test::test(){
    base_address = some_value; //initialise the variables
    size = some_other_value;
    table = another_value;
    //some code
}

void test::method1(){
    //some code
}

void test::method2(){
    //some code
}

This code works fine, and I get the desired output. My problem is that I wanted to make the variable “table” (currently declared and used throughout all the functions in the C file), as a class variable. When I remove it from the C file and put it under the private variables in the header file, it compiles fine but then my output blows up.
Any pointers to what I should look into.? (Sorry I know its hard without knowing the context, but thanks for the help.)

Answer 1

Is “another_value” supposed to represent a magic number or another vaiable? First things first, you should assign all primitives an initial value when they are defined. Don’t use magic numbers, used #define ANOTHER_VALUE 0, if this is the case.

When I remove it from the C file and put it under the private variables in the header file, it compiles fine but then my output blows up.

Probably because you need to extern “C” they variable, if you are pulling in the protoype for use from c code:

How do I use extern to share variables between source files?

Here’s a compiling example:

externTest.hpp

/*
 * externTest.hpp
 *
 *  Created on: Mar 15, 2012
 */

#ifndef EXTERNTEST_HPP_
#define EXTERNTEST_HPP_

class eTest
{
private:
    unsigned long m_test;

public:
    eTest();

    void testValue();
};


#endif /* EXTERNTEST_HPP_ */

externTest.cpp

//============================================================================
// Name        : externTest.cpp
// Author      : 
// Version     :
//============================================================================
#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include "externTest.hpp"

extern "C" {
#include "value.h"
}

using namespace std;

eTest::eTest() {
    m_test=global_table;
}

void eTest::testValue() {
    printf("Global C Value: %lu \n", global_table);
    cout << "Test global value: " << m_test << endl;
}

value.h

/*
 * value.h
 *
 *  Created on: Mar 15, 2012
 */

#ifndef VALUE_H_
#define VALUE_H_

extern unsigned long global_table;

void assign_value(unsigned long lValue);

void print_global_value();

#endif /* VALUE_H_ */

value.c

/*
 * value.c
 *
 *  Created on: Mar 15, 2012
 */

#include "value.h"

unsigned long global_table;

void assign_value(unsigned long lValue) {
    global_table=lValue;
}

void print_global_value() {
    printf("Global C Value: %lu \n", global_table);
}

If you want to use the physical address of table, just make m_test a pointer to global_table:

private:
    unsigned long *m_test;

Be sure to dereference your pointer for the value:

eTest::eTest() {
    m_test=&global_table;
}

void eTest::testValue() {
    printf("Global C Value: %lu \n", global_table);
    cout << "Test global value: " << *m_test << endl;
}