Logical and Physical Address Generation | OS

Question

MSQ

Consider execution of the following C code for different cases:

printf(“%d”,&a);

Consider the following statements. Which among these are TRUE:

1. Will print Logical Address in case of Local Variables, as they are static, compiled before runtime, and compiler generates LAS.
2. Will print Logical Address in case of Global Variables, as they are static, compiled before runtime, and compiler generates LAS.
3. Will print Physical Address in case of variables Dynamically allocated during the runtime, like malloc()
4. Will print Logical Address in case of variables Dynamically allocated during the runtime, like malloc()

Answer 1

Answer: ‘a’,’b’ and ‘d’
 

#include <stdio.h>

int main() {
    
    int a=33;
    int b = (int*)malloc(sizeof(int));
    b=37;
    if(fork()==0)
    {
    printf("Child\n");
    printf("a is %d and address is %d   \t",a,&a);
    printf("b is %d and address is %d",b,&b);
    printf("\n\n");
    }
    else
    {
    b=b+1;
    a=a+1;
    printf("Parent\n");
    printf("a is %d and address is %d   \t",a,&a);
    printf("b is %d and address is %d",b,&b);
    printf("\n\n");
    }
    

    return 0;
}

 

Comments

@Souvik33 why will it be Logical Address? Like at runtime the process is loaded onto the MM so LA → PA address translation should take place while execution of the process so shouldn’t the address assigned to the variables be that of the Physical Memory?

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The OS provides protection by not showing us the real physical address, even after MMU converts the LAS into PAS during execution. 

But, I read it somewhere that it is possible to do malicious things in case of dynamic memory allocation, as memory is allocated during runtime, the compiler cannot know before hand about the memory. But here as per this code, even malloc() is showing same address, which is not for sure the PAS, or else both would not have been the same, it’s also the LAS, maybe these days OS has some plan to deal with this problem.

Apart from this, you can refer this GATE 2005 question: https://gateoverflow.in/765/gate-cse-2005-question-72 → deals with this same thing, but local static variable.