Please create a memory diagram from this C code: #include #include #include stru

Question

Please create a memory diagram from this C code:

#include

#include

#include

struct person {

char *name;

int age;

};

void print (struct person *p)

{

printf ("Name=%s age=%d ", p->name, p->age);

}

int main ()

{

struct person **list;

int i;

list = (struct person **) malloc (sizeof(struct person*) );

for (i=0; i<3; i++) {

list [i] = (struct person*) malloc (sizeof(struct person));

}

list [0]->name = "R2-D2";

list [0]->age = 609;

list [1]->name = "Optimus Prime";

list [1]->age = 2700;

list [2]->name = "Wall-E";

list [2]->age = 210;

for (i=0; i < 3; i++){

print (list[i]);

}

for (i=0; i < 3; i++){

printf ("Heap Address %d : %u ", (i+1), list[i]);

}

}

Explanation / Answer

When we compile the above mentioned code, an executable is created called as a.out (default name)
Now this executable a.out is stored in the Text segment.

In C memory map the next segment comes is Initailized Data Segment (That stores the global/static variables assigned with some value by the user). In this given program we don't have such variables, so we can display them here in the memory map

After this comes the uninitialized Data Segment (That stores the global/static variables that are uninitialized). In this program we don't have such such variables, so we can display them here in the Memory Map.

Now, all the variables that have local scope i.e. they are present in a block of code (in between {}) or inside functions are stored in Stack Segment or Stack. The variable being stored in stack are per function/block of code. Every function gets some amount of memory on the stack to store the local state (i.e. the variables)

Taking firstly main() function,
int main ()
{
   struct person **list;
   int i;
   list = (struct person **) malloc (sizeof(struct person*) );
   for (i=0; i<3; i++) {
       list [i] = (struct person*) malloc (sizeof(struct person));
   }

   list[0]->name = "R2-D2";
   list[0]->age = 609;
   list[1]->name = "Optimus Prime";
   list[1]->age = 2700;
   list[2]->name = "Wall-E";
   list[2]->age = 210;
  
   for (i=0; i < 3; i++){
       print(list[i]);
   }
   for (i=0; i < 3; i++){
       printf ("Heap Address %d : %u ", (i+1), list[i]);
   }
   return 0;
}

Here local variables under main function are,
1. struck person **list or simply 'list'
2. int i or simply 'i'

Now moving second function print(),

void print (struct person *p)
{
   printf ("Name=%s age=%d ", p->name, p->age);
}

here we have following local variable,
1. struct person *p or just 'p'

Now coming back to next Memory segment which is Heap
Heap basically where C program gets it dynamically allocated memory i.e. memory that is reserved during run-time of program.
To identify heap we need to look into following code:

list = (struct person **) malloc (sizeof(struct person*));
for (i=0; i<3; i++) {
   list [i] = (struct person*) malloc (sizeof(struct person));
}

Here,
list = (struct person **) malloc (sizeof(struct person*));
With this we are allocating memory to a pointer that will be pointing an array of pointers.

Inside the loop we are allocating the memory to each pointer in array of pointers of type struct person.

Now this covers all the segements of the memory map.

Here is the final Memory Map:

On Stack we will have variables per function

Stack:

main:
Name        Contents
list       Address for the start of array of pointers  
i        Garbage Value (as we didn't initialize it intiially)

print:
Name         Contents
p             Stores the address passed by the calling function for variable of type struct person

Globals:

Name        Contents
No globals present in this code

For Heap , I have just added two printf to depict something, following is the code,

#include <stdio.h>
#include <stdlib.h>

struct person {
   char *name;
   int age;
};

void print (struct person *p)
{
   printf ("Name=%s age=%d ", p->name, p->age);
}

int main ()
{
   struct person **list;
   int i;
   list = (struct person **) malloc (sizeof(struct person*));
   printf ("Heap Address list : %u ", list); // Added
   for (i=0; i<3; i++) {
       list [i] = (struct person*) malloc (sizeof(struct person));
   }

   printf ("Heap Address list value: %u ", *list); // Added
   list[0]->name = "R2-D2";
   list[0]->age = 609;
   list[1]->name = "Optimus Prime";
   list[1]->age = 2700;
   list[2]->name = "Wall-E";
   list[2]->age = 210;
  
   for (i=0; i < 3; i++){
       print(list[i]);
   }
   for (i=0; i < 3; i++){
       printf ("Heap Address %d : %u ", (i+1), list[i]);
   }
   return 0;
}

Output:

./a.out
Heap Address list : 3569353520 <-- Address of list
Heap Address list value: 3569353616 <-- value stored in list, which is the start of array i.e. list[0]
Name=R2-D2 age=609
Name=Optimus Prime age=2700
Name=Wall-E age=210
Heap Address 1 : 3569353616 <--- list[0]
Heap Address 2 : 3569353632 <--- list[1]
Heap Address 3 : 3569362640 <--- list[2]

Heap:
Address            Contents
(Address of list)   (address of list[0] is stored inside)
3569353520           3569353616  

(list[0])
3569353616           R2-D2, 609

(list[1])
3569353632           Optimus Prime, 2700

(list[2])
3569353640           Wall-E, 210

Hope this Helps.

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